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Evolution

Set No. 7

Presentation MEDIA: Genetics & Evolution Series

OHT Title OHT Title

Index to OHT Titles

Set 7: Evolution

NEW ZEALAND:Biozone International LtdP.O. Box 13-034 HamiltonTelephone: +64 (7) 856-8104FAX: +64 (7) 856-9243E-mail: [email protected]

AUSTRALIA:Biozone Learning Media AustraliaP.O. Box 7523 GCMC 4217 QLDTelephone: +61 (7) 5575-4615FAX: +61 (7) 5572-0161E-mail: [email protected]

UNITED KINGDOM:Biozone Learning Media (UK)P.O. Box 16710, Glasgow G12 9WSTelephone: +44 (141) 337-3355FAX: +44 (141) 337-2266E-mail: [email protected]

© 1993-2001 Biozone International Ltd ISBN 0-909031-46-0

Presentation MEDIA

1 Evolution2 Evidence for Evolution3 Interpretation of Fossils4 Fossils in a Rock Profile5 The Fossil Record6 The History of Life on Earth7 Evolutionary History 18 Evolutionary History 29 Precambrian Life

10 Early Palaeozoic Life11 Late Palaeozoic Life12 Mesozoic Life13 Cenozoic Life14 Comparative Embryology15 Anatomical Similarity16 Homologous Structures17 Analogous Structures18 Vestigial Organs19 Vestigial Organs in Whales20 Biogeographical Evidence21 The Biogeography of the Camel Family22 Biogeographical Distribution23 Biogeography of Tristan da Cunha24 Island Colonisers 125 Island Colonisers 226 DNA Hybridisation27 DNA Hybridisation Method28 DNA Sequencing29 Amino Acid Sequencing 130 Amino Acid Sequencing 231 Immunological Techniques32 Immunological Evidence33 Early Contributions to Evolutionary Thought34 The Modern Theory of Evolution35 The Development of Darwin’s Ideas36 History of Evolutionary Thought37 The Concepts of Darwinism38 Darwin’s Theory of Natural Selection39 Species40 Limitations of the Species Concept41 Ring Species 142 Ring Species 243 Reproductive Isolating Mechanisms44 Prezygotic Isolating Mechanisms 145 Prezygotic Isolating Mechanisms 2

46 Prezygotic Isolating Mechanisms 347 Postzygotic Isolating Mechanisms48 Hybrids in the Horse Family49 Speciation50 Types of Speciation51 Allopatric Speciation 152 Allopatric Speciation 253 Allopatric Speciation 354 Allopatric Speciation 455 Sympatric Speciation 156 Sympatric Speciation 257 Sympatric Speciation 3 (polyploidy)58 Stages in the Formation of Species59 Life Cycles of Species60 Speciation in Australian Treecreepers61 Factors Affecting Distribution62 Changes in Sea Level63 Continental Drift 164 Continental Drift 265 Continental Drift 366 New Zealand as a Natural Lab for Evolution67 Interglacials in Ancient New Zealand68 Ice Ages in Ancient New Zealand69 Speciation in New Zealand Stag Beetles70 Speciation in New Zealand Flax Snails71 Phylogeny and Taxonomy72 Convergent Evolution73 Convergent Evolution in Mammals74 Divergent Evolution75 Gradualism76 Punctuated Equilibrium77 Macroevolution78 The Evolution of Novel Features79 Adaptive Radiation80 Adaptive Radiation in Mammals81 The Diversity of Mammal Orders82 Divergent Evolution of the Ratites83 Divergence and Radiation of the Ratites84 Extinction85 Background Extinction86 Mass Extinctions87 Causes of Mass Extinctions88 Micro- vs Macroevolution89 Factors Operating in Evolution 190 Factors Operating in Evolution 2

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Set 7: Evolution

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EvolutionDefinition

Evolution is the permanent genetic change in apopulation of individuals.

It does not refer to changes that occur to an individualwithin its own lifetime – individuals do not evolve, butpopulations can.

MicroevolutionSmall-scale changes within gene pools over generationsis called microevolution.

MacroevolutionEvolutionary change on a large scale, involving wholegroups of species and genera, is called macroevolution.

Macroevolution includes:1. Adaptive radiation of groups of species into different

environments.2. The origin of evolutionary novelties such as the wings

and feathers of birds, and the upright posture ofhumans.

The evolutionary history of a species or taxonomic groupis called its phylogeny.

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Evidence for EvolutionEvolutionary theory is now supported by a wealth ofobservations and experiments.

Although biologists do not always agree on the exactmechanisms of population change, the fact that evolutionhas taken place is well documented.

Evidence for evolution comes from a variety of sources:

1. Palaeontology: The identification, interpretation anddating of fossils. This gives us some of the most directevidence of evolution.

2. Embryology: The study of the embryonic developmentof different organisms.

3. Comparative Anatomy: The study of the structure ofparticular organs in different organisms.

4. Biogeography: The study of geographic distributionscan indicate where species may have originally arisen.

5. Artificial Breeding: Selective breeding of plants andanimals has shown that certain phenotypiccharacteristics can be ‘selected for’ in offspring.

6. Biochemistry: Similarities and differences in thebiochemical make-up of organisms can closely parallelsimilarities and differences in appearance.

7. Molecular Biology: Sequencing of DNA and proteinsindicates the degree of relatedness betweenorganisms.

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Interpretation of FossilsThe term fossil refers to any parts or impressions of aplant or animal that may survive after its death.

Fossils form best when organisms are buried quickly inconditions that slow the process of decay.

Fossils are most commonly found in sedimentary rock.

The age of a fossil may be determined by using one ofthe following absolute dating methods:

Dating Method Age Range (years ago) Material Dated

Electron SpinResonance

500,000 – 1,000 Bone, tooth enamel,cave deposits

Fission Track 1 million – 100,000 Volcanic rock

Obsidian Hydration 800,000 – present Obsidian (volcanic glass)

Amino acidracemization

1 million – 2,000 Bone

Thermoluminescence less than 200,000 Pottery, fired clay,bricks, burned rock

Uranium/Thorium Less than 350,000 Bone, tooth dentine

Carbon 14 1,000 – 50,000+ Bone, shell, charcoal

Potassium/Argon 10,000 – 100 million Volcanic rocks

A fossil trilobite, a primitivearthropod that dwelled in theseas of the Devonian period370 million years ago

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Fossils in a Rock ProfileLayers of sedimentary rock are arranged in the order inwhich they were deposited, with the most recent layersnearer the surface (unless they have been disturbed).

Interpretation of rock layers containing fossils allows usto arrange the fossils in chronological order (order ofoccurrence), but does not give their absolute date.aaaaaaaaaaaaaaaa Recent fossils are

found in recentsediments

Only primitive fossilsare found in oldersediments

New fossil types markchanges in environment

OldestSediments

Most recentSediments

Fossil types differ ineach sedimentaryrock layer

Numerousextinct speciesaaaa

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The Fossil RecordAlthough incomplete, the fossil record can tell us muchabout the history of life on Earth:

1. Fossil species are often similar to, but usually differ from,today's species.

2. Fossil types often differ between sedimentary rock layers.

3. Fossils can be dated to establish their approximateabsolute age.

4. Older sediments have older fossils while more recentsediments have more recent fossils.

5. Numerous extinct species are found as fossils.

6. New fossil types mark changes in the past environmentalconditions on the Earth.

7. Modern day species can be traced through fossil relativesto distant origins.

8. Rates of evolution can vary, with bursts of speciesformation followed by stable periods.

MammothPleistocene

ArchaeopteryxJurassic

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The history of life is divided up into eons, epochs, eras and periods:

The History of Life on Earth

3,000 2,500 2,000 1,500 1,000 500 03,5004,0004,500

Archean Eon Proterozoic Eon

Precambrian Enlarged Below

Formationof the earth4,600 mya

Oldest known microfossilsfound in 3,500 million yearold chert in Western Australia

Oxygen produced byplants accumulates in

the atmosphere

Millions of years ago


CenozoicMesozoicPalaeozoicPrecambrian

Qu

aternary

Cam

brian

Ord

ovician

Silu

rian

Devo

nian

Carb

on

iferou

s

Perm

ian

Triassic

Jurassic

Cretaceo

us

Tertiary

Pro

terozo

ic

600 500 400 300 200 100 0

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Evolutionary History 1

Bacteria and algae

Protists

Fungi

Sphenophytes (ferns etc.)

