Population GeneticsReconciling Darwin & Mendel

Darwin• Darwin’s main idea (evolution), was

accepted • But not the mechanism (natural selection)

– Scientists did not understand Darwin’s mechanism because there was no understanding of genetics

• Even once scientists grasped Mendel, genetics was viewed as an either/or – didn’t understand many traits are polygenic

• So how do you get the variation on which selection works?

Ideas About Evolution

• Orthogenesis – 1920’s – saw evolution as a predictable

progression to more & more elite forms of life

• Population Genetics – 1930’s – reconciled Darwin & Mendel

Genetics of Populations

• Population – a localized group of individuals belonging to

same species– The definition of a species not always clear  

• Gene pool = The total genes in a population

• Evolution on the smallest scale occurs when the relative frequency of alleles in a population changes over a succession of generations = microevolution 

Genetics of a Non-evolving Population

• The gene pool is in stasis• This is described by Hardy-Weinberg

Theorem:• The frequencies of alleles in a population’s

gene pool remain constant over the generations unless acted on by agents other than sexual recombination

• i.e. shuffling the deck has no effect on the overall genetic make-up of the population

The Hardy-Weinberg Theorem

• Example• In pink flowers (A), is dominant over

white flowers (a)• 2 alleles for at this locus• Sample 500 plants:• 20 white flowers (aa)• 480 pink [320 (AA); 160 (Aa)]• Therefore there are 1000 genes for

flower color in the population•

Example (Continued)

• The dominant allele accounts for 800 of these: – [(320 x 2) + (160 x 1)]

• Therefore:– the frequency of A in the population =

80%– the frequency of a = 20%

Predicting Change• How will genetic recombination

during sexual reproduction affect the frequencies in the next generation?

• If mating is random:

– the probability of picking 2 AA = (0.8 x 0.8) = .64

– the probability of picking 2 aa = (0.2 x 0.2) = .04

– and of heterozygotes = 2(0.8 x 0.2) = 0.32 • There are 2 heterozygote combinations: aA &

Aa• sperm or egg

Hardy-Weinberg Equilibrium

• This shows that the alleles are present in the gene pool in the same frequencies as they were in the previous generation

A: [0.64 + (0.32 2)] = 0.8 a: [0.04 + (0.32 2)] = 0.2

• The gene pool is at equilibrium• This is called Hardy-Weinberg

equilibrium

The Hardy-Weinberg Equation

• This example is the simplest case: – 2 alleles, one is dominant

• For this case: – if p = frequency of one allele – q the frequency of the other

• Then: p + q = 1probability of AA = p2

probability of aa = q2

probability of Aa = 2pq

• Therefore: p2 + 2pq + q2 = 1

Uses of Hardy-Weinberg

• Thus you can calculate the frequency of a gene in a population if you know the frequency of the genotypes

• This is important in genetic disease counseling

Relevance to Evolution

• A population at genetic equilibrium does not evolve

• Hardy-Weinberg tells us what to expect in non-evolving populations

• Therefore it is a baseline for comparing actual populations where gene pools may be changing.

• Can determine if the population is evolving

Genetic Equilibrium• Hardy-Weinberg equilibrium is

maintained only if the population meets all 5 of the following criteria:– Very large population size– Isolation from other populations

• migration can effect the gene pool– No net mutations– Random matings– No natural selection

• no difference in reproductive success)

• Describes an ideal that never exists in nature

Altering Genetic Equilibrium

• For evolution to take place something must upset the genetic equilibrium of the population:

• Factors that change genetic equilibrium are:– Genetic drift– Migration (Gene flow)– Non-randon mating (Isolation) – Mutation– Natural selection

Genetic Drift• Changes in gene frequency of a very

small population due to chance• Controlled by the laws of probability &

chance• Bottleneck effect

– Chance sampling error due to small population

• Founder’s effect – a few individuals colonize a remote spot – causes drift

Illustrating Genetic Drift

The Bottleneck Effect

Gene Flow (Migration)

• Movement of organisms into or out of a population

• Takes their genes out of the gene pool

• Most populations are not completely closed– gain & lose alleles

Non-random Mating

• More apt to mate with close neighbors

• Promotes inbreeding• Assortive mating

– seek mate like self (i.e. size)

Isolation

Mutation• A change in a

gene• An alteration of

DNA• The original

source of variation

• Raw material on which natural selection works

Natural Selection

• If one type produces more offspring than another, upsets the balance of equilibrium

• There are three types of natural selection:– Stabilizing Selection– Disruptive Selection– Directional Selection