EVOLUTION

Evolution• process that transformed life on Earth

from its earliest beginnings to diversity seen today

• unifying force for biology

• encompasses small & large scale evolution

–changes in gene frequency in a population from one generation to next

Evolution• produces pattern of relationships between

species

• as lineages evolve & split & modifications are inherited, evolutionary paths diverge

• produces branching pattern of relationships

• by studying inherited characteristics & historical evidence, can reconstruct evolutionary relationships & represent them on a family tree

• called a phylogeny

Phylogeny of Animalia lineage

Understanding Phylogeny• root of tree represents ancestral lineage

• tips of branches represent descendents of that ancestor

• as move from root to tipsmove forward in time

Which of These is Evolution

Pre-Darwin Theories• supernatural intervention• naturalistic models

– based on logic & philosophy

• 5th century BC-Greece– Plato (428-348 BC) & Aristotle (384-322 BC)– naturalistic view of origins– Anaximander

• believed life arose in water & simpler forms preceded more complex ones

• 1766 Georges Buffon– species represented by fossils could be an ancient version of groups of similar

living species

• Jean Baptiste Lamarck-1800s– relationship of fossils & current due to evolution

– proposed that by using or not using a body part individual develops specific characteristics which get passed to offspring

– inheritance of acquired characteristics

Charles Darwin• formalized theory of evolution-1859

• The Origin of the Species

– species living today descended from ancestral species

• Diversity is based on different modifications of common ancestors from which varieties of a species descended

• theory of how evolution works is natural selection

Theory Development• gathered evidence

during an around-the-world voyage

• HMS Beagle-1831-1836

• toured southern islands, South American coast & Australia

• collected specimens of fossils & living plants & animals

Theory Development• began to notice

organism made adaptations to live in a particular area

• Galapagos Islands • noted presence of

unique organisms that were like but different from organisms on mainland

Theory Development• while on the Beagle,

reading Principles of Geology by Charles Lyell (1797-1875)– Suggested

gradual changes in Earth’s surface

• proposed slow & subtle processes over vast amounts of time led to enormous changes

Theory Development• finches interested Darwin• normally seed-eating birds

adapted insect-eating habits• subtle form & habit changes of

these birds entrenched ideas of change over time

• seemed reasonable that organisms had been transformed over time

• new structures & habits had developed over time

• began to think species descended from ancestral species

Theory Development• 1840’s Darwin wrote essay on

evolutionary theory

• knew would create social furor

• delayed publishing

• middle 1850’s Alfred Wallace came up with similar theory

• Wallace asked Darwin to take a look at his essay to see if it should be published

• 1859 Darwin published On the Origin of Species by Means of Natural Selection

• Darwin proposed viable mechanism on how species evolve

• natural selection

Natural Selection• best theory on how

evolution occurs

• based on two observations

• Observation 1: species produce excessive numbers of offspring with limited natural resources

• too many individuals for natural resources to support struggle for existence ensues

• Observation 2: variation exists among individuals of a population

• no 2 are exactly the same

Natural Selection• Darwin inferred:

• "As many more individuals of each species are born than can possibly survive, and as, consequently, there is a frequently recurring struggle for existence, it follows that any being, if it vary, however slightly, in any manner profitable to itself under the complex and sometimes varying conditions of life, will have a better chance of surviving and thus be naturally selected”

Natural Selection• differential reproductive

success• individuals whose inherited

traits are better suited to a local environment are more likely than lesser fit individuals to survive & reproduce

• individuals who function best tend to leave more offspring

Evolution • Earth with liquid water has

existed for more than 3.6 billion years

• cellular life has been around for at least half that time

• organized multicellular life has existed for at least 800 million years

• major life forms now on Earth are not all represented in the past

• major life forms of past are no longer living on Earth

• all living forms come from previous living forms that were different

Methods & Evidence Used to Date Evolutionary

Events

Radiometric Datinghalf-life decay of radioactive elements allows scientists to date rocks & materials directly

