Organic EvolutionChapter 6

Evolution - DefinedEvolution – a change in the genetic

composition of a population over time. A change in the frequency of certain alleles.

On a larger scale, evolution can be used to refer to the gradual appearance of all biological diversity.

Darwin’s Revolutionary Theory

The Origin of Species focused attention on the diversity of life, similarities as well as differences, and the adaptations organisms have for particular environments.

Darwin’s Revolutionary Theory

Charles Darwin presented evidence that many modern organisms are descended from ancestral species that were different.

Darwin’s Revolutionary Theory

Prevailing view of the world was that the Earth was only a few thousand years old and that all life had been created at the beginning and remained unchanged.

Pre-Darwinian Evolutionary IdeasSeveral ancient Greek philosophers thought life

changed through time.

Aristotle recognized fossils as forms of ancient life.He developed the scala naturae (scale of nature). Each form of life had a rung on the ladder.Organisms were arranged in order of complexity.

The ancient Greeks didn’t propose an evolutionary mechanism.

Pre-Darwinian Evolutionary Ideas

Lamarck was the first to suggest an explanation for evolution.Inheritance of

acquired characteristics

Didn’t hold up to testing.

A Mechanism for EvolutionDarwin presented a mechanism for evolution

– natural selection.Organisms that are in some way more successful at

reproduction will pass on more of their genes.Over time the traits responsible for that success will

become widespread in the population.This theory holds up very well!!

Alfred Russell Wallace

Wallace independently developed a theory of natural selection.

He sent his manuscript to Darwin, spurring him to finally publish his ideas.

Both ideas were presented to the Linnean Society in 1858.

Darwin finished On the Origin of Species and published it in 1859.

UniformitarianismCharles Lyell’s

principle of uniformitarianism:Laws of physics &

chemistry present throughout history of Earth.

Past geological events similar to today’s events.

Principles of Geology

UniformitarianismNatural forces could

explain the formation of fossil-bearing rocks.

Lyell concluded the age of the earth must be millions of years.

He stressed the gradual nature of geological changes.

Uniformitarianism and the Age of Earth

Darwin studied the work of Lyell closely. He took the first volume of Lyell’s Principles of Geology on the Beagle. He received the second volume while on the voyage.He concluded that Earth must be much older than

6000 years.Perhaps these slow changes could work on living

things as well…..

Evolution in Need of a MechanismDarwin was not the first to have the thought

that organisms change through time.His grandfather, Erasmus Darwin, and others

suggested that life evolves as environments change.But a mechanism for that change was needed.

Darwin (1809 – 1882)Darwin had a

lifelong love of nature.

His father wanted him to study medicine.This was not what

Darwin wanted and he didn’t finish.

DarwinAfter leaving medical school he attended

Cambridge University with the intention of entering the clergy.

His mentor and botany professor, John Henslow, recommended him for a position as ship’s naturalist aboard the Beagle.

The Voyage of the BeagleDarwin started out on a five year trip around the

world aboard the Beagle in 1831. He was 22.

As ship’s naturalist he spent his time on shore collecting thousands of plant and animal specimens and making important observations.

The Voyage of the BeagleDarwin saw that the plants and animals that he

found in temperate areas of South America were more similar to tropical South American species than they were to temperate European species.

The Voyage of the BeagleThe fossils he found in South America were

more like modern South American species than European species.

The Voyage of the BeagleDuring the voyage he read Lyell’s Principles of

Geology.

He had Lyell’s ideas in mind as he traveled and observed the geology of South America.

The Voyage of the BeagleHe experienced an earthquake in Chile and

observed that the coastline had risen several feet.

He also found marine fossils high in the Andes Mountains.

Darwin concluded that the mountains were formed by a series of such earthquakes.

The Voyage of the Beagle

Darwin became interested in the geographic distribution of organisms after visiting the Galapagos Islands.

After the VoyageAfter returning,

Darwin realized that adaptation to the environment and the origin of new species were closely linked processes.

Galapagos finch species have evolved by adapting to specific conditions on each island.

Natural SelectionAfter reading a paper by

Thomas Malthus concerning the fact that human populations increase faster than limited food resources, Darwin noticed the connection between natural selection and this ability of populations to overreproduce.

Natural SelectionOnly a small fraction of all offspring produced

by any species actually reach maturity and reproduce.

Natural populations normally remain at a constant size.

Natural SelectionThose that survive may have heritable traits

that increased their chances of survival.They will pass those traits on.The frequency of those traits will increase.

