mechanisms of evolution-ii

Mechanisms of Evolution II

• Overproduction & competition

• Phenotypic variation

• Some variation is heritable

• Differential survival & reproduction

*Leads to changes in allele frequencies over time

Natural Selection Reminder

The Equilibrium Population:Assumptions of Hardy-Weinberg

• No mutation (no new genetic variation)

• No genetic drift (infinitely large population)

• No migration (no individuals entering or leaving the pop)

• No selection (genotypes have equal fitness)

• Random mating (dealing with a single population)

Example of Hardy-Weinbergp2 +2pq + q2 = 1

Recall: DNA replicates at every cell division => Opportunity for error and repair (mutation)

Recall: DNA replicates at every cell division => Opportunity for error & repair (mutation)

mismatchmust berepaired

if C =>T,a mutation

if A => G,no mutation

Mutation• alterations of the base DNA sequence

• ultimate source of all genetic variation• many types of mutation (e.g. point, chromosomal)• often reduce fitness (deleterious), but can be beneficial or neutral too

• weak force in changing allele frequencies over time (evolution)

• many genes per genome (~30,000 in humans), so odds of hitting any particular one is very low per

generation

The genetic code is degenerate => many mutations are silent

GUUGUCGUAGUG

Valine, so 3rd position mutations don’t affect protein sequence

Recall: DNA => mRNA => protein => => => phenotype

If mutation changes protein sequence, it may affect phenotype (e.g. Sickle cell anemia)

Recall: DNA => mRNA => protein => => => phenotype

single amino acid change

Genetic drift: an extreme example• Imagine a population of only 1 M and 1 F per generation (N = 2)

• Start with 2 heterozygotes (Aa), p & q = 0.5

• Simulate allele formation & random mating with coin flip

1.0

.50

.75

.25

alle

le fr

eq.

0 1 2 3 4 5 6 7 8 9 10

Genetic drift

• change in allele frequency between generations due to the random sampling of alleles

• large population, allele and genotype frequencies predictable

• smaller populations, random chance (drift) becomes important

Genetic drift...1. Changes allele frequencies of populations

2. Reduces genetic variation of populations

3. Is random: same starting point => different outcomes

4. Depends on population size (small pops have strong drift)

N = 4 N = 40 N = 400

Bottlenecks & founder effects

Bottleneck = drift due to a drastic reduction in population sizeFounder effect = bottleneck associated with the founding of a

new population

Migration = movement of individuals between populations

Gene flow = transfer of alleles from one population to another

Connectivity

AA

aaAAAA AA

AAAA aaAAAAAA

aaaaAA

aa

AAaa

Freq A = p = 1Freq a = q = 0

Freq A = p = 0Freq a = q = 1

Gene flow can be a strong evolutionary force


One migration event => deviation from H-W expectations

AAaaAAAA AA

AAAAaa

AAAAAA

aaaaAA aa

AAaa

Freq A = p = 0.82Freq a = q = 0.18Would predict 2pq = 0.30Actual frequ of Aa = 0

Freq A = p = 0.33Freq a = q = 0.67Would predict 2pq = 0.44Actual frequ of Aa = 0

AAAaAAAA Aa

AAAAAAAa

Aaaa

aa

Ongoing gene flow prevents population divergence & (eventually) homogenizes allele frequencies

AaaaAAaa

AAAa


Freq A = p = 0.67Freq a = q = 0.33Would predict 2pq = 0.44Actual frequ of Aa = 0.33

Freq A = p = 0.5Freq a = q = 0.5Would predict 2pq = 0.5Actual frequ of Aa = 0.33

Natural selection

beneficial mutation

(should increase in frequency)

Natural selection happensMany pathogens evolve resistance to antibiotics

Many pests evolve resistance to pesticidesNatural selection happens

Adaptation/diversification in higher eukaryotes - slower, but still going on

before selection

after selection(1977 drought)

mediumground

finch

Natural selection happens

Patterns of phenotypic selectionDirectional selection

trait value

frequ

ency

Stabilizing selection

trait value

frequ

ency

Patterns of phenotypic selection

Diversifying selection

trait value

frequ

ency


Frequency-dependent selection

Morphs of a single Heliconius speciesNon-poisonous mimics of poisonous butterflies

=> each has higher fitness when rare


Patterns of phenotypic selectionHeterozygous Advantage (Overdominance)

mechanisms of evolution-ii

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