• allows scientists to estimate dates

Stratigraphy &Fossil Record

• fossils-preserved remnants or impressions left by organisms that lived in the past– mostdiscovered in sedimentary

rocks– each layer contains fossils that

represent organisms that lived during particular part of Earth’s history

– position in layers reveals age• chronology of appearance in rock

layers leaves observable signs of evolution

• oldest are 3.5 billion years old • fossil record fits in with information

obtained with other evidences• fossil record is incomplete• in part because organisms with shells

or bony skeletons are more likely to be preserved than those without hard body parts

Indicators of Common Ancestry• Comparative Anatomy

–Homologous Structures

–Analogous Structures

–Vestigial Structures

• Comparative Embryology

• Molecular Biology

Comparative Anatomy• anatomical similarities exist

among species– same skeletal elements

make up forelimbs of humans, cats, whales & bats

• structural similarities are not surprising if all mammals came from common ancestor with prototype forelimb

• similarity in characteristics that are the result of common ancestry is homology

ANALOGOUS STRUCTURES• insect & bat wings are

analogous

– have separate evolutionary origins

• superficially similar because have evolved to do same job

• convergent evolution

• though bird & bat wings are analogous as wings, as forelimbs they are homologous

• birds and bats did not inherit wings from a common ancestor with wings, but did inherit forelimbs from a common ancestor with forelimbs

Vestigial Structures• features that were adaptations for

organism's ancestorevolved to be non-functional because organism's environment changed

• presence of such organs suggests common ancestry

• fish species that live in completely dark caves have vestigial, non-functional eye

• when sighted ancestors ended up living in caves, there was no longer any natural selection that maintained function of the eyes

• fish with better sight no longer out-competed fish with worse sight

• today, these fish still have eyes• not functional• by-products of fishes' evolutionary

history

Comparative Embryology• closely related organisms

have similar embryologic stages of development

• all vertebrates have structures on sides of throat during embryological development-pharyngeal pouches

• at this stage embryos of fish, frogs, snakes, birds & mammals look very similar

• in fish-pouches become gills

• in humans-part of ears & throat

Molecular Biology• study of DNA &

proteins• if two species have

genes & products of genes-proteins with similar sequences of nucleotides must have been copied from common ancestor

Molecular Biology• 98% of our DNA sequences

match chimpanzees• 95% of amino acids of

rhesus monkey hemoglobin are identical to human hemoglobin

• 14% of lamprey hemoglobin is similar to human hemoglobin– conclude-rhesus

monkeys are more closely related to humans than lampreys

– chimpanzees are more closely related to humans than Rhesus monkeys

Molecular Biology• master control genes

– regulate groups of other genes during embryological development

– very similar nucleotide sequences from fruit flies & mammals

• conclusion-genes arose in common ancestor

Human Evolution• did not evolve from

chimpanzees• share a recent

common ancestor • neither chimpanzee nor

human• humans are not higher

or more evolved than other living lineages

Natural Selection in Action• in some cases natural

selection can be observed

• data shows shape of finches' beaks on Galapagos Islands has tracked weather patterns

• after droughtsfinch have deeper, stronger beaks that lets them eat tougher seeds

Natural Selection in Action• human activity has

led to environmental changes that have caused populations to evolve through natural selection

• population of dark moths in 19th century England, rose & fell in parallel to industrial pollution

Natural Selection at Work

• How do antibiotics change the phenotype of bacteria and make them resistant?

