Lesson Overview

1. Lesson Overview 17.1 Genes and Variation

2. THINK ABOUT IT • Darwin developed his theory of evolution without knowing how heritable traits passed from one generation to the next or where heritable variation came from. • What would happen when genetics answered questions about how heredity works?

3. Genetics Joins Evolutionary Theory • How is evolution defined in genetic terms?

4. Genetics Joins Evolutionary Theory • How is evolution defined in genetic terms? • In genetic terms, evolution is any change in the relative frequency of alleles in the gene pool of a population over time.

5. Genetics Joins Evolutionary Theory • Researchers discovered that heritable traits are controlled by genes. • Changes in genes and chromosomes generate variation. • For example, all of these children received their genes from the same parents, but they all look different.

6. Genotype and Phenotype in Evolution An organism’s genotype is the particular combination of alleles it carries. An individual’s genotype, together with environmental conditions, produces its phenotype. Phenotype includes all physical, physiological, and behavioral characteristics of an organism.

7. Genotype and Phenotype in Evolution Natural selection acts directly on phenotype, not genotype. Some individuals have phenotypes that are better suited to their environment than others. These individuals produce more offspring and pass on more copies of their genes to the next generation.

8. Populations and Gene Pools A population is a group of individuals of the same species that mate and produce offspring. A gene pool consists of all the genes, including all the different alleles for each gene that are present in a population.

9. Populations and Gene Pools Researchers study gene pools by examining the relative frequency of an allele. The relative frequency of an allele is the number of times a particular allele occurs in a gene pool, compared with the number of times other alleles for the same gene occur.

10. For example, this diagram shows the gene pool for fur color in a population of mice.

11. Populations and Gene Pools Evolution is any change in the relative frequency of alleles in the gene pool of a population over time. Natural selection operates on individuals, but resulting changes in allele frequencies show up in populations. Populations, rather than individuals, evolve.

12. Sources of Genetic Variation • What are the sources of genetic variation?

13. Sources of Genetic Variation • What are the sources of genetic variation? • Three sources of genetic variation are mutation, genetic recombination during sexual reproduction, and lateral gene transfer.

14. Mutations Mutations that produce changes in phenotype may or may not affect fitness. Some mutations may be lethal or may lower fitness; others may be beneficial. Mutations matter in evolution only if they can be passed from generation to generation. The mutation must occur in the germ line cells that produce either eggs or sperm.

15. Genetic Recombination in Sexual Reproduction Mutations that produce changes in phenotype may or may not affect fitness. Some mutations may be lethal or may lower fitness; others may be beneficial. Mutations matter in evolution only if they can be passed from generation to generation. The mutation must occur in the germ line cells that produce either eggs or sperm.

16. Lateral Gene Transfer Lateral gene transfer occurs when organisms pass genes from one individual to another that is not its offspring. It can occur between organisms of the same species or organisms of different species. Lateral gene transfer can increase genetic variation in a species that picks up the “new” genes.

17. Single-Gene and Polygenic Traits • What determines the number of phenotypes for a given trait?

18. Single-Gene and Polygenic Traits • What determines the number of phenotypes for a given trait? • The number of phenotypes produced for a trait depends on how many genes control the trait.

19. Single-Gene Traits • A single-gene trait is a trait controlled by only one gene. Single-gene traits may have just two or three distinct phenotypes. • The most common form of the allele can be dominant or recessive.

20. Dominance of an allele for a single-gene trait does not necessarily mean that the dominant phenotype will always appear with greater frequency in a given population. An example of a single-gene trait is the presence of dark bands that appear on the shells of a certain species of snails. Even though the allele for shells without bands is dominant, a population may show a greater frequency of the “with bands” phenotype.

21. Polygenic Traits • Polygenic traits are traits controlled by two or more genes. • Each gene of a polygenic trait often has two or more alleles. • A single polygenic trait often has many possible genotypes and even more different phenotypes.

22. Polygenic Traits • Human height, which varies from very short to very tall, is an example of a polygenic trait. • The bell-shaped curve in the graph is typical of polygenic traits.

23. Lesson Overview 17.2 Evolution as Genetic Change in Populations

24. THINK ABOUT IT Insect populations often contain a few individuals that are resistant to a particular pesticide. Those insects pass on their resistance to their offspring and soon the pesticide-resistant offspring dominate the population. The relationship between natural selection and genetics explains how pesticide resistance develops.