Conifers

Cycads

Angiosperms

Cnidarians (Coelenterates)

Flatworms

Molluscs

Annelid worms

Insects

Crustacea

Diplopoda (centipedes etc.)

Arachnids (spiders etc.)

Echinoderms



Qu

ater

nar

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Cam

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Ord

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Silu

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Dev

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Car

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Per

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Tria

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Jura

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Cre

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Tert

iary

Pro

tero

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600 500 400 300 200 100 0

Inve

rteb

rate

sLa

nd P

lant

s

Based on fossil evidence and radio-isotope dating, theevolutionary history of non-chordates can be compiled:

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Evolutionary History 2

TunicatesAgnatha (jawless fish)

Sharks and RaysRay-finned fishLungfish

Amphibians

TurtlesCrocodiles & alligatorsRhyncocephalia (tuatara)

Squamata (lizards & snakes)

Birds

MonotremesMarsupialsPlacentals

Fish andtheirrelatives

Amphibians

Reptiles

Birds

Mammals



Qu

ater

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Silu

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Dev

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Car

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Tria

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Jura

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Cre

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Tert

iary

Pro

tero

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600 500 400 300 200 100 0

Based on fossil evidence and radio-isotope dating, theevolutionary history of chordates can be compiled:

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Precambrian Life

Algae

Cnidarians

Protozoans

Bacteriaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa4,600 mya: Origin of Earth.

4,600–3,800 mya: Chemical and molecular evolution leadingto origin of life: protocells to anaerobic bacteria.

3,800–2,500 mya: Origin of photosynthetic bacteria.

2,500–570 mya: Origin of protists, fungi, algae, and animals.

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Early Palaeozoic Life

Anomalocaris

Aysheaia

HallucigeniaOttoia

Wiwaxia

Pikaia

550-500 mya: Origin of animals with hard parts, whichappear as fossils in sedimentary rocks.

Simple marine communities become established.

A famous Canadian site with a rich collection of earlyCambrian fossils is known as the Burgess Shale deposits.Examples are shown below:

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Late Palaeozoic Life

Dimetrodon

Trilobite

Ammonite

Armouredfish

Earlyvascularplants

Early amphibians

Early insects

500-435 mya: Major adaptive radiations of marineinvertebrates and early fishes.

435-280 mya: Vast swamps with the first vascular plants.Origin and adaptive radiation of reptiles, insects, andspore bearing plants (including gymnosperms).

240 mya: Mass extinction of nearly all species on landand in the sea.

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Mesozoic Life

Torosaur

Early mammals

Deinonychus

Diplodocus

Mesosaur

Earlygymnosperms

240-205 mya: Recovery of surviving taxa, adaptive radiationof marine invertebrates, dinosaurs and fishes. Origin ofmammals. Gymnosperms become dominant land plants.

181-135 mya: Major radiations of dinosaurs.

135-65 mya: Major radiations of dinosaurs, fishes, andinsects. Origin of angiosperms.

65 mya: Apparent asteroid impact causes mass extinctionsof many marine species and all dinosaurs.

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Cenozoic Life

Sabretooth Tiger

Unitatherium

Deinotherium

Humans

Glyptodon

Diatryma

65-1.65 mya: Major shifts in climate. Major adaptiveradiations of angiosperms (flowering plants), insects, birdsand mammals.

3-5 mya: Early humans arise from ape ancestors.

1.65 mya: Modern humans evolve and their hunting andother activities accelerate.

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Comparative Embryology

Human

Gillslits

FertilisedEgg

LateCleavage

BodySegments

Limb Buds

Late Foetal

MonkeyBirdAmphibianDevelopmentalStage

Although the early developmental sequences betweenall vertebrates are similar, there are important deviationsfrom the general developmental plan in different species.

Similarity in development could be expected if allvertebrates were descended from a common ancestor.

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Anatomical Similarity

Forelimb Hind Limb

Femur(thigh)

Fibula

Tarsals(ankle)

Metatarsals(sole)

Phalanges(toes)

Tibia

Humerus(upper arm)

Radius

Carpals(wrist)

Metacarpals(palm)

Phalanges(fingers)

Ulna

The pentadactyl (5 digit) limb found on mostvertebrates has the same general bone structure.This similarity is called homology.

Forelimbs and hind limbs have different names forequivalent bones.

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Homologous Structures

Mole'sforelimb

Dog'sfront leg

Bird's wing

Human arm

Seal'sflipper

Bat's wing

Many mammals have limb bones that have been highlymodified to give a specialised locomotory function.

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Analogous Structures

Iris

Lens

Cornea

Retina

Iris

Lens

Cornea

RetinaOctopus Eye

Mammalian Eye

Not all similarity is inherited from a common ancestor.

Organs that have the same function in different organismsmay come from quite different origins.

Analogous structures do not imply an evolutionaryrelationship, but may illustrate convergence.

Examples: Eye structure of the octopus and mammals.Wing structure of a bird and a moth.

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Vestigial OrgansMany organisms have degenerate organs or structuresthat no longer perform the same function as in otherorganisms.

Presumably these organs were important in someancestral form, but became redundant in later species.

The selection pressure for their complete loss is weakso these structures remain in a reduced form.

Apparently, these vestigial organs have little use,although they often perform some secondary function.

Examples:

1. In humans, the appendix is often regarded as avestigial organ because it no longer has a digestivefunction as it does in many other mammals.

However, it may still have an immune function in itsdegenerate state.

2. The tiny pelvis and femur of whales are no longerused for the attachment of hind limbs.

3. The vestigial eyes of burrowing animals are nolonger used for vision.

4. The wisdom teeth of humans have little value inchewing.

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Vestigial Organs in Whales

Forelimb

Hindlimb

Pelvis

Femur

Whales are the descendants of large, four-legged landmammals that took up an aquatic existence some 60million years ago.

Over many millions of years, the pelvis and femur ofwhales have become very small and no longer fulfill alocomotory function.

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Biogeographical EvidenceThe study of plant and animal distribution is calledbiogeography.

The basic principle of biogeography is that each plant andanimal species originated only once. The place where thisoccurred is the centre of origin.

The range of a species can be very restricted or, as withhumans, almost the whole world (cosmopolitan).

Regions that have been separated from the rest of theworld for a long time, e.g. Australia and New Zealand,often have distinctive biota comprising of a large number ofendemic species (species that are found nowhere else).

General principles about the dispersal and distribution ofland animals are:

1. Closely related animals in different geographic areasprobably had no barrier to dispersal in the past.

2. The most effective barrier to dispersal in land animalswas sea (as when sea levels changed).

3. The discontinuous distribution of modern species maybe explained by movement out of the area theyoriginally occupied, or by extinction.

Oceanic islands often have species that are similar to, butdistinct from, those on neighbouring continents.

The occurrence of these species suggests that they wereisland colonisers that evolved in isolation differently to theirancestors on the mainland.

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The Biogeography of theCamel Family

Camel ancestor inNorth America 40million years ago

Four llama species:llama, alpaca,guanaco and vicuña

Land bridge acrossthe Bering Strait 1million years ago

Bactrian camelCamelus bactrianus

LlamaLama glama

Arabian camelCamelus dromedarius

Recent Distribution

Tertiary Distribution

VicuñaVicugna vicugna

GuanacoLama guanicoe

The camel family comprises of 6 modern-day species thathave survived on three continents.

