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