Population Genetics

1. Population Genetics

2. Populations • A group of the same species living in an area • No two individuals are exactly alike (variations) • More Fit individuals survive & pass on their traits

3. Species • Different species do NOT exchange genes by interbreeding • Different species that interbreed often produce sterile or less viable offspring e.g. Mule

4. Speciation • Formation of new species • One species may split into 2 or more species • A species may evolve into a new species • Requires very long periods of time

5. Modern Evolutionary Thought

6. Modern Synthesis Theory • Combines Darwinian selection and Mendelian inheritance • Population genetics - study of genetic variation within a population • Emphasis on quantitative characters

7. Modern Synthesis Theory • Today’s theory on evolution • Recognizes that GENES are responsible for the inheritance of characteristics • Recognizes that POPULATIONS, not individuals, evolve due to natural selection & genetic drift • Recognizes that SPECIATION usually is due to the gradual accumulation of small genetic changes

8. Microevolution • Changes occur in gene pools due to mutation, natural selection, genetic drift, etc. • Gene pool changes cause more VARIATION in individuals in the population • This process is called MICROEVOLUTION • Example: Bacteria becoming unaffected by antibiotics (resistant)

9. The Gene Pool • Members of a species can interbreed & produce fertile offspring • Species have a shared gene pool • Gene pool – all of the alleles of all individuals in a population

10. Allele Frequencies Define Gene Pools 500 flowering plants 480 red flowers 20 white flowers 320 RR 160 Rr 20 rr As there are 1000 copies of the genes for color, the allele frequencies are (in both males and females): 320 x 2 (RR) + 160 x 1 (Rr) = 800 R; 800/1000 = 0.8 (80%) R 160 x 1 (Rr) + 20 x 2 (rr) = 200 r; 200/1000 = 0.2 (20%) r

11. Gene Pools • A population’s gene pool is the total of all genes in the population at any one time.  • Each allele occurs with a certain frequency (.01 – 1).

12. The Hardy-Weinberg Theorem • Used to describe a non-evolving population. • Shuffling of alleles by meiosis and random fertilization have no effect on the overall gene pool.  • Natural populations are NOT expected to actually be in Hardy-Weinberg equilibrium.

13. The Hardy-Weinberg Theorem • Deviation from Hardy-Weinberg equilibrium usually results in evolution • Understanding a non-evolving population, helps us to understand how evolution occurs • .

14. Sources of genetic variation(Disruption of H-W law) • Mutations- if alleles change from one to another, this will change the frequency of those alleles • 2. Genetic recombination - crossing over; independent assortment • 3. Migration- immigrants can change the frequency of an allele by bringing in new alleles to a population. • - emigrants can change allele frequencies by taking alleles out of the population

15. Sources of genetic variation(Disruption of H-W law) • 4. Genetic Drift- small populations can have chance fluctuations in allele frequencies (e.g., fire, storm). • - bottleneck; founder effect • 5. Natural selection-if some individuals survive and reproduce at a higher rate than others, then their offspring will carry those genes and the frequency will change for the next generation.

16. Hardy-Weinberg Equilibrium The gene pool of a non-evolving population remains constant over multiple generations; i.e., the allele frequency does not change over generations of time. The Hardy-Weinberg Equation:                                     1.0 = p2 + 2pq + q2 where p2= frequency of AA genotype; 2pq = frequency of Aa plus aA genotype; q2 = frequency of aa genotype

19. But we know that evolution does occur within populations. • Evolution within a species/population = microevolution. • Microevolution refers to changes in allele frequencies in a gene pool from generation to generation. Represents a gradual change in a population. • Causes of microevolution: • 1)  Genetic drift • Natural selection (1 & 2 are most important) • Gene flow • Mutation

20. 1) Genetic drift • Genetic drift = the alteration of the gene pool of a small population due to chance. • Two factors may cause genetic drift: • Bottleneck effect may lead to reduced genetic variability following some large disturbance that removes a large portion of the population. The surviving population often does not represent the allele frequency in the original population. • Founder effect may lead to reduced variability when a few individuals from a large population colonize an isolated habitat.

23. *Yes, I realize that this is not really a cheetah.

24. 2) Natural selection As previously stated, differential success in reproduction based on heritable traits results in selected alleles being passed to relatively more offspring (Darwinian inheritance). The only agent that results in adaptation to environment. 3) Gene flow -is genetic exchange due to the migration of fertile individuals or gametes between populations.

26. 4) Mutation Mutation is a change in an organism’s DNA and is represented by changing alleles.  Mutations can be transmitted in gametes to offspring, and immediately affect the composition of the gene pool. The original source of variation.