There are no surviving species on their continent of origin,North America, where they emerged about 40 millionyears ago and later dispersed to other continents.

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The distribution of species around the world suggests thatmodern forms evolved from ancestral populations andspread out (radiated) out into new environments.

Good examples are found on islands offshore from largecontinental land masses.

Galapagos Islands

The Galapagos Islandshave species very similarto, but distinct from, theSouth American mainland.

Ancestral forms probablymigrated to the islandsfrom the mainland in thepast.

Cape Verde Islands

Species on the CapeVerde Islands have closerelatives on the WestAfrica mainland.

There are relatively fewanimals and noindigenous mammals.

Biogeographical Distribution

SouthAmerica

Galapagos Is

PacificOcean

800 km

WesternAfrica

Cape Verde Is

AtlanticOcean

450 km

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The Biogeography ofTristan da Cunha

African Origin2 Flowering Plants2 Ferns5 Liverworts

4,500 km3,000 km

Tristan da Cunha

SouthAmerica Africa

South AtlanticOcean

South American Origin7 Flowering Plants5 Ferns

30 Liverworts

Universal Origin19 Flowering Plants

The island of Tristan da Cunha, in the South AtlanticOcean, provides good evidence of the evolution of newspecies from ancestral ones.

Plant species on the island are of either South Americanorigin, African origin or both (universal origin).

Despite the fact that Africa is considerably closer, morespecies show South American affinities than African ones.

This is probably due to the predominant westerly tradewinds from the direction of South America.

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Island Colonisers 1Species may reach offshore islands (that provide newhabitats) in a number of ways:

Flight, rafting across water, being passively blown by theprevailing winds, etc.

Very mobile species such as seabirds with the ability forstrong, prolonged flight, are not easily isolated in onearea and are often widespread (cosmopolitan) in theirdistribution.

Very mobile terrestrial species may be isolated bychanges in the environment caused by major geologicalevents (e.g. volcanic activity, sea level rise).

Many species that become permanently isolated onoffshore islands have low mobility and weak powers ofdispersal.

They may be blown to an island by the prevailing windand be unable to return (e.g. bats, small birds, insects).

Other animals may be good colonisers because theycan survive periods of food and water deprivation whilemaking the journey to the island by sea (e.g. reptiles).

Successful colonisers of islands are often adaptable intheir food and habitat requirements.

In the new environment, species may later becomemore specialised as they exploit a new range ofresources.

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Island Colonisers 2

Rafting ondrifting vegetation

Planktoniclarvae

Swimming

Activeflight

Blown bystrong winds

Oceanic island

Deepocean

Amphibians cannot live awayfrom fresh water. They seldomreach offshore islands unless thatisland is a continental remnant.

Crustacean larvae drift to islands(e.g. crabs). Some crabs haveadapted to an island niche.

Sea Mammals have little difficulty inreaching islands (e.g. seals, sealions). They do not colonise theinterior of islands.

Seabirds fly to and from islands withrelative ease. Some adapt to life onland, (e.g. the flightless cormorantin the Galapagos Islands). Others,may treat the island as a stoppingplace (e.g. the frigate bird).

Small Birds, Bats, and Insects areblown to islands by accident. Theymust adapt to life there or perish.

Reptiles probably reach distantislands by floating in driftwood. Alow metabolic rate enables them tosurvive long periods without foodand water.

Land Mammals rarely coloniseislands. A high metabolic raterequires much food and water.Mammals cannot sustain themselveson long sea journeys.

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DNA Hybridisation

Flamingo Ibis Shoebill Pelican StorkNew World

Vulture0

10

20

30

40

50

Mill

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DN

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One way to reconstruct the evolutionary history of aspecies is using DNA hybridisation.

In this technique the DNA from different species is‘unzipped’ and recombined to form hybrid DNA.

Heat can be used to separate the hybridised strands. Theamount of heat required to do this is a measure of howsimilar the two DNA strands are (% bonding).

EXAMPLE:

The relationships among the New World vultures and storkshas been determined on the basis of DNA hybridisation.

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DNA Hybridisation Method

Unzip the DNA using heat(both human and chimpanzee

DNA unwinds at 86°C)

Mix strands toform a hybrid

Extract Human DNA Extract Chimpanzee DNA

Some of the opposingbases in the DNAsequence do not match

DNA is isolated from blood samples from each species:

The greater the similarity in DNA base sequence, thestronger the attraction between the two strands andtherefore they are harder to separate again.

By measuring how hard this hybrid DNA is to separate, acrude measure of DNA relatedness can be achieved.

The degree of similarity of the hybrid DNA can bemeasured by finding the temperature that it unzips intosingle strands again (in this case it would be 83.6°C).

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DNA Sequencing

0 20 40 60 80 100

DNA Similarity (%)

Chimpanzee 97.6%

Gibbon 94.7%

Rhesus Monkey 91.1%

Vervet Monkey 90.5%

Capuchin Monkey 84.2%

Galago 58.0%

Human 100%

Recent advanced techniques have enabled thesequence of DNA in different species to be determined.

Species thought to be closely related on the basis ofother evidence, were found to have a greater percentageof DNA sequences in common.

Humans and chimpanzees have a 97.6% similarity intheir DNA sequences and are very closely related.

An interesting finding was that the DNA of humans andchimpanzees is more closely matched than that ofchimpanzees and gorillas.

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Amino Acid Sequencing 1The sequences of amino acids in certain proteins (e.g.haemoglobin and cytochrome C) have revealed greatsimilarities and specific differences between species.

Closely related species have proteins with similar amino acidsequences.

Amino acid sequences are determined by inherited genesand differences are due to mutations.

The degree of similarity of these proteins is determined bythe number of mutations that have occurred. Distantly relatedspecies have had more time for differences to accumulate:

1. The greater the elapsed time since common ancestry,the greater the time for mutations to occur.

2. This in turn leads to a greater difference in amino acidsequences between species.

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Amino Acid Sequencing 2

9

8

3

1

Identical

No. of AminoAcids Differentfrom Humans

Squirrel monkey

Rhesus monkey

Gibbon

Gorilla

Chimpanzee

Primate Position ofChanged Amino Acids

5 6 9 21 22 56 76 87 125

9 13 33 50 76 87 104 125

80 87 125

104

–

The 'position of changed amino acids' is the point in the protein, composedof 146 amino acids, at which a different amino acid occurs.

SquirrelMonkey

RhesusMonkey

Gibbon

Gorilla

Chimpanzee

Amino acid differences for β-haemoglobin in primatescompared to the human sequence:

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Immunological Techniques

Human serumis injected intoa rabbit

Human Gorilla Baboon Lemur Rat

Rabbit serum with anti-human antibodies extracted

Precipitateforms

Decreasing recognition of anti-human antibodies to blood proteins

Rabbit serum added to blood of other species

1

2

3

Immunology indirectly measures the degree of similarity ofproteins in different species.

EXAMPLE: Anti-human antibodies are developed in arabbit and added to the blood of other species.

The greater the similarity between humans and the bloodof other species the greater the antibody-antigen reaction.

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Immunological EvidenceThe evolutionary relationships of a large number of differentanimal groups have been established on the basis ofimmunology.

The results support the phylogenies developed from otherareas of biology: biogeography, comparative anatomy,morphological studies, and fossil evidence.

Example: relatedness of tree frogs.

The phylogeny (evolutionary relationships)of tree frogs has been established byimmunology.

The immunological distance is the number ofamino acid substitutions between taxa.

01020304050


Immunological Distance

010

60

2030

NorthAmericantreefrogs

EuropeantreefrogsCricket frog

Chorus frog

Australiantreefrog

Australian treefrogs divergefrom other tree frogs morethan 35 million years ago

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Early Contributions toEvolutionary Thought

Contributors to the development of Darwin’s ideas were:

Jean Baptiste de Lamarck (1744-1829)Believed that organisms could pass ontraits acquired during their lifetime.– Discredited: when the mechanisms of

heredity became known.– Important: because he was the first to

propose that change over time was the resultof natural phenomena and not divineintervention.