Population & Evolution• individuals do not evolve

• populations do– group of individuals of

same species living in same place at same time

– smallest biological unit that can evolve

• evolutionary impact of natural selection can only be seen in tracking how a population changes over time

Population Genetics• tracks variations

in populations over time

• only traits that can be passed on in genes can affect evolution

Inheritable Traits• not all traits are

heritable• phenotype is result of

both genotype & environment

• only genetic characteristics can be inherited

• when population has two or more forms of one phenotype, they are polymorphic

Genetic Variation• basic mechanisms of

evolutionary change cannot operate without genetic variation

• two main sources• Sexual recombination• Mutation

Sexual Recombination• during ovum & sperm formation-meisois• genes are rearranged-prophase 1• original chromosomes are different from

chromosomes of parents

Sexual Recombination• Sex can introduce new gene• when organisms reproduce

sexually, some genetic "shuffling" occurs, bringing together new combinations of genes

• might have bushy eyebrows & big nose since mom had genes associated with bushy eyebrows & dad had genes associated with big nose

• combinations can be good, bad, or neutral

• shuffling can introduce new combinations of genes in every generation

Mutations• random changes in DNA• can be beneficial, harmful or

neutral• do not supply what is needed• mutation is unrelated to how

useful it is• not all matter to evolution• somatic mutations occur in

non-reproductive cells – will not be passed to offspring

• golden color on half of this • Red Delicious apple was

caused by somatic mutation• seeds do not carry mutation

Reproductive Cell Mutations• occur in reproductive cells-eggs

& sperm• germ line mutations• No change in phenotype

– mutation might occur in stretch of DNA with no function

– mutation occurs in protein-coding region, but ends up not affecting amino acid sequence of protein

• Small change in phenotype– single mutation caused cat's

ears to curl backwards slightly• Big change in phenotype

– some phenotypic changes are caused by single mutation

– can also have strong negative effects for organism or strong benefits

– mutations that cause death are lethals

Causes of Mutations• DNA fails to copy accurately

– most mutations that matter to evolution are naturally-occurring

– when cells divide-make copy of DNA

• sometimes copy is not quite perfect• small difference from original

sequence is a mutation • External influences

– exposure to specific chemicals or radiation

– cause DNA to break down– when cell repairs DNA, might

not do a perfect jobcell ends up with DNA slightly different than original DNA mutation

Sickle Cell Anemia

Analyzing Gene Pools• to study evolution at

population level focus is on gene pool– all alleles in all

individuals making up a population at a given time

• when frequencies of alleles in a population changes over a number of generations one can conclude that evolution is occurring

Hardy-Weinberg Equilibrium• gene frequencies & genotype

ratios in a randomly-breeding population remain constant from generation to generation

• tells us recessive alleles do not tend to disappear in a population

• if this were to happen population would soon become homozygous

• therefore sexual reproduction alone does not lead to evolution

• frequency of each allele in a gene pool will remain constant unless acted on by other agents

Gene Frequency• measures frequency of

particular gene relative to other genes at its locus in a population

• expressed as proportion (between 0 & 1) or percentage (between 0 &100%)

• if genotype contains two genes

• there are a total of 16 genes per locus in a population of eight individuals:

• Aa AA aa aa AA Aa AA Aa

Gene Frequency• Aa AA aa aa AA Aa AA Aa

• in above population

• frequency of A = 9/16 = 0.5625frequency of a = 7/16 = 0.4375

• use p to indicate frequency of A

• use q to indicate frequency of a

• p and q are gene frequencies

• frequencies for all possible genotypes = 1

• p + q = 1

Hardy-Weinberg Equation• (p + q)2 = p2 + 2pq + q2 = 1• formula allows us to determine frequencies by

observation of the population• when see an organism with dominant phenotype

do not know whether looking at (BB) or (Bb)• when see one with recessive phenotypeknow it

has genotype bb• if determine proportion of individuals in population

with bb it is a simple step to calculate frequency of b

Hardy-Weinberg Equation Uses• public health

professionals use it to calculate how many people carry certain alleles for inherited diseases

• can be used to calculate percentage of population carrying an allele for PKU-phenylketonuria

When Hardy-Weinberg Law Fails to Apply

• works in non evolving populations

• genetic equilibrium• law says-gene pools remain

constant over time • frequencies remain

unchanged• if equation does not equal

one-evolutionary change is occurring

Circumstances that lead to evolutionary change

•gene flow

•genetic drift

•natural selection

Gene Flow• occurs when individuals migrate &

mate outside original population

• many species make up local populations

• members tend to breed within their group

• each local population can develop gene pools distinct from that of other local populations

