Lecture #2 – Evolution of Populations

1. Lecture #2 – Evolution of Populations Image of a population of penguins

2. Key Concepts: • The Modern Synthesis • Populations and the Gene Pool • The Hardy-Weinberg Equilibrium • Micro-evolution • Sources of Genetic Variation • Natural Selection • Preservation of Genetic Variation

3. Review definitions • Species – individual organisms capable of mating and producing fertile offspring • Population – a group of individuals of a single species • Community – a group of individuals of different species Images – species, population, community

4. The Modern Synthesisintegrates our knowledge about evolution • Darwin’s natural selection • Mendel’s hereditary patterns • Particulate transfer (chromosomes) • Structure of the DNA molecule All explain how the genetic structure of populations changes over time

5. * * *** ***** * * ***** *** * ** ** * * * * KEY POINT Environmental factors act on the individual to control the genetic future of the population Individuals don’t evolve…..populations do

6. Population = a +/- localized group of individuals of one species Image – population of iris

7. Critical Thinking • How do we determine the boundaries of a population???

8. Critical Thinking • How do we determine the boundaries of a population??? • Boundaries are scale dependent • Some sub-populations overlap • Some are more isolated • We can look at populations at many different scales – micro to meta

9. Recall basic genetic principles: • In a diploid species (most are), every individual has two copies of every gene • One copy came from each parent • Most genes have different versions = alleles • Diploid individuals are either heterozygous or homozygous for each gene • Heterozygous = Aa • Homozygous = AA or aa

10. Recall basic genetic principles: • The total number of alleles for any gene in a population is the number of individuals in the population x 2 • If the population has 10 individuals, there are 20 copies of the A gene – some “A” alleles and some “a” alleles • All these alleles comprise the “gene pool”

11. Hardy-Weinberg Theorem • Gene pool = all alleles in a population • All alleles have a frequency in the population • There is a percentage of “A” and a percentage of “a” that adds up to 100% • Hardy-Weinberg Theorem demonstrates that allele frequencies don’t change through meiosis and fertilization alone

12. Hardy-Weinberg Theorem • A simple, mathematical model • Shows that repeated random meiosis and fertilization events alone will not change the distribution of alleles in a population • Even over many generations p2 + 2pq + q2 = 1 we will not focus on the math – you’ll work on this in lab

13. Hands On • The equation: p2+ 2pq + q2 = 1 (Page 2) • p = the frequency of one allele • q = the frequency of the other allele • p + q MUST = 1 = 100% of the gene pool

14. Hands On • If the allele frequencies are known, the HW equilibrium can be demonstrated by a Punnett square

15. Hands On • What are the frequencies of each allele in the F1 generation?

16. Hands On – Results • What are the frequencies of each allele in the F1 generation?

17. Hands On – Results • p2+ 2pq + q2 = 1 • .52 + 2x.5x.5 + .52 = 1 • 25% TT : 50% Tt : 25% tt • 50% T and 50% t = no change

18. Hands On – Results • p2+ 2pq + q2 = 1 • 25% TT : 50% Tt : 25% tt • 50% T and 50% t = no change • This goes on generation after generation • The phenotype remains 75% dominant and 25% recessive

19. Hands On • How do you determine the allele frequencies??? • How do you find p and q??? • In this example, how do you know the percentage of T and the percentage of t???

20. Hands On – Results • Remember that the recessive phenotype is tt • If you know the percentage of the population that expresses the recessive phenotype, then t (q) is the square root of that number • p2+ 2pq + q2 = 1 • 25% TT : 50% Tt : 25% tt • 75% express dominant; 25% express recessive • √.25 = .5; determine p by subtraction

21. Hands On • Clasp your hands

22. Hands On • Count right thumbs up vs. left thumbs up • Right thumb up is the recessive condition! • Determine the distribution of T and t in our class population • Type up a summary of your results and turn in tomorrow

23. Hardy-Weinberg Theorem • Meiosis and fertilization randomly shuffle alleles, but they don't change proportions • Like repeatedly shuffling a deck of cards • The laws of probability determine that the proportion of alleles will not change from generation to generation • This stable distribution of alleles is the Hardy-Weinberg equilibrium Doesn’t happen in nature!!!