Thomas Malthus (1766-1834)Believed that populations increased in sizeuntil checked by the environment, calledthe ‘struggle for existence’.

Charles Lyell (1797-1875) Developed the geological theory ofuniformitarianism: the physical features ofthe earth were the result of slow geologicalprocesses that still occur today.

Hebert Spenser (1820-1903)Introduced the concept of ‘Survival of theFittest’.

Jean Baptiste de Lamarck

Herbert Spencer

Charles Lyell

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The Modern Theory of EvolutionThe modern theory of evolution combines the following ideas:

– Darwin’s theory of the origin of species by natural selection.

– with an understanding of genetics (from Mendel).

– and the chromosomal basis of heredity (from Weismann).

Gregor Mendel August Weismann

This synthesis is called NEO–DARWINISM

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The Development ofDarwin’s Ideas

The first convincing case for evolution, The Origin ofSpecies, was published by Charles Darwin in 1859.

In The Origin of Species, Darwin argued that new speciesdeveloped from ancestral ones by natural selection.

Darwin developed his theory of “survival of the fittest” bybuilding on earlier ideas and supporting his views with alarge body of evidence he collected while voyagingextensively on the ship the ‘HMS Beagle’.

Alfred Russel Wallace, a young specimen collectorworking in the East Indies, developed a theory of naturalselection independently of Darwin. However, Darwinsupported the theory more extensively and receives mostof the credit for it.

Darwin’s theory was supported by data collected from:

1. The flora and fauna of South America. These showeddifferent adaptations for diverse environments butwere distinct from the European forms.

2. Observations of the fauna of the Galapagos Islandsconfirming his already formulated ideas from earlier inthe trip. He found that most of the Galapagos speciesare endemic, but resembled species on the SouthAmerican mainland.

3. Fossil finds of extinct species.

4. Evidence from artificial (selective) breeding.

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History of Evolutionary Thought

Alfred Russel Wallace1823 - 1913

‘Theory of Natural Selection’

John Baptiste deLamarck 1744 - 1829

First to publish a reasonedtheory of evolution.

Proposed idea of use anddisuse and inheritance ofacquired characteristics.

Rev. Thomas Malthus1766 - 1834

Wrote: ‘An Essay on thePrinciples of Population’,attempting to justify thesqualid conditions of the

poor.

Gregor Mendel1822 - 1884

Developed thefundamentals of the

genetic basis ofinheritance.

Julian Huxley 1887-1975

Ernst Mayr 1904 -

T. Dobzhansky 1900-1975

Collaborated to formulate themodern theory of evolution,

incorporating developments ingenetics, palaeontology andother branches of biology.

The New SynthesisNeo-Darwinism: The versionof Darwin’s theory refined and

developed in the light ofmodern biological knowledge

(especially genetics) in themid-20th century

Charles Lyell1797 - 1875

Major influence on Darwin.Lyell’s work ‘Principles of

Geology’ proposed that theearth is very old.

Erasmus Darwin1731 - 1802

Charles Darwin's grandfatherand probably an importantinfluence in developing his

thoughts on evolution.

Hebert Spencer1820 - 1903

Proposed concept of the‘survival of the fittest’

Charles Darwin1809 - 1882

‘Theory of Evolution byNatural Selection’

Pho

tos:

AT

August Weismann1834 - 1914

Proposed chromosomesas the basis of heredity,demolishing the theory

that acquiredcharacteristics could be

inherited.

R.A. Fisher 1890-1962

J.B.S. Haldane 1898-1964

Sewall Wright 1889-1988

Founding of population geneticsand mathematical aspects of

evolution and genetics.

OHT 37Produced by:


Set 7: Evolution



The Concepts of DarwinismDarwin’s view of life was of ‘descent with modification’ :descendants of ancestral forms adapted to differentenvironments over a long period of time.

The mechanism for adaptation is called ‘natural selection’,and is based on a number of principles:

1. OverproductionSpecies produce more young than will survive toreproductive age (they die before they have offspring).

2. VariationIndividuals vary from one another in manycharacteristics (even siblings differ). Some variations arebetter suited then others to the conditions of the time.

3. CompetitionThere is competition among the offspring for resources(food, habitat etc.).

4. Survival of the Fittest PhenotypeThe individuals with the most favourable combinationsof characteristics will be most likely to survive and passtheir genes on to the next generation.

5. Favourable Combinations IncreaseEach new generation will contain more offspring fromindividuals with favourable characters than those withunfavourable ones.

OHT 38Produced by:


Set 7: Evolution



Darwin’s Theory ofNatural Selection

InheritanceVariations are Inherited.The best suited variants

leave more offspring.

Natural SelectionNatural selection favours the

best suited at the time

OverproductionPopulations produce too

many young: many must die

VariationIndividuals show variation: someare more favourable than others

OHT 39Produced by:


Set 7: Evolution



Species

CoyoteCanis latrans

Red WolfCanis rufus

Grey WolfCanis lupus

Side-striped JackalCanis adjustus

DingoCanis familiaris dingo

Black-backed JackalCanis mesomelas

Domestic DogCanis familiaris

Golden JackalCanis aureus

Coyote–Red Wolf hybrids

No

Inte

rbre

edin

g

No

Inte

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edin

g

Inter-breeding

Inter-breeding

Inte

r-bre

edin

g Inter-breeding

Inter-breeding Inte

r-bre

edin

g

Inter-breeding

A biological species is a group of interbreeding (orpotentially interbreeding) individuals, reproductivelyisolated from other such groups.

Species are often composed of different populations(often in different habitats) that are quite distinct.

These are often called subspecies, races, and varietiesdepending on the degree of reproductive isolation.

The boundaries of a species gene pool can besometimes unclear, such as the genus to which all dogs,wolves, and related species belong:

OHT 40Produced by:


Set 7: Evolution



Definition of a biological species does not apply in all situations:1. The concept of a species being able to interbreed cannot

apply to extinct populations because this is unknown: extinctforms must usually be classified on morphological grounds.

2. Asexually reproducing organisms do not breed with eachother and so are assigned to species on the basis ofappearance or biochemical similarities.

Even for sexually reproducing organisms, a species may grade(show a gradual change) in phenotype over a geographic area.

Such a continuous gradual change is called a cline and oftenoccurs along the length of a country or continent.

All the populations are of the same species as long asinterbreeding populations link them.

A particular kind of clinal variation occurs with ring species.

Limitations of theSpecies Concept

Cline

OHT 41Produced by:


Set 7: Evolution



Ring Species 1A ring species is a special typeof cline that forms a loop,resulting in the two ends of thecline overlapping with eachother.

In such cases, the speciesoccupies a very wide geographicarea, where individuals at theends of the cline can be verydifferent from one another.

A classic example of a ringspecies is the herring gull andthe lesser black-backed gull.

– Their distribution forms acomplete circle around theNorth Pole.

– The two gull forms overlap inBritain and Western Europe,but rarely interbreed andbehave as two distinctspecies.

– However, they are connectedby a series of intermediate,interbreeding populations.

Lesser Black-Backed GullLarus fuscus

Herring GullLarus argentatus

Zone ofoverlap

OHT 42Produced by:


Set 7: Evolution



Ring Species 2

Gulls are recognisable as herringgulls and are classified as varioussubspecies of L.argentatus.

Gulls are recognisable as lesserblack-backed gulls and are classifiedas various subspecies of L.fuscus.

Zone of intermediate speciescapable of interbreeding withneigbouring populations.

Zone of overlap betweenthe gulls at the extremeends of the cline.

NorthAmerica

Asia

Europe

NorthPole

Siberia

2

1

3

4

5

6

7

Herring gulls fly eastwards acrossthe North Atlantic Ocean to Europe

75

1 4

Below is the geographical distribution of the herring gulland the lesser black-backed gull.

