15-3 Shaping Evolutionary Theory:

1. 15-3 Shaping Evolutionary Theory: Georgia Performance Standards: SB5b: Explain the history of life in terms of biodiversity, ancestry, and the rates of evolution. SB5d: Relate natural selection to changes in organisms. Essential Questions: Why is important to understand evolutionary theory? What is the role of natural selection in speciation?

2. Gene Pools: • All members of a population can interbreed, they share a common group of genes, called a gene pool. • A gene pool is the combined genetic information of all the members of a particular population. • Typically contains two or more alleles—or forms of a certain gene—for each inheritable trait. • The relative frequency of an allele is the number of times that allele occurs in a gene pool compared with the number of times other alleles occur.

3. Relative Frequencies of Alleles Section 16-1 Frequency of Alleles Sample Population allele for brown fur allele for black fur 48% heterozygous black 16% homozygous black 36% homozygous brown

4. Evolution as Genetic Change  • Natural selection on single-gene traits can lead to changes in allele frequencies and, thus, to evolution. • Ex: Color Mutations

5. Single-Gene and Polygenic Traits • Inheritable variation can be expressed in a variety of ways.   • The number of phenotypes produced for a given trait depends on how many genes control the trait

6. Trait controlled by asingle gene Variation in this gene leads to only two distinct phenotypes The number of phenotypes a given trait has is determined by how many genes control the trait. Single-gene trait In humans, having a widow’s peak or not having a widow’s peak is controlled by a single gene with two alleles. As a result, only two phenotypes are possible.  

7. Most traits are controlled by two or more genes and are, therefore, called polygenic traits. Each gene of a polygenic trait often has two or more alleles. As a result, one polygenic trait can have many possible genotypes and even more possible phenotypes. Polygenic Traits: EX: height (A bell-shaped curve is also called a normal distribution)

8. Natural selection does not act directly on genes, but on phenotypes. affects which individuals having different phenotypes survive and reproduce and which do not. determines which alleles are passed from one generation to the next. can change the relative frequencies of alleles in a population over time. Evolution as Genetic Change 

9. Exactly what factors change the relative frequencies of alleles in a population? • In genetic terms, any factor that causes alleles to be added to or removed from a population will change the relative frequencies of alleles. • Evolution is any change in the relative frequencies of alleles in apopulation’s gene pool. • Evolution acts on populations, not on individuals.

10. Natural Selection on Single-Gene Traits  • Natural selection on single-gene traits can lead to changes in allele frequencies and, thus, to evolution. • EX: Color Mutations (organisms of one color may produce fewer offspring than organisms of another color.

11. Equilibrium v/s Evolution • The Hardy-Weinbergprinciple states that allele frequencies in a population will remain constant unless one or more factors cause those frequencies to change. • The situation in which allele frequencies remain constant is called genetic equilibrium (juh-net-ik ee-kwih-lib-ree-um). • If the allele frequencies do not change, the population will not evolve.

12. Evolution Chapter 15 15.3 Shaping Evolutionary Theory • This equation allows us to determine the equilibrium frequency of each genotype in the population. • Homozygous dominant (p2) • Heterozygous (2pq) • Homozygous recessive (q2)

13. 13.8 The gene pool of a nonevolving population remains constant over the generations • Hardy-Weinberg equilibrium states that the shuffling of genes during sexual reproduction does not alter the proportions of different alleles in a gene pool • To test this, let’s look at an imaginary, nonevolving population of blue-footed boobies Webbing No webbing Figure 13.8A

14. We can follow alleles in a population to observe if Hardy-Weinberg equilibrium exists Phenotypes Genotypes WW Ww ww Number of animals(total = 500) 320 160 20 Genotype frequencies 320/500 = 0.64 160/500 = 0.32 20/500 = 0.04 Number of allelesin gene pool(total = 1,000) 640 W 160 W + 160 w 40 w Allele frequencies 800/1,000 = 0.8 W 200/1,000 = 0.2 w Figure 13.8B

15. Recombinationof alleles fromparent generation W sperm p = 0.8 W egg p = 0.8 SPERM EGGS WW p2 = 0.64 w sperm q = 0.2 w egg q = 0.2 WW qp = 0.16 Ww pq = 0.16 ww q2 = 0.04 Next generation: Genotype frequencies 0.64 WW 0.32 Ww 0.04 ww Allele frequencies 0.8 W 0.2 w Figure 13.8C

16. Understanding Evolution: Problem-based discussion Population genetics calculations and Hardy Weinberg 1) What are the genotype frequencies at the flower-color locus in this generation? 2) What are the allele frequencies at the flower-color locus in this generation? 3) If the individuals in this generation mate randomly, what would you expect the genotype frequencies to be in the next generation? Ribozyme structure comes from Scott, W.G., Finch, J.T., Klug, A. (1995) The crystal structure of an all-RNA hammerhead ribozyme: a proposed mechanism for RNA catalytic cleavage. Cell 81: 991-1002