Artificial SelectionArtificial selection

– people selectively breed organisms with desired traits.Darwin noticed that

considerable change can be achieved in a short period of time.

Natural SelectionNatural selection occurs when organisms with

particular heritable traits have more offspring that survive & reproduce.

Natural SelectionNatural selection

can increase the adaptation of an organism to its environment.

Natural SelectionWhen an environment changes, or when

individuals move to a new environment, natural selection may result in adaptation to the new conditions.Sometimes this results in a new species.

Natural SelectionIndividuals do not evolve; populations evolve.

Evolution is measured as changes in relative proportions of heritable variations in a population over several generations.

Natural SelectionNatural selection can only work on heritable

traits.Acquired traits are not heritable and are not subject

to natural selection.

Natural SelectionEnvironmental factors are variable.

A trait that is beneficial in one place or time may be detrimental in another place or time.

Darwinian Evolutionary Theory: Evidence

The main premise underlying evolutionary theory is that the living world is always changing.

Perpetual change in form & diversity of organisms over the last 700 million years can be clearly seen in the fossil record.

Fossils

Fossils are remnants of past life preserved in the earth.Complete remains –

insects in amber.Petrified skeletal parts

infiltrated with silica or other minerals.

Or traces of organisms such as molds, casts, impressions, trackways, or fossilized excrement.

The Fossil RecordFossils provide support

for the idea that life changes through time.Fossil intermediates

Whales descended from land mammals.

Birds descended from one branch of dinosaurs.

The oldest fossils are of prokaryotes.

Dating FossilsGeological time can be measured in

sedimentary rock layers.The Law of Stratigraphy

Dates oldest layers at the bottom and youngest at the top.

Time is divided into eons, eras, periods and epochs.

Dating FossilsRadiometric dating methods are based on the

decay of naturally occurring elements into other elements.Different methods used for different time

periods.

Dating Fossils - example40K has a half life of 1.3 billion years – meaning

half of the 40K will have decayed to 40Ar and 40Ca. Half of what remains will decay in the next 1.3 billion years.

Measure ratio of remaining 40K to the amount of 40K originally there (remaining 40K plus 40Ar and 40Ca).

Fossil Record

The fossil record of macroscopic organisms begins in the Cambrian period: 505–570 MYA.

Fossil bacteria and algae, casts of jellyfishes, sponges spicules, soft corals, and flatworms are found in Precambrian rocks.Mostly microscopic

Evolutionary TrendsThe fossil record shows that species arise and

go extinct repeatedly throughout geological history.

Trends appear in the fossil record – directional changes in features or patterns of diversity.

Evolutionary TrendsThe evolution of horses

from the Eocene epoch (57.8 MYA) to the present is a well studied trend.Body size – increasing Foot structure – fewer

toes Tooth structure –

larger grinding surface

Common DescentDarwin proposed that all organisms have

descended from a single ancestral form.

Life history is shown as a branching tree called a phylogeny.

HomologyThe phrase “descent with modification”

summarizes Darwin’s view of how Evolution works.All organisms descended from common ancestor.Similar species have diverged more recently.

Homology – when similar structures result from shared ancestry.

Anatomical HomologiesHomologous structures – variations on a

structural theme that was present in a common ancestor.

Example – vertebrate forelimbs have different functions, but share the same underlying structure.

Anatomical Homologies

Vertebrate embryos have a tail and pharyngeal pouches.

These structures develop into different but homologous structures in adults.Gills in fishesPart of ears & throat in humans.

Ontogeny & PhylogenyThere are many parallels between ontogeny

(an individual’s development) and phylogeny (evolutionary descent).Embryological similaritiesFeatures of an ancestors ontogeny can be shifted

earlier or later in a descendant's ontogeny.

Ontogeny & PhylogenyHeterochrony – evolutionary

change in timing of development.Characteristics can be added

late in development and features are then moved to an earlier stage.

Ontogeny can be shortened in evolution.

Terminal stages may be deleted causing adults of descendants to resemble youthful ancestors.

PaedomorphosisRetention of ancestral

juvenile characters by descendant adults.

Developmental Modularity and Evolvability

Heterotopy – a change in the physical location of a developmental process in an organism’s body.Process must be compartmentalized into semi-

autonomous modules to be expressed in new location

Ex: Location of toepad development in geckos.

Developmental Modularity and Evolvability

Evolvability – denotes the great evolutionary opportunities created by semi-autonomous developmental modules whose expression can be moved from one part of the body to another.