OHT 43Produced by:


Set 7: Evolution



Reproductive IsolatingMechanisms

Reproductive Isolating Mechanisms (RIMs) preventsuccessful breeding between different species. Theyare barriers to gene flow.

A single barrier may not completely isolate a gene pool,but most species have more than one isolatingmechanism operating to maintain a distinct gene pool.

Geographical barriers prevent species interbreeding butare not considered to be RIMs because they are notoperating through the organisms themselves.

Reproductive isolating mechanisms can be categorisedaccording to when and how they operate:

1. Prezygotic (pre-fertilisation) mechanisms include:

– habitat preference

– behavioural incompatibility

– structural incompatibility

– physiological incompatibility

2. Postzygotic (post-fertilisation) mechanisms include:

– zygote mortality

– poor hybrid fitness

– hybrid sterility

OHT 44Produced by:


Set 7: Evolution



Prezygotic IsolatingMechanisms 1

Desert Habitat Coastal Habitat

These closely relatedsnakes of the same genusdo not interbreed because

they do not get theopportunity to meet

Prezygotic isolating mechanisms act before fertilisationto prevent successful reproduction.

Ecological or HabitatDifferent species may occupy different habitats withinthe same region e.g. aquatic and terrestrial species.

OHT 45Produced by:


Set 7: Evolution




Breeding seasonfor Species A

J F M A M J J A S O N D

Breeding seasonfor Species B

J F M A M J J A S O N D

Temporal (Time)Species may have different activity patterns – they maybe nocturnal or diurnal, or breed at different seasons.

In the hypothetical example below, the two species of butterfly will never mate because they are sexually active at different times of the year:

OHT 46Produced by:


Set 7: Evolution



BehaviouralSpecies may have specificcalls, rituals, postures etc.that enable them to recognise potential mates (many bird species have elaborate behaviours).

StructuralFor successful mating, species must have compatible copulatory apparatuses, appearance,and chemical make-up(odour, chemical attractants).

Gamete MortalityIf sperm and egg fail to unite, fertilisation will be unsuccessful.

Lyre bird

Bird ofParadise

Peacock

Insects have very specificcopulatory organs thatact like a lock and key

Attemptedfertilisation

Sperm

Egg


OHT 47Produced by:


Set 7: Evolution



Hybrid InviabilityThe fertilised egg may fail to develop properlyFewer young may be produced and they mayhave a low viability (survivability).

Hybrid SterilityThe hybrid of two species may be viable but sterile,unable to breed (e.g. themule).

Hybrid BreakdownThe first generation maybe fertile but subsequent generations are infertile or non-viable.

Postzygotic IsolatingMechanisms

Postzygotic isolating mechanisms act after fertilisationto prevent successful reproduction.

Species A Species BX

Hybrid AB Hybrid ABX

Hybrid AB

Reduced viability

Non-viable or sterile

F1

F2

Reduced viability

This mule is ahybrid between ahorse and a donkey

OHT 48Produced by:


Set 7: Evolution



Hybrids in the Horse FamilySterile hybrids are common among the horse family.

The chromosomes of the zebra and donkey parents differin number and structure, producing a sterile “zebronkey”.

Zebra stallion (2n = 44) Donkey mare (2n = 62)

Y

Karyotype of ‘Zebronkey’ offspring (2n = 53)

X

Chromosomes contributedby donkey mother

Chromosomes contributedby zebra father

X

OHT 49Produced by:


Set 7: Evolution



SpeciationSpeciation refers to the process by which new species areformed.

Speciation occurs when gene flow has ceased betweenpopulations where it previously existed.

Speciation is brought about by the development ofreproductive isolating mechanisms which maintain theintegrity of the new gene pool.

Taking a very simple view, speciation can happen in one oftwo ways:

– Splitting: A species could split fairly equally into twopopulations that evolve differently until they eventuallybecome separate species.

– Budding: A small part of the species population could“bud off” from the main part and evolve rapidly (ingeological time-scale terms) to form a new species whileleaving most of the original species population unchanged.

A

B C A B

Splitting Budding

ASmall population

buds offSpecies

A

SpeciesC

SpeciesA

SpeciesB

SpeciesA

SpeciesB

Populationsplits eqally

OHT 50Produced by:


Set 7: Evolution



Types of SpeciationThere are several models proposed to account for the formationof new species among sexually reproducing organisms:

1. Allopatric speciation: Populations become geographicallyseparated, each being subjected to different natural selectionpressures, and finally establishing reproductive isolatingmechanisms.

2. Sympatric speciation: Occurs when a population forms anew species within the same area as the parent species.

– There is no geographical separation between thespeciating populations.

– All individuals are, in theory, able to meet each otherduring the speciation process.

Several models have been proposed for this type ofspeciation, involving one of the following:

– A change in host preference, food preference or habitatpreference.

– The partitioning of an essential but limiting resource.

– Instant speciation as a result of polyploidy (particularlyamong plants as in the evolution of wheat).

3. Parapatric speciation: The speciating populations are onlypartially separated geographically, so some individuals oneach side are able to meet across a common boundaryduring the speciation process.

Sympatric speciation is thought to be rarer than allopatricspeciation among animals, although it is probably a majorcause of speciation among plants.

OHT 51Produced by:


Set 7: Evolution



Allopatric Speciation 1STAGE 1: Moving Into New Environments

The parent population expands its range and occupiesnew parts of the environment.

Expansion of the range may be due to competition.

The population has a common gene pool with regulargene flow (any individual has potential access to allmembers of the opposite sex for the purpose of mating).

Parent Population

OHT 52Produced by:


Set 7: Evolution



Allopatric Speciation 2STAGE 2: Geographical Isolation

Gradual formation of physical barriers may isolate partsof the population at the extremes of the species range

As a consequence, gene flow between these isolatedpopulations is prevented or becomes rare.

Agents causing geographical isolation include:continental drift, climatic change, and changes in sealevel (due to ice ages).

IsolatedPopulation C

IsolatedPopulation B

Mountain barrierprevents gene

flow

River barrierpreventsgene flow

IsolatedPopulation A

Some naturalvariation exists ineach population

OHT 53Produced by:


Set 7: Evolution



Allopatric Speciation 3STAGE 3: Formation of a Subspecies

The isolated populations may be subjected to quitedifferent selection pressures.

These selection pressures will favour those individualswith traits suited to each environment.

Allele frequencies for certain genes change and thepopulations take on the status of a subspecies(reproductive isolation is not yet established).

Sub-species A Sub-species A

Sub-species C

Drier climate

Cooler climateWetter climate

OHT 54Produced by:


Set 7: Evolution



Allopatric Speciation 4STAGE 4: Reproductive Isolation

Each separated subspecies undergoes changes in itsgenetic makeup and behaviour. This will prevent matingwith individuals from the other populations.

Each subspecies’ gene pool becomes reproductivelyisolated from the others and they attain species status.

Even if geographical barriers are removed to allowmixing of the populations, genetic isolation is complete.

Species A

Species B

Sympatric species

Allopatricspecies

Mountain barrier remains

River barrierremoved

Species A

Sympatric species: Closely related species with overlapping distribution

Allopatric species: Closely related species still geographically separated

OHT 55Produced by:


Set 7: Evolution



Sympatric Speciation 1Habitat Preference

It is not uncommon for some insect species to beconditioned to lay eggs on the plant species on whichthey themselves were reared.

– If the normally preferred plant species is unavailable, then the insect may be forced to choose another species to lay eggs on.

– A few eggs surviving on this new plant will give rise to a new population with a new plant species preference.

Original Host Plant Species New Host Plant Species

An insect forced to lays its eggson an unfamiliar plant species

may give rise to a new populationof flies isolated from the original

population

OHT 56Produced by:


Set 7: Evolution



Sympatric Speciation 2Establishing Reproductive Isolation

If mating and rearing of offspring takes place entirelywithin the habitat, then the population will becomereproductively isolated.

