Evolution of Populations:

1. Evolution of Populations: 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? Why are there species alive now that were not found in the past fossil record? How does fossil and biochemical evidence support the evolutionary theory?

2. 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?

3. 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.

4. Evolution Chapter 15 15.3 Shaping Evolutionary Theory Mechanisms of Evolution • Hardy-Weinberg principle states that when allelic frequencies remain constant, a population is in genetic equilibrium.

5. 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)

6. Hardy-Weinberg Equation: • Used to calculate the frequency of alleles p2 + 2pq + q2 = 1 • Frequency of WW + Frequency of Ww + Frequency of ww = 1 • The combined frequencies of all alleles must be 100%

7. The population is very large The population is isolated Mutations do not alter the gene pool Mating is random All individuals are equal in reproductive success Evolution v/s Equilibrium Five conditions are required to maintain genetic equilibrium from generation to generation Five conditions are required for Hardy-Weinberg equilibrium

8. Evolution Chapter 15 15.3 Shaping Evolutionary Theory

9. Hardy-Weinberg principle • 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.

10. 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?

11. Microevolution • When the relative frequencies of alleles in a population change over a number of generations, evolution is occurring on its smallest scale (microevolution)

12. 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

13. There are several potential causes of microevolution • Genetic drift is a change in a gene pool due to chance • Genetic drift can cause the bottleneck effect • Gene flow can change a gene pool due to the movement of genes into or out of a population • Mutation changes alleles • Nonrandom mating • Natural selection leads to differential reproductive success

14. 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.

15. Genetic Drift • Natural selection is not the only source of evolutionary change. • The smaller a population is, the farther the results may be from what the laws of probability predict. This kind of random change in allele frequency is called genetic drift. • How does genetic drift take place? • In small populations, individuals that carry a particular allele may leave more descendants than other individuals do, just by chance. • Over time, a series of chance occurrences of this type can cause an allele to become common in a population.

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

17. 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.

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

19. 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

20. 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.

21. 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

22. 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)

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

24. 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

25. 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.  

26. 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)

27. 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?

28. 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 

29. 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.

30. 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.

31. 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

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

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

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

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

36. 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

37. 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

38. 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

39. 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

40. 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.

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: • behavioral isolation = occurs when two populations are capable of interbreeding but have differences in courtship rituals or other types of behavior. • 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 • 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. Unrelated Related Intense environmental pressure Similar environments Inter-relationshiops Small populations Different environments Convergent evolution Punctuated equilibrium Adaptive radiation Coevolution Extinction Section 17-4 Species that are form in under under in in can undergo can undergo can undergo can undergo can undergo Go to Section:

45. 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?