Allows for “experimentation” with the construction of many new structures.

Vestigial OrgansVestigial organs – remnants of structures that

served important functions in an ancestor.Remnants of pelvis and leg bones in snakesAppendix in humans

Molecular HomologiesSimilarities can be found at the molecular level,

too.The genetic code is universal - it is likely that all

organisms descended from a common ancestor.Different organisms share genes that have been

inherited from a common ancestor.Often, these genes have different functions, like

the mammalian forelimbs.

Homologies & the Tree of LifeDarwin’s evolutionary tree of life can explain

homologies.The genetic code is shared by all species because it

goes back deep into the ancestral past.More recent homologies are shared by only smaller

branches of the tree.

Homologies & the Tree of LifeHomologies result in

a nested pattern.All life shares the

deepest layer.Each smaller group

adds homologies to those they share with larger groups.

SpeciationSpeciation refers to the formation of new

species.

Defining a species is difficult…Descent from common ancestral population.Ability to interbreed.Maintenance of genotypic & phenotypic cohesion.

Reproductive barriers prevent species from interbreeding.Where do they come from?

Allopatric SpeciationAllopatric (another land) populations occupy

separate geographic areas.Separated geographically, but able to interbreed if

brought together.

Over time, reproductive barriers may evolve so that they could not interbreed.Allopatric speciation

Allopatric SpeciationThe geographical separation can arise in two

ways:Vicariant speciation is initiated when climatic or

geological changes fragment a species’ habitat, forming impenetrable barriers.

Founder events occur when a small number of individuals disperse to a distant place where no other members of their species exist.

HybridsMuch can be

learned by studying what happens when previously isolated populations come into contact again.Hybrids are

offspring of members of two closely related species.

HybridsSpecies eventually become different enough

that they can’t produce hybrids.Premating barriers prevent mating from occurring in

the first place.Postmating barriers impair growth, survival, or

reproduction of the offspring.

Sympatric SpeciationSympatric (same land) speciation occurs when

speciation occurs in one geographic area – a lake for example.

Individuals within the species become specialized on a food type, shelter, part of the lake etc. Eventually reproductive barriers evolve.

Parapatric SpeciationParapatric Speciation – geographically

intermediate between allopatric and sympatric speciation.Two species are parapatric if their geographic

ranges are primarily allopatric but make contact along a borderline that neither species successfully crosses.

Adaptive RadiationAdaptive radiation – the production of

ecologically diverse species from a common ancestral stock.Common in lakes & islands – sources of new

evolutionary opportunities.

Adaptive RadiationArchipelagoes increase opportunities for both

founder events and ecological diversification.Entire archipelago isolated from the continent.Each island is geographically isolated from the others.Ex: Galápagos Islands

GradualismDarwin’s theory of

gradualism proposes that small differences accumulate over time producing the larger changes we see over geologic time.Certainly, this process

is always at work, but probably does not account for all changes.

Punctuated EquilibriumPunctuated

equilibrium states that phenotypic evolution is concentrated in relatively brief events of branching speciation followed by periods of stasis.

Populations EvolveVariation exists within

any population.

When natural selection acts to favor one trait over another that trait will increase in the population.

The population has evolved, not any one individual.

The Modern SynthesisPopulation Genetics – the study of how

populations change over time.Dependent on both Darwin’s theory of natural

selection and Mendel’s laws of inheritance.All heritable traits have a genetic basis, some are

controlled by multiple genes – not as simple as in Mendel’s studies.

The Modern SynthesisThe modern synthesis is a comprehensive

theory of evolution that brings in ideas from many fields.R. A. Fisher (statistician)J. B. S. Haldane (biologist)Theodosius Dobzhansky (geneticist)Sewall Wright (geneticist)Ernst Mayr (biogeographer)George Gaylord Simpson (paleontologist)G. Ledyard Stebbins (botanist)

PopulationsPopulation – a localized, interbreeding group

of individuals of a particular species.Separate populations of a species may be isolated

from each other.

PopulationsSometimes the

populations overlap, but little interbreeding occurs.

MicroevolutionMicroevolution – evolutionary changes in the

frequency of alleles in a population.Polymorphism – occurrence of different allelic

forms of a gene in a population.If there is only one allele for a gene in the population

– every individual is homozygous for the trait – it is fixed in the population.

All alleles of all genes possessed by all members of a population form a gene pool.

