1. Chapter 19 Evolutionary Genetics 18 and 20 April, 2004
Evolution and fate of genes
18 and 20 April, 2004

3. 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.

5. 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

6. 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.

7. 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

9. 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

14. Variation is stable when m>1/N or m>1/N.
105 individuals is a reasonable population to avoid loss of heterozygosity.

15. 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

20. 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

21. A canalized character

22. 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

24. 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

25. 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

26. 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)

28. Origin of new genes Polyploidy Duplications Imported DNA
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

33. 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

34. 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

35. 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

39. 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

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