18 and 20 April, 2004

Chapter 19

Evolutionary Genetics

Evolution and fate of genes

Overview• Evolution consists of continuous heritable change both within and

between lines of descent. • Mutations in DNA causing heritable variation in physiology,

development, and behavior provide the raw material for evolutionary change. Natural selection is the differential reproduction of genotypes that differ in these traits.

• Both natural selection and random events contribute to evolutionary change.

• Genes have similar DNA sequences as a result of common descent, though the degree of similarity may vary considerably. Genes underlying animal development are highly conserved.

• Species consist of populations that can exchange genes.• Acquisition of new DNA makes evolutionary novelties possible.

Evolution• Evolution was an accepted fact among

many scholars prior to Darwin• Darwin provided a plausible explanation for

evolution: natural selection• All living organisms are related through

descent from common ancestor– homologous features have the same

developmental origin inherited from a common ancestor

– analogous features have independent origin– similarities of DNA and protein sequences

allow inferences about evolutionary origin

Darwinian evolution

• Principle of variation. Among individual members of a population there is variation in morphology, physiology, and behavior.

• Principle of heredity. Offspring resemble their parents more than they resemble individuals to which they are unrelated.

• Principle of selection. Some variants are more successful at surviving and reproducing than other variants in a given environment. Such individuals are naturally selected.

Evolutionary history

•Phyletic evolution: change within a continuous line of descent

•Diversification: many different contemporaneous species evolved from common ancestor (branching)

•Natural selection converts heritable variation among members of a population into heritable differences among populations

Synthesis of evolutionary forces• Adaptive evolutionary change is a balance between forces

of breeding structure, mutation, migration, and selection

• Forces that increase or maintain variation within populations prevent differentiation of populations (e.g., migration, mutation, balancing selection)

• Divergence of populations is a result of forces that make each population homozygous (e.g., inbreeding, founder effect, directional selection)

• Evolution requires genetic variation in order to occur; direction of change unpredictable

Variation is stable when m>1/N or >1/N.

105 individuals is a reasonable population to avoid loss of heterozygosity.

Multiple adaptive peaks

• Often multiple ways for selection to produce different genotypes with same phenotype

• Adaptive surface (landscape): plot of mean fitness (reproductive success) for all possible allele frequencies– under identical conditions of natural selection, two populations may arrive

at two different genetic compositions

– selection carries population from low fitness to high fitness peaks

Heritability of variation• For evolutionary change, phenotypic

variation must be heritable• Not all variable traits are heritable

– e.g., metabolic responses to stress– e.g., behavior versus structure– not always easy to determine heritability

• In some cases, there is substantial genetic variability and no morphological variation– such characters are canalized characters– genetic differences revealed by stress

A canalized character

Variation within and between human populations

• Within populations:– ~33% of protein-encoding loci are polymorphic– additional nucleotide diversity in introns,

regulatory sequences, flanking sequences

• Between populations– frequencies of alleles may vary, especially for

morphological traits– in humans, most (~85%) of total genetic

variation is found within populations

Speciation (1)

• A species is a group of organisms than can exchange genes among themselves but not with other groups

• In some species, local populations constituting geographical races may exist– genetically distinguishable with different allele

frequencies– often arbitrary– human “races” denote groups with different

skin color, but few other biological differences

Speciation (2)• All species related to each other through common ancestry

• Species arise from previously existing species and become genetically distinct– theoretically, one or a small number of mutations could result in

speciation

– polyploidization can “instantly” form new species

– usually species form through geographical isolation of populations, which eventually become reproductively isolated

• referred to as allopatric speciation

• diverge by mutation, selection, and genetic drift

Biological isolating mechanisms•Prevent successful reproduction between groups

•Prezygotic isolation–separation in times or places of sexual activity–behavioral or physical incompatibility–gametic incompatibility

•Postzygotic isolation–failure of hybrid to develop

–hybrid sterility (F1 or F2)

Origin of new genes• Polyploidy• Duplications

– small sections of DNA containing one or more genes

– duplicated sequence may diverge in function• e.g., hemoglobins

• Imported DNA– e.g., origin of chloroplasts and mitochondria

through endosymbiosis– horizontal transfer through viruses and

transposons

Functional change and mutation

•Two extremes with regard to mutation and functional change

–virtually all amino acids can be replaced while maintaining original function–single mutation may give rise to new function

•When >1 mutation is required for new function, order of mutational events may be important

–many evolutionary failures

Rate of molecular evolution (1)

•Mutations can have three effects on fitness–deleterious, reducing or eliminating reproduction–increase fitness–no effect on fitness, i.e. neutral

•An important question is how much molecular evolution is adaptive (selected) and how much is random fixation of effectively neutral alleles

•Rate of neutral replacement is mutation rate

Rate of molecular evolution (2)

•Constant rate of neutral substitution predicts that evolution should proceed according to a molecular clock

–nonsynonymous substitution rate may be different than synonymous substitution rate–different proteins will have different clock rates

•Difficult to determine how much of nonneutral molecular evolution is adaptive

Genetic evidence of common ancestry

•Near universality of genetic code and conservation of translation mechanism

•Conservation of homeodomain control of development in animals

•Comparative synteny maps

•Analysis of protein and DNA sequences–comparative genomics and proteomics–conserved sequences are most informative

Assignment: Concept map, Solved Problems 1-3, All Basic and Challenging Problems.