Further differentiation of the two populations is likelyas each becomes increasingly adapted to theirrespective habitats.

Ultimately, the two groups will diverge to berecognised as separate species.

Geneflow

Original host plant species New host plant species

Nogeneflow

Each host plant will attract flies that werereared on that plant where they will matewith other flies with a similar preference

OHT 57Produced by:


Set 7: Evolution



Sympatric Speciation 3Polyploidy involves the multiplication of whole sets ofchromosomes (each set being the haploid number n).

Polyploids occur frequently in plants and in some animalgroups such as rotifers and earthworms.

When such individuals spontaneously arise, they areinstantly reproductively isolated from their parent population.

As many as 80% of flowering plant species may haveoriginated as polyploids. Different species of Chrysanthemum(some examples are shown below) have arisen as a result ofpolyploidy. They have chromosome numbers (2n) that aremultiples of 18: 2n = 18, 36, 54, 72, and 90.

OHT 58Produced by:


Set 7: Evolution



Stages in theFormation of Species

Different types of isolating mechanisms operate anddifferent amounts of gene flow take place as twopopulations diverge to form new species:

Population A

Race A

Subspecies A

Species A

Evo

lutio

nary

Dev

elop

men

t

Population B

Race B

Subspecies B

Species B

Population splits

Geographicisolation

Prezygoticisolation

Postzygoticisolation

Geographicisolation

Prezygoticisolation

Postzygoticisolation

HomogeneousAncestral Population

Gene flowcommon

Gene flowuncommon

Gene flowvery rare

No geneflow

OHT 59Produced by:


Set 7: Evolution



Life Cycle of SpeciesSpecies undergo a ‘life cycle’ of developmental stagesincluding formation, maturation and final extinction.

It has been estimated from the fossil record that this cyclemay take, on average, from between 1 to 20 million years,depending on the species.

Origin Extinction

Favourable mutationsor new combinationscan lead to formation

of new species

IncreasingGeneticVariability

Reduction insubspeciation

ReducingGeneticVariability

ReproductiveIsolation

Many Subspecies Fewer Subspecies

1 NeospeciesIncreasing range

and numbers

2 MesospeciesRange stable - subspeciesand individuals abundant

3 EuspeciesRange stable - individuals fairlyabundant, few or no subspecies

4 TelospeciesReduction in range

and numbers

OHT 60Produced by:


Set 7: Evolution



Speciation in AustralianTreecreepers

Treecreeper species populations are isolated by largeregions of inhospitable habitat (desert and dry areas).

These geographical barriers (circles below) arethought to have contributed to their speciation.

C. picumnuspicumnus

C. picumnusmelanota

C. melanuramelanura

C. melanurawellsi

C. rufa

Zone of sympatrydue to secondaryrange expansion

Secondary rangeexpansion

Circles denote physicalbarriers to expansion ofthe species range

Plumage Colour Key

Black

Brown

Rufous

OHT 61Produced by:


Set 7: Evolution



Factors Affecting DistributionChanges in Sea Level

Oscillations between periods of climatic cooling (iceages) and periods of warming (interglacials) cause sealevels to rise and fall.

Ice AgesDuring ice ages, vast amounts of sea water are lost assnowfall on the land and polar caps. This has the effectof causing a large drop in sea level (60 m).Areas previously isolated by water may develop landbridges, allowing species to move from one region toanother.

Warm PeriodsIn interglacial periods species may again be isolated bya rise in sea level.Separated on either side of a now submerged landbridges, they may evolve in response to very differentselection pressures.

Continental Drift

The crust of the Earth is in constant motion, movingwhole continents and sea floors.

The movement of continents, sometimes joining togetherand at other times separating, has had a powerful effecton the distribution of organisms.

OHT 62Produced by:


Set 7: Evolution



Changes in Sea LevelFalls in sea level or during ice ages create importantland bridges while rises in sea level during interglacialperiods create geographical barriers:

Warm Interglacial period

Present-day sea level

Populations on the mainland andthe offshore island are separatedby the physical barrier of the sea

Mainlandpopulations Island populations

Ice Age

After the sea bed has been exposed for thousands of years,recolonisation by terrestrial plants and animals may occur, allowingdispersal of island species to to the mainland and vice versa.

Sea level drops by60 metres (200 feet)

Snow andice fields

Mainland and islandcommunities remix

These isolated populations may be subjected todifferent natural selection pressures and mayultimately give rise to separate species.

OHT 63Produced by:


Set 7: Evolution



Continental Drift 1The movement of continents moved whole assemblagesof plants and animals across the surface of the Earth

As well as creating geographical barriers in the form ofoceans, it brought new combinations of species together:

NorthAmerica Europe Asia

Africa

Antarctica

Australia

India

Madagascar

SouthAmerica

TurgaiStrait

Late Palaeozoic

250 million years agoThe continents formed asingle landmass; dinosaursand mammal ancestorsroamed the earth.

Laurasia

Gondwanaland

Palaeocene – Mid Eocene

65-46 million years agoMost continents were separated,North America and Europe joined;

Mammals underwent adaptiveradiation into many forms.

Late Eocene

46-38 million years agoEurope split away from NorthAmerica and joined Asia. Indiawas still an island, with SouthAmerica joined to Antarctica.

NorthAmerica Europe Asia

AfricaIndia

MadagascarAustralia

Antarctica

SouthAmerica

OHT64

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Continental Drift 2The map of the present day world below shows the continents and islands thatonce comprised the supercontinent of Gondwana, and evidence supporting howthey once formed this single landmass

Madagascar

North America AsiaEurope

India

aaaaaaaaaaaaaaaaaaaaaaaaAfrica

AntarcticaaaaaaaaaaaaSouth

America

aaaaaaaaAustralia

NewGuinea

NewZealandaaaaaaaaaaaa aaaaaaaaaaOld Precambrian

(> 650 mya)Precambrian

(650-570 mya)Late Mesozoic(160-70 mya)

aaaaaaEarly Paleozoic(570-350 mya)

Paleozoic-Mesozoic(350-160 mya)

Distribution of Glossopteris

Distribution of Lystrosaurus

Geomagnetic pole direction 150 million years ago

Direction of ice sheet movement 350-230 mya

OHT65

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Continental Drift 3The map below shows how the continents and islands that once comprised thesupercontinent of Gondwana may be fitted together to form the single landmass.aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa aaaaaaaaAfrica aaaaaaaaaaaa aaaaaaaaaaaaaaaaAntarctica

aaaaaaaaaaaaaaaaaaaaaaaaSouth America aaaaaaaaaaaa aaaAustraliaMadagascar

Geomagnetic South Pole150 million years ago

South Pole 350-230million years ago

New Guinea

New CaledoniaNew Zealand

India

Gondwana supercontinentcoastline about 250-150million years ago

OHT 66Produced by:


Set 7: Evolution



New Zealand as a NaturalLaboratory for EvolutionThe islands of New Zealand in the South Pacific oceanprovide an excellent case study to examine the effects ofgeophysical events on evolutionary processes.

Consisting of two large islands and a smaller one to thesouth, the group of islands were subjected tochanges in sea level as ice ages came and went.

During the ice ages, the drop in sea level caused theislands to be united as a single land mass.

During periods of climate warming (interglacials), the sealevel rose to create more islands and archipelagos,particularly in the northern part of New Zealand.

New Zealand

Australia

Antarctica

Asia

PacificOcean

IndianOcean

OHT 67Produced by:


Set 7: Evolution



Interglacials in New ZealandDuring the warm interglacials, the sea level rose tocreate more islands and archipelagos in the northernpart of New Zealand.

Adjacent terrestrial populations which were once linked,became isolated from each other.

This isolation of gene pools may have assisted in theprocess of speciation in New Zealand.