27. Genetic Variation, the Substrate for Natural Selection Genetic (heritable) variation within and between populations: exists both as what we can see (e.g., eye color) and what we cannot see (e.g., blood type). Not all variation is heritable. Environment also can alter an individual’s phenotype [e.g., the hydrangea we saw before, and… …Map butterflies (color changes are due to seasonal difference in hormones)].

29. Variation within populations Most variations occur as quantitative characters (e.g., height); i.e., variation along a continuum, usually indicating polygenic inheritance. Few variations are discrete (e.g., red vs. white flower color). Polymorphism is the existence of two or more forms of a character, in high frequencies, within a population.  Applies only to discrete characters.

30. Variation between populations Geographicvariations are differences between gene pools due to differences in environmental factors.  Natural selection may contribute to geographic variation.  It often occurs when populations are located in different areas, but may also occur in populations with isolated individuals.

31. Geographic variation between isolated populations of house mice. Normally house mice are 2n = 40. However, chromosomes fused in the mice in the example, so that the diploid number has gone down.

32. Cline, a type of geographic variation, is a graded variation in individuals that correspond to gradual changes in the environment.  Example:  Body size of North American birds tends to increase with increasing latitude. Can you think of a reason for the birds to evolve differently? Example: Height variation in yarrow along an altitudinal gradient. Can you think of a reason for the plants to evolve differently?

34. Mutation and sexual recombination generate genetic variation a.  New alleles originate only by mutations (heritable only in gametes; many kinds of mutations; mutations in functional gene products most important). - In stable environments, mutations often result in little or no benefit to an organism, or are often harmful. - Mutations are more beneficial (rare) in changing environments.  (Example:  HIV resistance to antiviral drugs.)  b.  Sexual recombination is the source of most genetic differences between individuals in a population. - Vast numbers of recombination possibilities result in varying genetic make-up.

35. Diploidy and balanced polymorphism preserve variation a.  Diploidy often hides genetic variation from selection in the form of recessive alleles. Dominant alleles “hide” recessive alleles in heterozygotes. b.  Balancedpolymorphism is the ability of natural selection to maintain stable frequencies of at least two phenotypes. Heterozygoteadvantage is one example of a balanced polymorphism, where the heterozygote has greater survival and reproductive success than either homozygote (Example: Sickle cell anemia where heterozygotes are resistant to malaria).

37. Frequency-dependentselection = survival of one phenotype declines if that form becomes too common. • (Example:  Parasite-Host relationship. Co-evolution occurs, so that if the host becomes resistant, the parasite changes to infect the new host. Over the time, the resistant phenotype declines and a new resistant phenotype emerges.)

40. Neutralvariation is genetic variation that results in no competitive advantage to any individual. - Example:  human fingerprints.

41. A Closer Look: Natural Selection as the Mechanism of Adaptive Evolution Evolutionary fitness - Not direct competition, but instead the difference in reproductive success that is due to many variables. Natural Selection can be defined in two ways: a.  Darwinian fitness- Contribution of an individual to the gene pool, relative to the contributions of other individuals. And,

42. b.  Relative fitness • - Contribution of a genotype to the next generation, compared to the contributions of alternative genotypes for the same locus. • Survival doesn’t necessarily increase relative fitness; relative fitness is zero (0) for a sterile plant or animal. Three ways (modes of selection) in which natural selection can affect the contribution that a genotype makes to the next generation. •  a.  Directionalselection favors individuals at one end of the phenotypic range. Most common during times of environmental change or when moving to new habitats.

43. Directional selection

44. Diversifyingselection favors extreme over intermediate phenotypes.  - Occurs when environmental change favors an extreme phenotype. Stabilizingselection favors intermediate over extreme phenotypes.  - Reduces variation and maintains the current average. - Example = human birth weights.

45. Diversifying selection

47. Natural selection maintains sexual reproduction • -Sex generates genetic variation during meiosis and fertilization. • Generation-to-generation variation may be of greatest importance to the continuation of sexual reproduction. • Disadvantages to using sexual reproduction: Asexual reproduction produces many more offspring. • -The variation produced during meiosis greatly outweighs this disadvantage, so sexual reproduction is here to stay.

48. All asexual individuals are female (blue). With sex, offspring = half female/half male. Because males don’t reproduce, the overall output is lower for sexual reproduction.

49. Sexual selection leads to differences between sexes • a.  Sexual dimorphism is the difference in appearance between males and females of a species. • Intrasexualselection is the direct competition between members of the same sex for mates of the opposite sex.  • This gives rise to males most often having secondary sexual equipment such as antlers that are used in competing for females. • -In intersexualselection (mate choice), one sex is choosy when selecting a mate of the opposite sex.  • -This gives rise to often amazingly sophisticated secondary sexual characteristics; e.g., peacock feathers.