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Population Genetics (Ch. 16)

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Presentation on theme: "Population Genetics (Ch. 16)"— Presentation transcript:

1 Population Genetics (Ch. 16)

2 Why should ecologists care about evolution and genetics?
Genetics influences who lives, who dies, who mates, whether populations grow or shrink Changes in the environment produce evolutionary responses that can influence ecological responses Genetics can be a big problem for small populations

3 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

4 Dominant alleles are expressed whether there is one copy or two
Recessive alleles must be present in two copies to be expressed

5 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

6 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

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

8 Hardy-Weinberg equilibrium
Frequencies of alleles and genotypes will stay the same from generation to generation given large N random mating no selection no mutation no migration between populations If

9 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



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

13 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


15 2 times when genetic drift can be important:
Founder events – small number of individuals found a new population contain a reduced sample of the alleles in the parent population

16 Population bottleneck – a large population is reduced to small numbers
can occur with highly endangered species

17 Inbreeding – mating with close relatives
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

18 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

19 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


21 Optimal outcrossing distance

22 Ecotypes – genetically distinct subpopulations in different locations

23 Cline – gradual change in a trait over distance

24 Geographic variation without a cline

25 Geographic barriers

26 Today: Finish population genetics Start community ecology (consumer-resource interactions)

27 Three kinds of natural selection:
Stabilizing – intermediate phenotypes are best selection for an optimal phenotype

28 Directional – more extreme phenotypes (in one direction) do best
new optimum for the population

29 Disruptive – more extreme phenotypes (both directions) do best
selection for specialization






35 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

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