MicroevolutionPopulation geneticists measure the relative

frequencies of alleles in the population.Allelic frequency

Genetic EquilibriumAccording to Hardy-Weinberg equilibrium,

the hereditary process alone does not produce evolutionary change.Allelic frequency will remain constant generation to

generation unless disturbed by mutation, natural selection, migration, nonrandom mating, or genetic drift.Sources of microevolutionary change.

Frequency of AllelesEach allele has a frequency (proportion) in the

population.

Example population of 500 wildflowers.CRCR = red; CRCW = pink; CWCW = white

320 red, 160 pink, 20 whiteFrequency of CR =

(320 x 2) + 160 / 1000 = 800/1000 =.8 = 80%

Frequency of Allelesp is the frequency of the most common allele

(CR in this case).p = 0.8 or 80%

q is the frequency of the less common allele (CW in this case).

p + q = 1

q = 1- p = 1 – 0.8 = 0.2 or 20%

Hardy-Weinberg TheoremPopulations that are

not evolving are said to be in Hardy-Weinberg equilibrium.

As long as Mendel’s laws are at work, the frequency of alleles will remain unchanged.

Hardy-Weinberg TheoremThe Hardy-Weinberg theorem assumes

random mating.

Generation after generation allele frequencies are the same.

Hardy-Weinberg TheoremAt a locus with two alleles, the three genotypes

will appear in the following proportions:

(p + q) x (p + q) = p2 + 2pq + q2 = 1

Hardy-Weinberg TheoremConditions:

Very large populationNo gene flow into or out of the populationNo mutationsRandom matingNo natural selection

Departure from these conditions results in evolution.

Practice with Hardy WeinbergHardy & Weinberg studied the frequencies of

alleles in populations.Frequency – the proportion of individuals in a

category in relation to the total number of individuals. 100 cats, 84 black, 16 white – frequency of black = 84/100 = 0.84, white =0.16.

Two alleles – p is common, q is less common.p+q = 1

Practice with Hardy Weinberg(p + q)2 = p2 + 2pq + q2

Individuals homozygous for allele B

Individuals heterozygous for alleles B & b

Individuals homozygous for allele b

Practice with Hardy WeinbergUsed to calculate allele frequencies (p & q) in a

simple way.

100 cats, 16 white (bb) so q2 = 0.16

q = square root of 0.16 = 0.40.

Since p + q = 1; p = 1 – q = 0.60.

p2 = 0.36; so 36 homozygous dominant (BB)

2pq = 0.48; 48 heterozygous (Bb)

Where Does Variation Come From?Natural selection acts on the variation that is

already present in the population.But, where did that variation come from?

Where Does Variation Come From?Two processes provide the variation in gene

pools.MutationSexual recombination

MutationNew genes or

alleles only result by mutations.

Mutations are changes in the nucleotide sequence of DNA.

Point MutationsPoint mutation – a change in a single base

pair.

Often harmlessMuch of the DNA does not code for protein products.Genetic code is redundant.

CGU, CGA, CGC, CGG all code for argenine.

Occasionally significantSickle cell disease.

MutationsBeneficial mutations of any kind are very rare.

Mutations that alter gene number or sequence are almost always harmful.

Gene DuplicationGene duplication occasionally provides an

expanded genome with new loci that may take on new functions as selection continues.

New genes can also appear when non-coding introns get shuffled into the coding portion of the genome.

Sexual RecombinationSexual recombination is a much more

common way of producing variation in populations.Reshuffling of allele combinations already present in

the population is how variation is maintained in populations.

Sexual reproduction rearranges alleles into fresh combinations every generation.

Natural SelectionWhen natural selection is occurring, some

individuals are having better reproductive success than others.Alleles are being passed to the next generation in

frequencies that are different from the current generation.

Hardy-Weinberg equilibrium is upset.

Genetic DriftThe smaller the sample, the greater the chance

of deviation from expected results.These random deviations from expected

frequencies are called genetic drift.Allele frequencies are more likely to deviate from the

expected in small populations.

Genetic DriftWhich allele was lost is due to random chance.

Over time, drift tends to reduce genetic variation through random loss of alleles.

The Bottleneck EffectSometimes a

catastrophic event can severely reduce the size of a population.The random

assortment of survivors may have drastically different allele frequencies.

Bottleneck effect

The Bottleneck EffectThe actions of people

sometimes cause bottlenecks in other species.N. California elephant seal

population reduced to 20-100 individuals in the 1890s.

