Population Genetics (Ch. 16)

Why should ecologists care about evolution and genetics?

1. Genetics influences who lives, who dies, who mates, whether populations grow or shrink

2. Changes in the environment produce evolutionary responses that can influence ecological responses

3. Genetics can be a big problem for small populations

• Genes are made of DNA and carry instructions for making proteins

• Most organisms have two sets of each chromosome – therefore, two copies of each gene– genes come in different variants (alleles)– individuals can have one or two alleles for

any gene• two of the same allele = homozygous• two different alleles = heterozygous

• Dominant alleles are expressed whether there is one copy or two

• Recessive alleles must be present in two copies to be expressed

Two sources of new variation in genes:

• Mutation – accidental change in genetic code, usually during cell division– most mutations are harmful– rate of mutation is very low

• Recombination – exchange of genetic material between different chromosomes during meiosis

• Gene pool – the sum of all genes in a population

• Imagine a gene with 2 alleles, A & B:– 20 AA homozygotes = 40 A alleles– 10 BB homozygotes = 20 B alleles– 15 AB heterozygotes = 15 A, 15 B alleles

• Total gene pool: 55 A, 35 B alleles– frequency of A = 55/90 = 0.61– frequency of B = 35/90 = 0.39

The gene pool determines the proportions of genotypes in the next generation.

Hardy-Weinberg equilibrium

Frequencies of alleles and genotypes will stay the same from generation to generation given

1. large N

2. random mating

3. no selection

4. no mutation

5. no migration between populations

• With 2 alleles:– frequency of allele A = p– frequency of allele B = q – q + p = 1, so… p = 1 – q

• At equilibrium, genotype frequencies can be predicted based on allele frequencies:

Genotype AA AB BB

Frequency p2 2pq q2

Most populations deviate from Hardy-Weinberg equilibrium due to:– genetic drift– assortative mating– gene flow– selection

• Genetic drift – change in allele frequencies due to random chance– results in loss of uncommon alleles– most important in very small populations

• Importance of genetic drift is proportional to 1/N

2 times when genetic drift can be important:

1. Founder events – small number of individuals found a new population

– contain a reduced sample of the alleles in the parent population

2. Population bottleneck – a large population is reduced to small numbers

– can occur with highly endangered species

• Assortative mating – when individuals chose mates nonrandomly with respect to genotype– positive assortative mating – like with like– negative assortative mating – prefer

different genotype

• Inbreeding – mating with close relatives

• Positive assortative mating and inbreeding – reduce the number of heterozygotes– lead to expression of harmful, recessive

mutations

• Inbreeding is a major problem for small, isolated populations

Spatial variation in gene frequencies• Gene flow – movements of alleles

between subpopulations

• Subpopulations often have different allele frequencies due to– barriers to gene flow– different selection pressures

• result in locally adapted genotypes

Optimal outcrossing distance

Ecotypes – genetically distinct subpopulations in different locations

Cline – gradual change in a trait over distance

Geographic variation without a cline

Geographic barriers

Today:

1. Finish population genetics

2. Start community ecology (consumer-resource interactions)

Three kinds of natural selection:

1. Stabilizing – intermediate phenotypes are best• selection for an optimal phenotype

2. Directional – more extreme phenotypes (in one direction) do best

– new optimum for the population

3. Disruptive – more extreme phenotypes (both directions) do best

– selection for specialization

Summary of factors influencing genetic variation

• Forces increasing genetic variation– recombination– mutation– migration

• Forces reducing genetic variation– genetic drift– directional and stabilizing selection– inbreeding– positive assortative mating