Lecture 6. Gene frequencies in populations: Hardy-Weinberg equilibrium (no evolution) and random genetic drift (evolution without selection)

1. Genes in populations

a. Individuals vs. populations

b. Important terms in populations

i. Gene pool

ii. Gene (allele) frequency

iii. Genotype frequency

c. Evolution = change in gene frequencies in populations across generations

i. Evolution is NOT a change within individuals

ii. populations evolve, individuals do not

iii. Observing a change in phenotype frequencies is NOT enough to say that evolution has occurred (why??)

2. Evolution vs. equilibrium

a. Evolution = change in gene frequencies across generations

b. Equilibrium = NO change in gene frequencies across generations

c. Sexual reproduction alone, or the simple action of Mendels laws will NOT change gene frequencies!!

d. What are the conditions for gene frequencies to remain the same??

i. No mutation

ii. No migration (input of genes from another population)

iii. Population must be large enough to avoid random genetic drift (changes in gene frequency by chance)

iv. Random mating

v. No selection (differential survival or reproduction)

e. If these 5 conditions are true, there will be NO EVOLUTION (Hardy and Weinberg figured this out . . . the "Hardy-Weingberg Equilibrium")

f. Violating any of these 5 conditions will lead to evolution

4. Hardy-Weinberg equilibrium--how it works and why it's important

a. Gene frequencies add to 1: p + q = 1

b. Using a Punnett square to calculate genotype frequencies

c. Genotype frequencies add to 1: Under H-W, p2 + 2pq + q2 = 1

d. So what??

i. Population will go to H-W proportions after ONE generation of random mating

ii. For a population in H-W proportions, we can easiliy calculate gene frequencies if we know frequency of recessive phenotype (example: the frequency of a rare recessive disease is 0.049 in the Amish population of Lancaster county, PA. What are the gene frequencies for this disease in that population?)

iii. We can also use the assumption of H-W to calculate genotype frequencies: what is the frequency of "carriers" (heterozygotes) for this genetic disease in the Amish?

iv. What if we know only the frequency of unaffected individuals?

e. If we know conditions for equilibrium, can focus on how violations ---> evolution!

5. Violations of the assumptions of the H-W equilibrium: conditions that DO lead to evolution.

a. Discovering that genotype frequencies deviate significanlty from Hardy-WEinberg proportions is evidence that some violation of assumptions has occurred. BUT, it doesnn't tell you which assummption has been violated!

b. One evolutionary agent leads to change in genotype frequencies without changein gene frequencies: non-reandom mating

i. Inbreeding (mating with relatives), or selfing

ii. What does inbreeding do to genotype frequencies??

--increase in homozygotes

--deviation from H-W equilibrium

c. Three evolutionary agents change gene frequencies without natural selection

i. Mutation

ii. Migreation

iii. Random genetic drift in small populations (changes in gene frequencies by chance alone)

--bottleneck

--founder effect (ex: Amish)

--importance of random genetic drift in conservation

iv. Note: IN small populations, inbvreeding may also increase (fewer choices of mates)

d. Only one evolutionary force leads to adaptive evolution--natural selection! (more on this next week)

Next time: Natural selection--how does it work