Current population > 30,000.

Variation drastically reduced – 24 genes with 1 allele.

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The Founder EffectFounder effect – When a small group of

individuals becomes separated from the population and form a new population, the allele frequencies in their gene pool may be different than the original population.

Gene FlowThe population can gain or lose new alleles

through gene flow.

When individuals move into or out of a population, they may carry the only copy of certain alleles in the gene pool with them.

Gene flow usually reduces differences between populations.

Natural Selection & Adaptation

Natural selection is the only one of these ways of altering the gene pool that results in adaptation.

Selection depends on variation.

Genetic VariationVariation in a population is always present.

Heritable variation is the raw material of natural selection.

Genetic Variation

Not all genetic variation is heritable.

Environmental influences sometimes effect phenotype.

Polymorphism

Different versions of discrete characters are called morphs.

When a population has two or more morphs that are common in the population, it is called polymorphic.This is phenotypic

polymorphism

Protein PolymorphismDifferent allelic forms of a gene code for

slightly different proteins – protein polymorphism.

If the difference affects the protein’s net electric charge, the different forms can be separated using protein electrophoresis.

Quantitative VariationQuantitative traits are those that show

continuous variation.Influenced by many genes.Height in humans, tail length in miceWhen trait values for a population are graphed, they

follow a bell shaped curve.

Modes of Selection Stabilizing –

removes the extremes.

Directional – variants at one of the extremes are favored.

Disruptive – variants at both extremes are favored.

Evolutionary FitnessFitness – the contribution an individual makes

to the gene pool of the next generation.

Relative fitness – the contribution of one genotype relative to the contribution of other genotypes at the same locus.

Natural selection acts on phenotypes.

Preserving VariationSome variation is hidden from the natural

selection process in the form of recessive alleles in heterozygotes.

Less favorable recessive alleles can be maintained in the population because they do not harm heterozygous individuals.

Sexual SelectionSexual selection –

natural selection for mating success.May result in

sexual dimorphism – differences between the sexes.

Secondary sexual characteristics – not directly involved in reproduction.

Intrasexual SelectionIntrasexual selection – selection within the

same sex – results when individuals of one sex are competing with each other for members of the other sex.Features that make the male a better fighter

or more intimidating to other males would be favored.

Intersexual SelectionIntersexual

selection – mate choice – individuals of one sex are choosy in selecting a mate.Features that make

an individual more attractive to the opposite sex would be favored.

Intersexual SelectionShowiness that results from mate choice can

be risky.Flashy tails of guppies make them more visible to

predators.

Benefits of finding a mate outweigh potential costs.

Showiness may reflect overall health.

MacroevolutionMacroevolution refers to grand scale events

in evolution.Evolution of new structuresMajor trends in the fossil record

Gould’s Tiers of TimeStephen Jay Gould

recognized three tiers of time for evolutionary processes:Tens to thousands of

years – population genetic processes.

Millions of years – speciation and extinction can be measured and compared among different groups of organisms.

Tens to hundreds of millions of years – marked by episodic mass extinctions.

Speciation and Extinction Through Geological Time

A species has two possible fates:Become extinct orGive rise to new species.

Speciation and extinction rates vary among species.

Lineages with high speciation and low extinction produce the greatest diversity.

Speciation and Extinction Through Geological Time

Species Selection Differential survival and multiplication of species

based on variation among lineages.Species-level properties include mating rituals, social

structuring, migration patterns, geographic distribution, etc.

Mass Extinctions

Mass extinctions are episodic events where many species go extinct at the same time.Permian extinction – 225 MYA – half the families of

shallow-water marine invertebrates and 90% of the marine invertebrate species went extinct over a few million years.

Cretaceous extinction – 65 MYA – marks the end of the dinosaurs as well as many other species.

Mass ExtinctionsMany possible

explanations for mass extinctions have been suggested.

Alvarez hypothesis – bombardment of the earth by asteroids would send debris into the atmosphere, altering climate.Search for evidence

CratersAtypical iridium

concentrations

Mass ExtinctionsCatastrophic species selection would result

from selection by these events. Mammals were able to use resources due to

dinosaur extinction.

Endurance of Darwin’s TheoryThe beauty of Darwin’s theory is that it explains

so many different kinds of observations: anatomical and molecular homologies that match patterns in space (biogeography) and time (fossil record).

"Nothing in biology makes sense except in the light of evolution." Theodosius Dobzhansky, Geneticist