New ZealandToday

Equivalent area on right

New Zealand Duringthe Pliocene

(10 million years ago)

NorthlandIsland

NorthIsland

Great BarrierIsland

Poor Knights Is.

Manukau

Str

ait

Ahipar

a

Str

ait

Three Kings Is.

AupouriIsland

OHT 68Produced by:


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Ice Ages in New ZealandDuring the colder climates of the ice ages, the sea levelsfell, exposing a larger land mass.

Islands that were once separated were then connectedby exposed sea bed.aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa a aaaaaaaaaaaaaaaaaaaaaaaaaaaaaa a aaaaaaaaaaa a aa aaaaa aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaPuysegur Is.

Traps Is.

Mernoo Is.

Ranfurly Is.

Poor Knights Is.

Three Kings Is.

WoodyVegetation

SubalpineGrassland

Steppe LoessZone (Grassland)

Sea level 60 m belowpresent level, revealinglarge areas of sea beda

Glaciers and Snow

Tundra Zone

Grassland

Woody Vegetation

New ZealandToday

New ZealandDuring an Ice Age

OHT 69Produced by:


Set 7: Evolution



Speciation in New ZealandStag Beetles

Stag beetles shared a common ancestor more than 10million years ago, before the Pliocene sea level rise.

Rises in the sea level created island chains separated bystraits (see map bottom left).

The isolated populations formed distinct species.

The present distribution of four species.is shown below:

A Staghorn Beetle(genus Lissotes)

A Stagh(genus

North Cape

Cape Mariavan Diemen

Lissotesplanus

Lissotes oconnori

Lissotesmangonuiensis

Lissotesreticulatus

Zone of overlap(sympatricdistribution)

Whangarei

distAuckland

NorthlandIsland

NorthIsland

Manukau

Str

ait

Ahipar

a

Str

ait


OHT 70Produced by:


Set 7: Evolution



Speciation in New ZealandFlax Snails

Flax snails, which are not very mobile, evolved in isolationin different regions of New Zealand during periods ofraised sea level (e.g. Pliocene).

The isolated populations formed three distinct species andhave evolved adaptations to particular habitats.

Auckland

Whangarei

PoorKnights Is.

Three Kings Is.

North Cape

NorthlandIsland

NorthIsland

Manukau

Str

ait

Ahipar

a

Str

ait


Placostylusbollonsi

Placostylusambagiosus

Placostylushongii

OHT71

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Phylogeny and TaxonomyThe evolutionary history of a group of related species is called phylogeny.

Reconstructing phylogenies involves identifying and classifying species to showtheir evolutionary relatedness: a discipline (or area of study) called taxonomy.

Similarity of form due to a shared ancestry is called homology.

EXAMPLE: Classification and Phylogeny of Order Carnivora.

Dog Wolf Coyote Fox Cat Puma Lion Tiger Cheetah

Canis Vuples Felis Panthera Acinonyx

Canidae Felidae

Carnivora

Genus

Family

Order

NOTE: This is a simplified diagram - there are additional families, genera, and species not represented here.

(The dog family) (The cat family)

OHT 72Produced by:


Set 7: Evolution



Convergent EvolutionNot all similarity is inherited from a common ancestor:

Species from different evolutionary branches mayresemble each other if they have similar ecological roles.

This is called convergent evolution.

Similarity due to convergence is not a basis for includingspecies in the same taxonomic group.

EXAMPLE: The swimming carnivore niche.

This niche was exploited by a number of unrelatedvertebrate groups at different times in the history of life.

The selection pressures of this niche produced fins orflippers and a streamlined body shape for rapidmovement through the water.

Reptile: Icthyosaur (extinct)Fish: Shark

Bird: PenguinMammal: Dolphin

OHT 73Produced by:


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Convergent Evolution in MammalsMarsupial and placental mammals have evolved separatelyto occupy similar niches – they are ecological equivalents.

Marsupialmole

Wombat

Flyingsquirrel

Rabbit

Mole

Woodchuck

Mouse

Wolf

Flyingphalanger

Marsupialmouse

Tasmanianwolf

Long-earedbandicoot

Flyingphalanger

Placental MammalsNorth America

Marsupial MammalsAustralia

OHT 74Produced by:


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Divergent EvolutionThe diversification of an ancestral group into two or morespecies in different habitats is called divergent evolution.

When divergent evolution involves the formation of a largenumber of species to occupy different niches this is calledan adaptive radiation.

A hypothetical family tree showing divergence fromcommon ancestors on two occasions:

Tim

e

Extinction

Speciationby budding

Species D

Speciationby splitting

Species W

Genetic changesaccumulate to forma new species

Little genetic change- species remainsrelatively unchanged

Changes in thegenetic make-up

of the two species

Species B

Species HSpecies P

Species D

Common Ancestor

Common Ancestor

OHT 75Produced by:


Set 7: Evolution



GradualismGradualism assumes that populations slowly divergefrom one another by accumulating adaptive characteristicsin response to different selective pressures.

If species evolve by gradualism there should betransitional forms seen in the fossil record, as with theevolution of the horse.

New speciesdiverges from theparent species

New Species Parent Species

Each speciesundergoes

gradualchanges in its

genetic makeupand phenotype

A typical pattern

OHT 76Produced by:


Set 7: Evolution



Punctuated EquilibriumThere is abundant evidence in the fossil record that,instead of gradual change, species stayed much the samefor long periods (stasis) and had short bursts of evolutionthat produced new species quite rapidly.

According to the punctuated equilibrium theory, most ofa species’ existence is spent in stasis and little time isspent in active evolutionary change.

New species 'bud off'from the parent speciesand undergo rapidchange, followed by along period of stability

New Species Parent Species New Species

A typical pattern

OHT 77Produced by:


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MacroevolutionMacroevolution refers to evolutionary changes above thelevel of the species: changes in genera or orders.

Macroevolution is concerned with changes in the kinds ofspecies over evolutionary time and includes:

1. The origin of unusual features (evolutionary novelties).

2. The origin of evolutionary trends (e.g. increased brainsize in primates).

3. Adaptive radiation (a form of divergent evolution).

4. Extinction.

Example of evolutionary trend: brain size in hominids

A. afarensis:450cc

Neanderthal:1400cc

Increasing Brain Size

H. habilis:675cc

H. sapiens:1800cc

OHT 78Produced by:


Set 7: Evolution



The Evolution of Novel FeaturesA change in the basic design of an organismcan produce a unique feature.

EXAMPLES:

1. Flowers of angiosperms.

2. Feathers of birds.

3. Wings of insects.

These new structures or preadaptations areusually variations of some existing structurealready in use for some other function.

EXAMPLES:

1. The feathers of birds have evolved from thescales of reptiles.

2. The bones of the middle ear in mammalshave been modified from equivalent bonesof the reptilian jaw.

Novel adaptations are thought to arisethrough changes to regulatory genes.

This can result in changes tobody structure, or to the rate ortiming of development.

ReptilianScale

Bird'sFeather

OHT 79Produced by:


Set 7: Evolution



Adaptive RadiationWhen an organism develops a new adaptive feature, anew niche may become available to it.

Once new niches are made available, an ancestralspecies can radiate from its point of origin and newspecies develop to exploit different habitats.

Adaptive radiation is more common in periods of majorenvironmental change e.g. cooling climates.

These changes may be the driving force for adaptiveradiation or they may have an indirect effect by increasingthe extinction rate.

EXAMPLE: The radiation of the mammals occurred afterthe extinction of the dinosaurs, which has made nichesavailable for exploitation.

