1. 23 Charles Darwin’s Theory of Evolution Observations made on his 5 year trip helped Darwin formulate his theory of evolution, which had two major components.  First, species are not immutable, but change, or adapt, over time.  Second, the agent that produces the changes is natural selection.

2. Figure 23.1 Darwin and the Voyage of the Beagle (Part 2)

3. 23 Charles Darwin’s Theory of Evolution Fourteen years after Darwin first made the observations, Alfred Russel Wallace came to similar conclusions independently. On July 1, 1858, Darwin’s and Wallace’s ideas were presented to the Linnaean Society of London. In 1859 Darwin published The Origin of Species.

4. 23 Charles Darwin’s Theory of Evolution Darwin observed that slight variations among individuals can significantly affect the chance that a given individual will survive and the number of offspring it will produce. Darwin called this differential reproductive success of individuals natural selection. (See handout) Darwin clearly understood a fundamental principle of evolution—that populations, not individuals, evolve and become adapted to the environments in which they live.

5. 23 Charles Darwin’s Theory of Evolution The rediscovery of Gregor Mendel’s publications gave rise to the study of population genetics which provides a major underpinning for Darwin’s theories. What is population genetics?

6. 23 Genetic Variation within Populations Population Genetics studies variation within and among species in order to understand the processes that result in evolutionary changes in species through time.

7. 23 Genetic Variation within Populations For a population to evolve, its members must possess heritable, genetic variation, which is the raw material on which agents of evolution act. We observe phenotypes in nature, the physical expressions of genes. The genetic constitution that governs a trait is called its genotype. A population evolves when individuals with different genotypes survive or reproduce at different rates.

8. 23 Genetic Variation within Populations Genes have different forms called alleles. A single individual has only some of the alleles found in the population to which it belongs. The sum of all the alleles in a population is the gene pool. The gene pool contains the variation (different alleles) that produces the differing phenotypes on which agents of evolution act.

9. Figure 23.3 A Gene Pool

10. 23 Genetic Variation within Populations Natural populations possess genetic variation. For example, selection for traits in a wild mustard has produced many important crop plants. What happened with this?

11. Figure 23.4 Many Vegetables from One Species

12. 23 Genetic Variation within Populations The study of the genetic basis of evolution is difficult because genotypes do not uniquely determine phenotypes. Dominance can lead to a particular phenotype being expressed by more than one genotype. Different phenotypes can also be produced by a given genotype, depending on environmental conditions encountered during development. Ex. Leaf size

13. In-Text Art p. 464

14. 23 Genetic Variation within Populations A locally interbreeding group within a geographic population is called a Mendelian population. The relative proportions, or frequencies, of all alleles in a population are a measure of that population’s genetic variation. Biologists can estimate allele frequencies for a given locus by measuring numbers of alleles in a sample of individuals from a population.

15. 23 Genetic Variation within Populations Measurements of allele frequencies range from 0 to 1, and the sum of all allele frequencies at a locus is 1. An allele’s frequency (p) is calculated by dividing the number of copies of the allele in a population by the sum of alleles in the population. If only two alleles (A and a) for a given locus are found among the members of a diploid population, they may combine to form three different genotypes: AA, Aa, and aa.

16. 23 Genetic Variation within Populations Allele frequencies can be calculated using mathematics with the following variables:  N AA = the number of individuals that are homozygous for the A allele (AA)  N Aa = the number of individuals that are heterozygous (Aa)  N aa = the number of individuals that are homozygous for the a allele (aa)  Note that N AA + N Aa + N aa = N, the total number of individuals in a population.

17. 23 Genetic Variation within Populations The total number of alleles in a population is 2N because each individual is diploid (in this case, either AA, Aa, or aa). p = the frequency of allele A. q = the frequency of allele a. For each population, p + q = 1.

18. Figure 23.6 Calculating Allele Frequencies

19. 23 Genetic Variation within Populations The two populations in this example have the same allele frequencies for A and a, but they are distributed differently. Therefore, the genotype frequencies of the two populations are different. Genotype frequency is the number of individuals with the genotype divided by the total number of individuals in the population. The frequencies of different alleles at each locus and the frequencies of different genotypes in a Mendelian population describe its genetic structure.

20. 23 Homework Read pp 465- 470. Answer problems on hand out.

21. 23 The Hardy–Weinberg Equilibrium A population of sexually reproducing organisms in which allele and genotype frequencies do not change from generation to generation is said to be at Hardy–Weinberg equilibrium. Five assumptions must be made in order to meet Hardy–Weinberg equilibrium.  Mating is random.  Population size is very large.  There is no migration between populations.  There is no mutation.  Natural selection does not affect the alleles under consideration.

22. 23 The Hardy–Weinberg Equilibrium If the conditions of the Hardy–Weinberg equilibrium are met, two results follow. The frequencies of alleles at a locus will remain constant from generation to generation. After one generation of random mating, the genotype frequencies will not change. The second result can be stated in the form of the Hardy–Weinberg equation: p 2 + 2pq + q 2 = 1.

23. Figure 23.7 Calculating Hardy–Weinberg Genotype Frequencies (Part 1)

24. Figure 23.7 Calculating Hardy–Weinberg Genotype Frequencies (Part 2)

25. 23 The Hardy–Weinberg Equilibrium The most important message of the Hardy– Weinberg equilibrium is that allele frequencies remain the same from generation to generation unless some agent acts to change them. The equilibrium also shows the distribution of genotypes that would be expected for a population at genetic equilibrium. The Hardy–Weinberg equilibrium allows scientists to determine whether evolutionary agents are operating and their identity (as evidenced by the pattern of deviation from the equilibrium).