17. Understanding Evolution: Problem-based discussion Population genetics calculations and Hardy Weinberg 4) If the individuals in this generation mate randomly, what would you expect the allele frequencies to be in Generation 2? 5) If the resulting individuals in Generation 2 mate randomly, what would you expect the allele frequencies to be in Generation 3? 6) Is this population evolving? Why or why not? Ribozyme structure comes from Scott, W.G., Finch, J.T., Klug, A. (1995) The crystal structure of an all-RNA hammerhead ribozyme: a proposed mechanism for RNA catalytic cleavage. Cell 81: 991-1002

18. Evolution Chapter 15 15.3 Shaping Evolutionary Theory

19. Process of Speciation: • Factors such as natural selection and chance events can change the relative frequencies of alleles in a population. • But how do these changes lead to the formation of new species, orspeciation?

20. Process of Speciation: • A species as a group of organisms that breed with one another and produce fertile offspring. • They share a common gene pool. • A genetic change that occurs in one individual can spread through the population as that individual and its offspring reproduce. • If a genetic change increases fitness, that allele will eventually be found in many individuals of that population.

21. Genes and Variation: • Genetics, molecular biology, and evolutionary theory work together to explain how inheritable variation appears and how natural selection operates on that variation

22. Checkpoint Questions: • Describe how natural selection can affect traits controlled by single genes. • Describe three patterns of natural selection on polygenic traits. Which one leads to two distinct phenotypes? • How does genetic drift lead to a change in a population’s gene pool? • What is the Hardy-Weinberg principle? • How are directional selection and disruptive selection similar? How are they different?

23. Evolution Chapter 15 15.3 Shaping Evolutionary Theory Genetic Drift • A change in the allelic frequencies in a population that is due to chance • In smaller populations, the effects of genetic drift become more pronounced, and the chance of losing an allele becomes greater. Examples: Founder Effect & Bottleneck

24. Genetic Drift Sample of Original Population Descendants Founding Population A Founding Population B

25. Understanding Evolution: Problem-based discussion Population genetics simulation of drift 1) Explain what is shown on the x- and y-axes. 2) Choose two lines on graph A, one that goes to the top of the graph and one that goes to the bottom. For each line, explain what the line represents and how it changes over time. Also, explain what it means when a line goes to the top of the graph versus what it means when a line goes to the bottom of the graph. Ribozyme structure comes from Scott, W.G., Finch, J.T., Klug, A. (1995) The crystal structure of an all-RNA hammerhead ribozyme: a proposed mechanism for RNA catalytic cleavage. Cell 81: 991-1002

26. Understanding Evolution: Problem-based discussion Population genetics simulation of drift 3) Explain the difference between the simulations that generated graphs A and B. 4) Why do y-values fluctuate in each graph? Ribozyme structure comes from Scott, W.G., Finch, J.T., Klug, A. (1995) The crystal structure of an all-RNA hammerhead ribozyme: a proposed mechanism for RNA catalytic cleavage. Cell 81: 991-1002

27. Understanding Evolution: Problem-based discussion Population genetics simulation of drift 5) In graph A, how many of the trials resulted in a frequency of A = 1.0 and how many resulted in a frequency of a = 1.0? Why might this occur? 6) The populations represented by graph A and graph B demonstrate very different behavior. What concept regarding genetic drift does this illustrate? Ribozyme structure comes from Scott, W.G., Finch, J.T., Klug, A. (1995) The crystal structure of an all-RNA hammerhead ribozyme: a proposed mechanism for RNA catalytic cleavage. Cell 81: 991-1002

28. Understanding Evolution: Problem-based discussion Population genetics simulation of drift 7) If one allele were under selection in graph B, how would this graph differ? 8) For each population represented on graph A, would you expect the population to be in Hardy-Weinberg equilibrium? For graph B? Ribozyme structure comes from Scott, W.G., Finch, J.T., Klug, A. (1995) The crystal structure of an all-RNA hammerhead ribozyme: a proposed mechanism for RNA catalytic cleavage. Cell 81: 991-1002

29. Understanding Evolution: Problem-based discussion Population genetics simulation of drift 9) How would each of these graphs differ if starting allele frequencies in the simulations were not 50/50—for example, if the populations began with 30% A alleles and 70% a alleles? Would drift occur? Ribozyme structure comes from Scott, W.G., Finch, J.T., Klug, A. (1995) The crystal structure of an all-RNA hammerhead ribozyme: a proposed mechanism for RNA catalytic cleavage. Cell 81: 991-1002

30. Evolution Chapter 15 15.3 Shaping Evolutionary Theory Founder Effect • Occurs when a small sample of a population settles in a location separated from the rest of the population • Alleles that were uncommon in the original population might be common in the new population.