24. Conditions for H-W Equilibrium: • No natural selection • Large population size • Isolated population • Random mating • No mutation Doesn’t happen in nature!!! The violation of each assumption acts as an agent of microevolution

25. The value of H-W??? • It provides a null hypothesis to compare to what actually happens in nature • Allele frequencies DO change in nature • BUT, they change only under the conditions of microevolution • In nature, all the H-W assumptions are violated • Result – populations DO evolve

26. Critical Thinking • What are the limitations of the Hardy-Weinberg theorem???

27. Critical Thinking • What are the limitations of the Hardy-Weinberg theorem??? • The H-W model considers just one trait at a time, and assumes that just one gene with 2 alleles (one completely dominant) controls that trait • Recall your basic genetics – is this realistic???

28. Critical Thinking • Reality is much more complex for most traits in most organisms • Incomplete dominance or codominance • More than 2 alleles for many genes • Pleiotropy – one gene affects multiple traits • Polygenic traits – multiple genes affect one trait • Epistasis – one gene affects expression of another gene • Environmental effects on phenotypic expression • Reproductive success depends on the way all genes and phenotypic traits interact

29. Individuals Do Not Evolve • Individuals vary, but populations evolve • Natural selection pressures make an individual more or less likely to survive and reproduce • But, it is the cumulative effects of selection on the genetic makeup of the whole population that results in changes to the species The environment is a wall; natural selection is a gate

30. * * *** ***** * * ***** *** * ** ** * * * * ***** ***** The environment is the wall; natural selection is the gate ?

31. Micro-evolution:population-scale changes in allele frequencies • Natural Selection • Genetic Drift • Gene Flow • Selective Mating • Mutation Image – natural variation in flower color; same image for all these summary slides

32. Natural Selection – the essence of Darwin’s theory Differential reproductive success is the only way to account for the accumulation of favorable traits in a population Cartoon – beaver with chainsaw paws  “natural selection does not grant organisms what they “need”” More on this later…. More on this later….

33. Micro-evolution:population-scale changes in allele frequencies • Natural Selection • Genetic Drift • Gene Flow • Selective Mating • Mutation

34. Parent pop = 10% blue Smaller pop = 100% blue Larger pop = ~29% blue Genetic Drift – random changes in allele frequency from generation to generation • Reproductive events are samples of the parent population • Larger samples are more representative than smaller samples (probability theory)

35. Parent pop = 10% blue Segregated pop = 100% blue Segregated pop = ~29% blue Genetic Drift – random changes in allele frequency from generation to generation • More pronounced in smaller and/or more segregated populations • Bottleneck effect • Founder effect

36. Bottlenecking = extreme genetic drift Diagram – bottlenecking

37. Critical Thinking • What events could cause a bottleneck???

38. Critical Thinking • What events could cause a bottleneck??? Bottlenecks occur when there is an extreme and indiscriminate reduction in the reproducing population • Disease • Herbivory • Malnutrition • Major disturbance (flood, fire) • Human intervention

39. Conservation implications – cheetahs are a bottlenecked species Image – cheetah

40. Extreme range reduction due to habitat destruction and poaching + Cheetahs were naturally bottlenecked about 10,000 years ago by the last major ice age (kinked tail) The species is at risk of extinction Maps – historic and current range of cheetahs

41. Australian Flame Robin, California Condor, Mauritian Kestrel…..and many more, all driven nearly to extinction….. Images – bottlenecked and now endangered species Some colorful results of a quick web search on “bottlenecked species”

42. Founder Effect = extreme genetic drift • Occurs when a single individual, or small group of individuals, breaks off from a larger population to colonize a new habitat • Islands • Other side of mountain • Other side of a river… • This small group may not represent the allele distribution of the parent population

43. Founder Effect

46. Long distance dispersal events can lead to the founder effect Image – a founding population of seeds; possibly also the bird if it’s a gravid female

47. Critical Thinking • What do you think follows long distance dispersal to a new ecosystem???

48. 1 Founding Population 2 3 4 Critical Thinking • What do you think follows long distance dispersal to a new ecosystem??? • Adaptive radiation frequently leads to many new, closely related species as the organisms adapt to new habitat zones in their new home

49. Hands On • Genetic drift is random • Some drift is expected with every generation • Genetic drift is not necessarily extreme • Use the beads to explore this idea (Page 4) • Count out 50 beads each of 2 colors • Each bead represents an allele in the gene pool • Since B and b are in equal proportion, what is the phenotypic makeup of the diploid population??? • p2 + 2pq + q2 = 1

50. Hands On – Results • Genetic drift is random • Some drift is expected with every generation • Genetic drift is not necessarily extreme • Use the beads to explore this idea • Count out 50 beads each of 2 colors • Each bead represents an allele in the gene pool • Since B and b are in equal proportion, what is the phenotypic makeup of the diploid population??? • 75% express dominant; 25% express recessive