Hardy-Weinberg Principle

1

Hardy-Weinberg Principle

• Hardy-Weinberg - original proportions of genotypes in a population will remain constant from generation to generation

– Sexual reproduction (meiosis and fertilization) alone will not change allelic (genotypic) proportions.

2

Hardy-Weinberg Equilibrium

Population of cats n=10016 white and 84 blackbb = whiteB_ = black

Can we figure out the allelic frequencies of individuals BB and Bb?

3

Hardy-Weinberg Principle

• Necessary assumptions

Allelic frequencies would remain constant if…

– population size is very large– random mating– no mutation– no gene input from external sources– no selection occurring

4

Hardy-Weinberg Principle

• Calculate genotype frequencies with a binomial expansion

(p+q)2 = p2 + 2pq + q2

• p2 = individuals homozygous for first allele• 2pq = individuals heterozygous for alleles• q2 = individuals homozygous for second allele

5

p2 + 2pq + q2

and

p+q = 1 (always two alleles)

• 16 cats white = 16bb then (q2 = 0.16)• This we know we can see and count!!!!!• If p + q = 1 then we can calculate p from q2

• Q = square root of q2 = q √.16 q=0.4• p + q = 1 then p = .6 (.6 +.4 = 1)• P2 = .36• All we need now are those that are heterozygous

(2pq) (2 x .6 x .4)=0.48

• .36 + .48 + .16

Hardy-Weinberg Principle

6

Hardy-Weinberg Equilibrium

7

Five Agents of Evolutionary Change

• Mutation– Mutation rates are generally so low they

have little effect on Hardy-Weinberg proportions of common alleles.

ultimate source of genetic variation• Gene flow

– movement of alleles from one population to another

tend to homogenize allele frequencies

8

Five Agents of Evolutionary Change

• Nonrandom mating– assortative mating - phenotypically similar

individuals mate Causes frequencies of particular

genotypes to differ from those predicted by Hardy-Weinberg.

9

Five Agents of Evolutionary Change

• Genetic drift – statistical accidents.– Frequencies of particular alleles may

change by chance alone. important in small populations

founder effect - few individuals found new population (small allelic pool)

bottleneck effect - drastic reduction in population, and gene pool size

10

Genetic Drift - Bottleneck Effect

11

Five Agents of Evolutionary Change

• Selection – Only agent that produces adaptiveevolutionary change

– artificial - breeders exert selection – natural - nature exerts selection

variation must exist among individuals variation must result in differences in

numbers of viable offspring produced variation must be genetically inherited

natural selection is a process, and evolution is an outcome

12

Five Agents of Evolutionary Change

• Selection pressures:– avoiding predators– matching climatic condition– pesticide resistance

13

Measuring Fitness

• Fitness is defined by evolutionary biologists as the number of surviving offspring left in the next generation.

– relative measure Selection favors phenotypes with the

greatest fitness.

14

Interactions Among Evolutionary Forces

• Levels of variation retained in a population may be determined by the relative strength of different evolutionary processes.

• Gene flow versus natural selection– Gene flow can be either a constructive or

a constraining force. Allelic frequencies reflect a balance

between gene flow and natural selection.

15

Natural Selection Can Maintain Variation

• Frequency-dependent selection– Phenotype fitness depends on its frequency

within the population. Negative frequency-dependent selection

favors rare phenotypes. Positive frequency-dependent selection

eliminates variation.• Oscillating selection

– Selection favors different phenotypes at different times.

16

Heterozygote Advantage

• Heterozygote advantage will favor heterozygotes, and maintain both alleles instead of removing less successful alleles from a population.

– Sickle cell anemia Homozygotes exhibit severe anemia,

have abnormal blood cells, and usually die before reproductive age.

Heterozygotes are less susceptible to malaria.

17

Sickle Cell and Malaria

18

Forms of Selection

• Disruptive selection– Selection eliminates intermediate types.

• Directional selection– Selection eliminates one extreme from a

phenotypic array.• Stabilizing selection

– Selection acts to eliminate both extremes from an array of phenotypes.

19

Kinds of Selection

20

Selection on Color in Guppies

• Guppies are found in small northeastern streams in South America and in nearby mountainous streams in Trinidad.

– Due to dispersal barriers, guppies can be found in pools below waterfalls with high predation risk, or pools above waterfalls with low predation risk.

21

Evolution of Coloration in Guppies

22

Selection on Color in Guppies

• High predation environment - Males exhibit drab coloration and tend to be relatively small and reproduce at a younger age.

• Low predation environment - Males display bright coloration, a larger number of spots, and tend to be more successful at defending territories.

– In the absence of predators, larger, more colorful fish may produce more offspring.

23

Evolutionary Change in Spot Number

24

Limits to Selection

• Genes have multiple effects– pleiotropy

• Evolution requires genetic variation– Intense selection may remove variation

from a population at a rate greater than mutation can replenish.

thoroughbred horses• Gene interactions affect allelic fitness

– epistatic interactions