31. Evolution Chapter 15 15.3 Shaping Evolutionary Theory Bottleneck • Occurs when a population declines to a very low number and then rebounds

32. Evolution Chapter 15 15.3 Shaping Evolutionary Theory Gene Flow • Increases genetic variation within a population and reduces differences between populations Nonrandom Mating • Promotes inbreeding and could lead to a change in allelic proportions favoring individuals that are homozygous for particular traits

33. Sources of Genetic Variation • The two main sources of genetic variation are mutations and the genetic shuffling that results from sexual reproduction. • Sexual reproduction can thus produce many different phenotypes, but this does not change the relative frequency of alleles in a population. (Card deck analogy)

34. Checkpoint Questions: • What two processes can lead to inherited variation in populations? • How does the range of phenotypes differ between single-gene traits and polygenic traits? • What is a gene pool? How are allele frequencies related to gene pools? • How could you distinguish between a species in which there is a lot of variation and two separate species?

35. Natural Selection on Polygenic Traits Fitness varies in polygenic traits. Where fitness varies, natural selection can act.   Natural selection can affect the distributions of phenotypes in any of three ways: directional selection stabilizing selection disruptive selection. Evolution as Genetic Change

36. Evolution Chapter 15 15.3 Shaping Evolutionary Theory Natural Selection • Acts to select the individuals that are best adapted for survival and reproduction

37. Evolution Chapter 15 15.3 Shaping Evolutionary Theory • Stabilizing selectionoperates to eliminate extreme expressions of a traitwhen the average expression leads to higher fitness.

38. Evolution Chapter 15 15.3 Shaping Evolutionary Theory • Directional selectionmakes an organism more fit.

39. Evolution Chapter 15 15.3 Shaping Evolutionary Theory • Disruptive selectionis a process that splits a population into two groups.

40. Evolution Chapter 15 15.3 Shaping Evolutionary Theory • Sexual selection operates in populations where males and females differ significantly in appearance. • Qualities of sexual attractiveness appear to be the opposite of qualities that might enhance survival. Natural Selection

41. Isolating Mechanisms • What happens to a gene pool as one species evolves into one or more species?   • As new species evolve, populations become reproductively isolated from each other. • When the members of two populations cannot interbreed and produce fertile offspring, reproductive isolation has occurred. • At that point, the populations have separate gene pools.

42. Reproductive Isolation • Develops in a variety of ways: • Postzygotic isolation (behavioral isolation) = occurs when two populations are capable of interbreeding but have differences in courtship rituals or other types of behavior. • Allopatric Speciation (geographic isolation) = two populations are separated by geographic barriers such as rivers, mountains, or bodies of water. • do not guarantee the formation of new species • Prezygotic isolation (temporal isolation)= two or more species reproduce at different times.

43. Geographic isolation Behavioral isolation Temporal isolation Physical separation Behavioral differences Different mating times Section 16-3 Reproductive Isolation results from Isolating mechanisms which include produced by produced by produced by which result in Independentlyevolving populations which result in Formation ofnew species

44. Courtship ritual in blue-footed boobies is an example of one kind of prezygotic barrier, behavioral isolation • Many plant species have flower structures that are adapted to specific pollinators • This is an example of mechanical isolation, another prezygotic barrier Figure 14.2A, B

45. Hybrid sterility is one type of postzygotic barrier • A horse and a donkey may produce a hybrid offspring, a mule • Mules are sterile Figure 14.2C

46. Evolution Chapter 15 15.3 Shaping Evolutionary Theory • Prezygotic isolationprevents reproduction by making fertilization unlikely. • Prevents genotypes from entering a population’s gene pool through geographic, ecological, behavioral, or other differences Eastern meadowlark and Western meadowlark

47. Evolution • Postzygotic isolationoccurs when fertilization has occurred but a hybrid offspring cannot develop or reproduce. Chapter 15 15.3 Shaping Evolutionary Theory • Prevents offspring survival or reproduction Liger

48. Evolution Chapter 15 15.3 Shaping Evolutionary Theory Allopatric Speciation • A physical barrier divides one population into two or more populations. Kaibab squirrel Abert squirrel

49. Evolution Chapter 15 15.3 Shaping Evolutionary Theory Sympatric Speciation • A species evolves into a new species without a physical barrier. • The ancestor species and the new species live side by side during the speciation process.

50. Checkpoint Questions: • How is reproductive isolation related to the formation of new species? • What type of isolating mechanism was important in the formation of different Galápagos finch species? 3. Explain how behavior can play a role in the evolution of species. 4. Leopard frogs and tree frogs share the same habitat. Leopard frogs mate in April; tree frogs mate in June. How are these species isolated from each other?