25. How Natural Selection Works How does natural selection affect single-gene and polygenic traits?

26. How Natural Selection Works How does natural selection affect single-gene and polygenic traits? Natural selection on single-gene traits can lead to changes in allele frequencies and, thus, to changes in phenotype frequencies. Natural selection on polygenic traits can affect the distributions of phenotypes in three ways: directional selection, stabilizing selection, or disruptive selection.

27. How Natural Selection Works Evolutionary fitness is the success in passing genes to the next generation. Evolutionary adaptation is any genetically controlled trait that increases an individual’s ability to pass along its alleles.

28. Natural Selection on Single-Gene Traits Natural selection for a single-gene trait can lead to changes in allele frequencies and then to evolution. For example, a mutation in one gene that determines body color in lizards can affect their lifespan. So if the normal color for lizards is brown, a mutation may produce red and black forms.

29. Natural Selection on Single-Gene Traits: The example of Lizard Color

30. Natural Selection on Single-Gene Traits If red lizards are more visible to predators, they might be less likely to survive and reproduce. Therefore the allele for red coloring might not become common.

31. Natural Selection on Single-Gene Traits Single-Gene Traits: The allele for red coloring might not become common.

32. Natural Selection on Single-Gene Traits Black lizards might be able to absorb sunlight. Higher body temperatures may allow the lizards to move faster, escape predators, and reproduce.

33. Natural Selection on Single-Gene Traits Single-Gene Traits: The allele for black color might become more common.

34. Natural Selection on Polygenic Traits Polygenic traits have a range of phenotypes that often form a bell curve. The fitness of individuals may vary from one end of the curve to the other. Natural selection can affect the range of phenotypes and hence the shape of the bell curve.

35. Directional Selection Directional selection occurs when individuals at one end of the curve have higher fitness than individuals in the middle or at the other end. The range of phenotypes shifts because some individuals are more successful at surviving and reproducing than others.

36. For example, if only large seeds were available, birds with larger beaks would have an easier time feeding and would be more successful in surviving and passing on genes. Directional Selection

37. Stabilizing Selection Stabilizing selection occurs when individuals near the center of the curve have higher fitness than individuals at either end. This situation keeps the center of the curve at its current position, but it narrows the overall graph.

38. Stabilizing Selection For example, very small and very large babies are less likely to survive than average-sized individuals. The fitness of these smaller or larger babies is therefore lower than that of more average-sized individuals.

39. Disruptive Selection Disruptive selection occurs when individuals at the upper and lower ends of the curve have higher fitness than individuals near the middle. Disruptive selection acts against individuals of an intermediate type and can create two distinct phenotypes.

40. Disruptive Selection For example, in an area where medium-sized seeds are less common, birds with unusually small or large beaks would have higher fitness. Therefore, the population might split into two groups—one with smaller beaks and one with larger beaks.

41. Genetic Drift What is genetic drift?

42. Genetic Drift What is genetic drift? In small populations, individuals that carry a particular allele may leave more descendants than other individuals, just by chance. Over time, a series of chance occurrences can cause an allele to become more or less common in a population.

43. Genetic Drift Genetic drift occurs in small populations when an allele becomes more or less common simply by chance. Genetic drift is a random change in allele frequency.

44. Genetic Bottlenecks The bottleneck effect is a change in allele frequency following a dramatic reduction in the size of a population. For example, a disaster may kill many individuals in a population, and the surviving population’s gene pool may contain different gene frequencies from the original gene pool.

45. The Founder Effect The founder effect occurs when allele frequencies change as a result of the migration of a small subgroup of a population.

46. The Founder Effect Two groups from a large, diverse population could produce new populations that differ from the original group.

47. Evolution Versus Genetic Equilibrium What conditions are required to maintain genetic equilibrium?

48. Evolution Versus Genetic Equilibrium What conditions are required to maintain genetic equilibrium? According to the Hardy-Weinberg principle, five conditions are required to maintain genetic equilibrium: (1) The population must be very large; (2) there can be no mutations; (3) there must be random mating; (4) there can be no movement into or out of the population, and (5) no natural selection.

49. Evolution Versus Genetic Equilibrium A population is in genetic equilibrium if allele frequencies in the population remain the same. If allele frequencies don’t change, the population will not evolve.

50. The Hardy-Weinberg Principle The Hardy-Weinberg principle describes the conditions under which evolution does not occur. The Hardy-Weinberg principle states that allele frequencies in a population remain constant unless one or more factors cause those frequencies to change.