Genes Within Populations

1. Genes Within Populations Chapter 21

2. Outline • Gene Variation • Hardy Weinberg Principle • Agents of Evolutionary Change • Measuring Fitness • Interactions Among Evolutionary Forces • Forms of Selection • Selection on Color in Guppies • Limits to Selection

3. Gene Variation is Raw Material • Evolution - change over time • Evolution is descent with modification • Darwin • Through time, species accumulate differences such that ancestral and descendent species are not identical.

4. Gene Variation is Raw Material • Natural selection and evolutionary change • Some individuals in a population possess certain inherited characteristics that play a role in producing more surviving offspring than individuals without those characteristics. • The population gradually includes more individuals with advantageous characteristics.

5. Darwin versus Lamarck

6. Gene Variation In Nature • Measuring levels of genetic variation • blood groups – 30 blood grp genes • Enzymes – 5% heterozygous • Enzyme polymorphism • A locus with more variation than can be explained by mutation is termed polymorphic. • Natural populations tend to have more polymorphic loci than can be accounted for by mutation. • 15% Drosophila • 5-8% in vertebrates

7. Hardy-Weinberg Principle • Population genetics - study of properties of genes in populations • blending inheritancephenotypically intermediate (phenotypic inheritance) was widely accepted • new genetic variants would quickly be diluted

8. 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.

9. Hardy-Weinberg Equilibrium Population of cats n=100 16 white and 84 black bb = white B_ = black Can we figure out the allelic frequencies of individuals BB and Bb?

10. 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

11. 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

12. Hardy-Weinberg Principle 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

13. Hardy-Weinberg Equilibrium

14. 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

15. 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.

16. 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

17. Genetic Drift - Bottleneck Effect

18. Five Agents of Evolutionary Change • Selection – Only agent that produces adaptive evolutionary 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

19. Five Agents of Evolutionary Change • Selection pressures: • avoiding predators • matching climatic condition • pesticide resistance

20. 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.

21. 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.

22. 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.

23. 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.

24. Sickle Cell and Malaria

25. 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.

26. Kinds of Selection

27. 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.

28. Evolution of Coloration in Guppies

29. 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.

30. Evolutionary Change in Spot Number

31. 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

32. Summary • Gene Variation • Hardy Weinberg Principle • Agents of Evolutionary Change • Measuring Fitness • Interactions Among Evolutionary Forces • Forms of Selection • Selection on Color in Guppies • Limits to Selection