ArborealHerbivore

Niche

UndergroundHerbivore

Niche

Marine PredatorNiche

FreshwaterPredator

Niche

FlyingPredator/Frugivore

Niche

TerrestrialPredator

Niche

Browsing/GrazingNiche

Megazostrodon - anearly mammal ancestor

OHT 80Produced by:


Set 7: Evolution



Adaptive Radiation in MammalsThe mammals have diversified widely, from an ancestralshrew-like form, into many different niches:

Early Mammals

PlacentalsDiverge

0.01

1.8

5

25

37

53

65

134

200

GeologicalTime Scale

Mar

supi

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Ant

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rs, s

loth

s

Har

es, r

abbi

ts

Rod

ents

Prim

ates

Bat

s

Inse

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Car

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Wha

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& d

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Odd

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Ele

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Mon

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mes

Sea

-cow

s

Aar

dvar

k

Hyr

axes

Col

ugos

Pan

golin

s

Sea

ls

Holocene

Pleistocene

Pliocene

Miocene

Oligocene

Eocene

Palaeocene

Cretaceous

Jurassic

Triassic

mya

OHT 81Produced by:


Set 7: Evolution



The Diversity of Mammalian Orders

Modern mammals are represented by 17 orders:

Insectivora Sirenia HyracoideaPholidota

Cetacea

Lagomorpha Pinnipedia DermopteraProbiscidea

Tubulidentata Artiodactyla Perissodactyla

Primates Edentata Chiroptera Carnivora Rodentia

OHT 82Produced by:


Set 7: Evolution



Divergent Evolutionof the Ratites

The ratites are an ancient group of birds that haveevolved from one ancestor and lost their powers of flightearly in their evolutionary development.

Their distribution can be explained by continental driftfollowing the break-up of Gondwana.

ElephantbirdMadagascar (extinct)

EmuAustralia

KiwiNew Zealand

CassowaryAustralia/New Guinea

RheaSouth America

MoaNew Zealand (extinct)

OstrichAfrica

OHT83

Produced by:

BIO

ZO

NE

INT

ER

NA

TIO

NA

L©

1993 – 2001

Set 7: Evo

lutio

n

Prin

ting

on

to P

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ibite

d

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us

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to g

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Tra

ns

pa

ren

cie

s (O

HT

s).

Divergence and Radiationof the Ratites

All otherliving birds

Brown Kiwi

Emu

Cassowary

Ostrich

Rhea 2

Moa 1: Anomalopteryx

Tinamou (can fly)

RATITES

Mesozoic Era Cenozoic Era

Ratites diverge fromthe line to the rest ofthe birds about 100

million years ago

Birds evolved from adinosaur ancestor about

150 million years ago

Rhea 1

Great Spotted Kiwi

Little Spotted Kiwi

Moa 2: Pachyornis

Moa 3: Dinornis

Moa 4: Megalapteryx

Fossil evidence suggests thatratite ancestors possesseda keeled breastbone and anarchaic palate (roof of mouth)

C

F

B

D

EG

A

K

H

J

I

L

A Letters indicatecommon ancestors

Elephantbird

OHT 84Produced by:


Set 7: Evolution



ExtinctionMost species that have ever lived are now extinct.

Entire lineages that were once dominant have nowdisappeared or have dwindled in numbers as other radiationshave flourished.

Often, the extinction of one group has allowed another toundergo extensive radiation into free niches: large scalespecies changes are probably opportunistic events.

Radiations may follow extinctions but are rarely the cause ofextinctions.

Background Rates of Extinction

A background rate of extinction describes the steady rate ofspecies extinction that is a natural feature of evolvingtaxonomic groups.

The causes of such extinctions vary depending on thespecies. In general, larger and more complex organisms havehigher rates of background extinction than simpler organisms.

Mass Extinction

Superimposed on average rates of background extinction arefive events in the Earth’s history when the rate of extinctionrose markedly. These are called mass extinctions.

Mass extinctions refer to an abrupt increase in extinctionrates affecting huge numbers of species at the same time.

During these episodes the diversity of major groups declines.

OHT 85Produced by:


Set 7: Evolution



Studies of the fossil record have revealed that eachtaxonomic group has species with a characteristic“duration of existence” before becoming extinct.

This provides the average background extinction rate forthe taxonomic group.

Background Extinction

Examples of the Estimated

Average Durations of Species

Taxonomic Species Duration

Group (millions of years)

Mosses and Liverworts 20 +

Higher Plants 8-20 +

Bivalves 11-14

Snails 10-13

Ammonites 1-15

Trilobites 1 +

Beetles 2 +

Freshwater Fishes 3

Snakes 2 +

Mammals 1-2 +

(After Stanley, 1985)

OHT 86Produced by:


Set 7: Evolution



Global history has been characterised by five majorextinction events and two of these have been intensivelystudied by palaeontologists:

The Permian Extinction (225 million years ago)This extinction event marks the Palaeozoic-Mesozoicboundary– over 90% of marine species perished and probablymany terrestrial ones also.

The Cretaceous Extinction (65 million years ago)Marks the Mesozoic-Cenozoic boundary andexterminated more than half the marine species andmany families of terrestrial plants and animals, includingnearly all the dinosaur species (the birds may bedinosaur descendants!)

Mass Extinctions

700

600

500

400

300

200

100

Millions of Years Ago

Palaeozoic Mesozoic Cenozoic

600 500 400 300 200 100 0

Nu

mb

ers

of

Fam

ilies Cretaceous

Triassic

PermianDevonian

Ordovician

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Set 7: Evolution



Mass extinctions are the result of more widespread andpossibly catastrophic events than those that affectindividual species.

The causes of mass extinctions are the topic of muchheated debate among palaeontologists today.

Theories include:

1. Changes in climate due to continental drift (slow). Sealevel changes caused by successive ice ages andinterglacials are implicated in most cases (slow).

2. Dust from volcanic activity on a huge scale causinga cooling of the planet (faster).

3. Asteroid/comet impact causing a dust cloud andcooling effect (immediate).

The Permian extinctions occurred at about the time thecontinents merged to form the super-continent ofPangaea. This probably caused a major disturbance ordestruction of many habitats, and altered the climate.

There are many more theories to account for theCretaceous extinctions, but one is most promising:

– There is good evidence for an asteroid or comet strikeat this time, which may have created a cloud of dust,blocking light and inducing a “nuclear winter” effect.

– A probable site for the impact crater lies just off thecoast of the Yucatan Peninsula in Mexico.

Causes of Mass Extinctions

OHT 88Produced by:


Set 7: Evolution



The mechanisms of gene pool change and naturalselection represent the modern synthesis of evolution.

Many biologists believe that given enough time, smallmicroevolutionary processes are sufficient to account forlarge evolutionary changes or macroevolution.

Over long periods of time (millions of years) newspecies will give rise to new genera, families, ordersand phyla (the gradualist view).

Other biologists believe (in the punctuated equilibriumview) that:

1. While natural selection is the force behind permanentgene pool changes and “fine-tunes” organisms totheir environment .....

2. Most morphological change occurs during abrupt,chance speciation events and once in existence,species then change very little.

However, the debate is not about whether or notevolution occurs. It is only about the relative importanceof different evolutionary mechanisms.

Micro- vs Macroevolution

OHT 89Produced by:


Set 7: Evolution



At the molecular level:

– Point mutations

– Control of gene expression

– Rate of protein synthesis

Forces Operating in Evolution 1

At the chromosomal level:

– Crossing over

– Block mutations

– Polyploidy

– Aneuploidy

– Independent assortment

– Recombination

At the cellular level:

– Gamete viability

– Fertilisation

– Genotype expression

UV Light

Sperm

Egg

Various “forces” or phenomenon have a part to play in theevolutionary process:

OHT 90Produced by:


Set 7: Evolution



At the organism level:

– Environmentalmodification of phenotype

– Reproductive success

– Selection pressures

– 'Fitness' of the phenotype

Forces Operating in Evolution 2

At the population level:

– Genetic drift and population size

– Natural selection altering genefrequencies

– Mate selection

– Intraspecific competition

– Founder effect

– Immigration/emigration (gene flow)

– Population bottlenecks

At the species level:

– Geographical barriers

– Reproductive isolation(prezygotic and postzygotic)

– Selection pressures

– Interspecific competition

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