Evolution Exam #2

5 Microevolutionary Forces

1. Mutation

2. Gene Flow (migration)

3. Genetic drift

4. Non-Random mating

5. Natural selection

Assortative mating

non-random mating due to mate choice

positive assortative mating

individuals with similar traits/genotypes choose to mate with each other more often than expected by chance

negative assortative mating

occurs when individuals choose mates with dissimilar or opposite traits

Whalund effect

Reduction in heterozygosity within a total population when separate subpopulations have different allele frequencies and are mistakenly sampled or grouped as a single large population

gene flow

change in allele frequency due to migration

Mainland/island migration model

a population biology and genetics framework that describes unidirectional gene flow from a large, stable "mainland" source population into a small, potentially unstable "island" sink population

Finite Island migration model

All populations are discrete "islands" (no mainland)

● same migration rate in both directions

● same size populations

● different allele frequencies in different populations

● will reach equilibrium

genetic drift

accumulation of chance changes in allele frequency due to random sampling; smaller populations are more affected by genetic drift than larger populations

Fixation

when one allele reaches a frequency of 100%; The more generations that occur, the more likely at some point this will happen

Genetic bottleneck

Period during which only a few individuals survive to continue the existence of the population

Founder effect

Populations descended from a small founder group may have low genetic variation or by chance have a high or low frequency of particular alleles

The Mutation Balance effect

at equilibrium, the rate at which neutral mutations are born is equal to the rate at which they are lost to drift

At equilibrium, the expected neutral diversity in a diploid population is:

Θ = 4Neμ

Θ

level of genetic diversity

Ne

effective population size; number of individuals you could expect from the level of diversity observed

Nc

census size; total number of individuals in a population

Identity by descent

when both copies of the allele come from the same common ancestor

F_IS

inbreeding coefficient; F_IS= 1 - H_I / H_S

• H_I is observed heterozygosity within subpopulations

• H_S is average expected heterozygosity within subpopulations (HWE)

F_IS = 0

nonrandom mating

F_IS = 1

no observed heterozygosity

F_IS > 0

inbreeding

F_IS < 0

outbreeding

Where is population structure important to consider?

• conservation (pocket mouse example)

• human health (genetic marker example)

• pathogen monitoring (C. auris example)

F_ST

a measure of genetic differences between subpopulations; F_ST = 1 - H_S / H_T

• H_T is the expected heterozygosity in the total population

• H_S is the average expected heterozygosity within subpopulations

F_ST = 0

subpopulations have the same structure

F_ST = 1

complete differentiation

F_ST > 0

detectable differences

Panmixia

random mating across all subpopulations, subpopulations acting like one big group

Isolation by distance (IBD)

Physically closer populations are more genetically similar, physically distant populations are less genetically similar

How Population structure leads to speciation

• reduction in gene flow

• genetic differences build up

• gene flow stops completely

Prezygotic barriers (Before Fertilization)

● Habitat / Spatial / Temporal Isolation

● Behavioural

● Mechanical

● Genetic difference prevent fertilization

Postzygotic barriers (after fertilization)

● Hybrid inviability

● Hybrid sterility

● 2nd generation breakdown

The Bateson-Dobzhansky-Muller Model

● ancestor is AABB genotype

● due to unique mutations, new species are aaBB and AAbb

● hybrids AaBb are a bad combination, so hybrids have poor fitness.

Gene linkage

genes located close together on the same chromosome are inherited together at a higher frequency than expected by chance

Sex linkage

The trait/gene/allele is found on the sex chromosomes, causes unique inheritance patterns

Haplotype

is a physical grouping of genomic variants that tend to be inherited together, reflects a unique combination of variants that reside near each other on a chromosome

Linkage equilibrium

allele combinations occur at frequencies that equal the exact mathematical product of their individual separate frequencies, total independence (4 different gametes)

Linkage disequilibrium

associations between genes are preserved, linked genes (2 different gametes)

Total Linkage disequilibrium

P(AB) = P(A) = P(B)

Total Linkage Equilibrium

P(AB) = P(A) x P(B)

D-statistic

P(AB) - P(A)xP(B)

Recombination Frequency

# of recombinants / total # of offspring

What breaks down linkage disequilibrium?

Time and recombination

Negative D-statistic

alleles are found together less often than by random chance

D-statistic > 0

genes are found together more often than you would expect by chance

D-statistic = 0

the alleles are found together as often as you would expect by chance (at equilibrium)

What causes linkage disequilibrium?

• Mutation

• Migration

• Nonrandom mating

• Selection (genetic hitchhiking, selective sweeps, coevolution)

Selective sweep

a beneficial mutation quickly increases in frequency and becomes fixed in a population, reducing or eliminating genetic variation near the mutation site

Genetic hitchhiking

neighboring DNA sequences (linked alleles) are dragged along positively selected allele, also becoming more common or fixed

Coevolution

a process where two or more interacting species, genes, or biological systems exert reciprocal evolutionary pressure on each other

Why is the average amount of observed genetic diversity in humans so much lower than one would expect for such a big population?

Its because our population expanded rapidly from a much smaller original population, and there simply has not been enough time for new mutations to build up, to reach mutation-drift equilibrium conditions.

Identity by descent

The property that two alleles can be traced back to the same recent ancestor

Hidden population structure effect on heterozygosity

Hidden population structure would cause a deficit of heterozygotes, because the different genotypes in separate populations would be less like to come into contact

H_s

the average expected heterozygosity within subpopulations

H_I

observed heterozygosity within subpopulations

H_T

expected heterozygosity in the total population