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