26. 23 Evolutionary Agents and Their Effects Evolutionary agents cause changes in the allele and genotype frequencies in a population. These are observed as a deviations from the Hardy–Weinberg equilibrium. The known evolutionary agents are mutation, gene flow, random genetic drift, nonrandom mating, and natural selection.

27. 23 Evolutionary Agents and Their Effects The origin of genetic variation is mutation. A mutation is any change in an organism’s DNA. Most mutations appear to be random and are harmful or neutral to their bearers. Some mutations can be advantageous. Mutation rates are low; one out of a million loci is typical. Although mutation rates are low, they are sufficient to create considerable genetic variation.

28. 23 Evolutionary Agents and Their Effects One condition for Hardy–Weinberg equilibrium is that there is no mutation. Although this condition is never met, the rate at which mutations arise at single loci is usually so low that mutations result in only very small deviations from Hardy–Weinberg expectations. If large deviations are found, it is appropriate to dismiss mutation as the cause and look for evidence of other evolutionary agents.

29. 23 Evolutionary Agents and Their Effects Gene flow results when individuals migrate to another population and breed in their new location. Immigrants may add new alleles to the gene pool of a population, or they may change the frequencies of alleles already present if they come from a population with different allele frequencies. No immigration is allowed for a population to be in Hardy–Weinberg equilibrium.

30. 23 Evolutionary Agents and Their Effects Genetic drift is the random loss of individuals and the alleles they possess. In very small populations, genetic drift may be strong enough to influence the direction of change of allele frequencies even when other evolutionary agents are pushing the frequencies in a different direction. Organisms that normally have large populations may pass through occasional periods when only a small number of individuals survive (a population bottleneck).

31. Figure 23.8 A Population Bottleneck

32. 23 Evolutionary Agents and Their Effects During a population bottleneck, genetic variation can be reduced by genetic drift. Populations in nature pass through bottlenecks for numerous reasons; for example, predation and habitat destruction may reduce the population to a very small size, resulting in low genetic variation.

33. 23 Evolutionary Agents and Their Effects When a few pioneering individuals colonize a new region, the resulting population will not have all the alleles found among members of the source population. The resulting pattern of genetic variation is called a founder effect. Ex. Polydacdaly.

34. 23 Evolutionary Agents and Their Effects Nonrandom mating occurs when individuals mate either more often with individuals of the same genotype or more often with individuals of a different genotype. The resulting proportions of genotypes in the following generation differ from Hardy–Weinberg expectations. If individuals mate preferentially with other individuals of the same genotype, homozygous genotypes are overrepresented and heterozygous genotypes are underrepresented in the next generation. Conversely, individuals may mate preferentially with individuals of a different genotype.

35. 23 Start 9-27-07

36. 23 Evolutionary Agents and Their Effects For adaptation to occur, individuals that differ in heritable traits must survive and reproduce with different degrees of success. Alleles that help an organism in a particular environment would more likely be passed on and change the frequency of the allele. This process is known as natural selection.

37. 23 Evolutionary Agents and Their Effects The reproductive contribution of a phenotype to following generations relative to the contributions of other phenotypes is called its fitness. The fitness of a phenotype is determined by the average rates of survival and reproduction of individuals with that phenotype.

38. 23 The Results of Natural Selection Most traits (Characters) are influenced by alleles at more than one locus. For example, the size of individuals in a population is influenced by genes at many loci, and distribution of sizes is likely to be a bell- shaped curve. Natural selection can act on traits with quantitative variation in three ways:  Stabilizing selection  Directional selection  Disruptive selection

39. 23 The Results of Natural Selection Stabilizing selection preserves the characteristics of a population by favoring average individuals. Ex. Human Birth Weight

40. Figure 23.13 Human Birth Weight Is Influenced by Stabilizing Selection

41. 23 The Results of Natural Selection Directional selection changes the characteristics of a population by favoring individuals that vary in one direction from the mean of the population. Directional selection occurs when one extreme of a population contributes more offspring to the next generation.

42. Figure 23.12 Natural Selection Can Operate on Quantitative Variation in Several Ways (Part 2)

43. 23 The Results of Natural Selection Disruptive selection changes the characteristics of a population by favoring individuals that vary in both directions from the mean of the population. Disruptive selection occurs when individuals at both extremes of a population are simultaneously favored.

44. Figure 23.12 Natural Selection Can Operate on Quantitative Variation in Several Ways (Part 3)

45. 23 The Results of Natural Selection Sexual selection was Darwin’s explanation for the evolution of apparently useless but conspicuous traits in males of many species. such as bright colors, long tails, etc. Ex. In widowbirds, males with longer tails attract significantly more females than do males with shorter tails. Source of non-random mating.

46. 23 Assessing the Costs of Adaptations Adaptations generally are good and bad. Recombinant DNA techniques allow investigators to compare individuals that differ only in the genetically based adaptation of interest.

47. 23 Assessing the Costs of Adaptations Plasmid transfer techniques have been used to measure the cost associated with the resistance to an herbicide conferred by a single allele in Arabidopsis thaliana. Plants with the resistance allele produce 34 percent fewer seeds than nonresistant plants, indicating a high cost for resistance to the herbicide.

48. 23 Maintaining Genetic Variation Genetic drift, stabilizing selection, and directional selection all tend to reduce genetic variation. However, when organisms reproduce sexually, variation is amplified. Random assortment of chromosomes during meiosis, crossing over, and the cellular component of each gamete contribute to the diversity of offspring.