• members of one population may breed with an occasional immigrant from an adjacent population of same species

• introduces new genes or alters existing gene frequencies in residents

• gene flow reduces genetic differences between populations

• gene immigration increases variability of a gene pool

Genetic Drift• change in relative frequency in which an allele

occurs in a population due to chance• change in gene pool due to chance• more impact in small populations

Genetic Drift• two situations that

can shrink populations down to a size where genetic drift can take place

• bottle neck effect

• founder effect

Bottle-Neck Effect• result of a disaster

– flood, earthquake or fire

• drastically reduces population size

• certain alleles may be present at higher frequency in surviving population than in original one

• effect usually reduces overall genetic variability of a population

Founder Effect• a few individuals

colonize an isolated area

• smaller the colony less its genetic makeup will represent gene pool of larger population from which colony migrated

Natural Selection• process by which variation,

introduced by mutation, gene flow, or genetic drift, is refined

• individuals with phenotypes not appropriate for an environment will not pass genes on

• selection produces adaptations• if individuals having certain

genes are better able to produce mature offspring than those without a gene then frequency of genes will increase

• expression of natural selection is seen in alterations in gene pools

What about fitness?• fitness describes how good a particular

genotype is at leaving offspring in the next generation

• if brown beetles consistently leave more offspring than green ones because of their color, you would say brown beetles have higher fitness

Fitness• genotype's fitness

depends on environment in which organism lives

• fittest genotype during ice ageprobably not fittest once ice age is over

• fittest individual is not necessarily strongest, fastest, or biggest

• genotype's fitness includes its ability to survive, find a mate, produce offspring

• ultimately leave its genes in next generation

Fitness• caring for offspring

• producing thousands of young

• sporting fancy feathers that attract females

• these strategies increase fitness because help parents get more of their offspring into next generation

Natural Selection• results from differential mortality

and/or differential fecundity• differential mortality could be

termed mortality selection meaning that certain genotypes are less successful than others in surviving to end of their reproductive period

• evolutionary impact of mortality selection can be felt anytime from formation of new zygote to end of organism's period of fertility

• another way of describing Darwin's criteria of fitness: survival

Natural Selection• Fecundity selection• certain phenotypes (thus

genotypes) may make disproportionate contributions to gene pool of next generation by producing disproportionate number of young

• way of describing another criterion of fitness described by Darwin: family size

• in each of these examples certain phenotypes are better able than others to contribute genes to next generation

• By Darwin's standards they are more fit

Natural Selection• population does not "want" or

"try" to evolve• natural selection cannot try to

supply what organism needs • selects among whatever

variations exist in population • result is evolution• natural selection is sometimes

interpreted as random• misconception• genetic variation that occurs in a

population because of mutation is random

• selection acts on that variation in a non-random way

• genetic variants that aid survival & reproduction are much more likely to become common than variants that don't

Outcomes of Natural Selection

• Directional

• Diversifying

• Stabilizing

Directional Changes• shift phenotypic curve of

population by selecting in favor of extreme phenotypes

• most common when local environment changes or when organisms migrate to new environment

• when challenged species must adapt or become extinct

• populations that survive often change enough to be considered a new species

• most common-extinction

Diversifying or Disruptive Changes

• lead to balance between 2 or more contrasting phenotypic forms in a population

Stabilizing Changes• maintain variations for

a particular trait within a narrow range

• selects against extreme phenotypes

• stabilizing outcomes persist most of time

• lead to well adapted populations

Evolution Exercise

• Beetles on a diet

• Beetles of a different color

Evolution Exercise Answer• difference in weight in first situation occurred

due to environmental influences-low food supply-not because of a change in frequency of genes

• example 1 is not evolution• because small body size was not genetically

determined, this generation of small-bodied beetles will produce beetles that will grow to normal size if they have a normal food supply

• changing color in example 2 is evolution• these two generations of the same population

are genetically different• How did it happen?