Population Genetics and the Gene Pool

1. Population Genetics

2. 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

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

4. 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

5. 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

6. 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

7. 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)

8. Population - a localized group of individuals of the same species.  Species - a group of populations whose individuals have the ability to breed and produce fertile offspring.  Individuals near a population center are, on average, more closely related to one another than to members of other populations.

10. A population’s genepool is the total of all genes in the population at any one time. If all members of a population are homozygous for a particular allele, then the allele is fixed in the gene pool.

11. 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

12. 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.

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

16. 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 between populations.

18. 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.

19. Genetic Variation, the foundation of Natural Selection Genetic (inheritable) 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). 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)].

22. 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.

23. 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.

24. 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?

26. 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.)

27. 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).

29. Sickle Cell and Malaria

32. Relative fitness  a.  Directionalselection favors individuals at one end of the phenotypic range. Most common during times of environmental change or when moving to new habitats.

33. Directional selection

34. Diversifying/Disruptiveselection 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.

35. Diversifying selection

39. Genes Within Populations

40. 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.

41. Kinds of Selection

43. global warming survival Why is genetic variation important? variation EXTINCTION!! no variation

44. north south north south Why is genetic variation important? variation no variation

45. north south north south Why is genetic variation important? divergence variation NO DIVERGENCE!! no variation

46. Resistance to antibacterial soap Generation 1: 1.00 not resistant 0.00 resistant Natural selection

47. Natural selection Resistance to antibacterial soap Generation 1: 1.00 not resistant 0.00 resistant

48. Natural selection Resistance to antibacterial soap Generation 1: 1.00 not resistant 0.00 resistant Generation 2: 0.96 not resistant 0.04 resistant mutation!

49. Natural selection Resistance to antibacterial soap Generation 1: 1.00 not resistant 0.00 resistant Generation 2: 0.96 not resistant 0.04 resistant Generation 3: 0.76 not resistant 0.24 resistant

50. Natural selection Resistance to antibacterial soap Generation 1: 1.00 not resistant 0.00 resistant Generation 2: 0.96 not resistant 0.04 resistant Generation 3: 0.76 not resistant 0.24 resistant Generation 4: 0.12 not resistant 0.88 resistant