Chapter 23

1. Chapter 23 The Evolution of Populations

2. A Common Misconception • Many people think individuals evolve. • This is not true. • Populations evolve as a result of natural selection acting on each individual within a given population. • Those individuals better fit to survive are more likely to reproduce and pass on genes that will benefit future generations.

3. Microevolution • Evolution on a small scale. • The change in the genetic make up of a population from generation to generation.

4. Darwin’s “Problem” • Darwin’s problem was that there were many limitations of science at the time. • He did not have a good explanation for how such heritable variations that were required for natural selection appeared in a population. • Nor did he have an explanation for how they were transmitted from organisms to their offspring.

5. Recall the Blending Hypothesis: • At the time, the blending hypothesis was what people used to explain why offspring look like both parents. • Darwin and others realized this was wrong because it would eliminate variation within a population. • Ironically, shortly after Darwin published Origin, Gregor Mendel published his paper on genetics.

6. Mendel’s paper went unnoticed for nearly 50 years. • In the early 20th century, as scientists uncovered the work of Mendel, it became apparent that its implications and relatedness to Darwin’s idea were profound.

7. Population Genetics • Scientists so began drawing parallels between Darwin and Mendel and melded them into what is known as population genetics--the study of how populations change over time.

8. The Modern Synthesis • As more was learned about Darwin and Mendel, scientists developed the Modern Synthesis--a comprehensive theory of evolution that incorporates many fields of science.

9. Populations • Populations are groups of individuals that can breed with one another and are localized in certain regions. • Some populations are isolated from others. • Still others can easily mix with other members of a population.

10. Populations • Within a population, all of the genes are called the gene pool and it consists of all alleles at a given locus. • If only one allele exists in a population, it is said to be fixed and all individuals are homozygous. • If more than one allele exists, then individuals are either homozygous or heterozygous.

11. Allele Frequency • Consider a population of 500 with 2 alleles, CR and CW • CRCR gives Red • CWCW gives White • CRCW gives Pink

12. Allele Frequency • Our Population Breakdown: • 320 red, CRCR • 160 Pink, CRCW • 20 White, CWCW • These numbers suggest a blending hypothesis • Why can’t we use blending?

13. The Hardy-Weinberg Theorem • This theorem is a way to examine how allele frequencies change over time when only segregation and independent assortment are working on the alleles. • The properties of a non-evolving gene pool--in the absence of natural selection. • The theorem states that the frequencies of the alleles will remain constant in a population when it is not evolving.

14. The Hardy-Weinberg Theorem • The theorem describes Mendelian inheritance in non-evolving populations. • It also helps us to understand long-term evolutionary change--that is, the preservation of genetic variation gives the opportunity for natural selection to occur.

15. Hardy-Weinberg Frequency • In our population, there are 1000 copies of genes (500 individuals, 2 copies). • 800 of them are CR • 200 of them are CW • When we have 2 alleles, by convention we represent them as p and q. • p = CR = 800/1000 = 0.8 or 80% • q = CW = 200/1000 = 0.2 or 20%

16. The Hardy-Weinberg Theorem • To determine the probabilities in our wild flower example: • The chance of CRCR is: • p • p = p2 = 0.8 • 0.8 = 0.64 64% • The chance of CRCW is: • p • q = 2pq = 0.8 • 0.2 = 0.32 32% • The chance of CWCW is: • q • q = q2 = 0.2 • 0.2 = 0.04 4%

17. The Hardy-Weinberg Equation • The Hardy-Weinberg Equation becomes: p2 + 2pq +q2 = 1 • Again, with a non-evolving gene pool, the frequencies of alleles will remain constant if mating is random. You can think of it like a deck of cards, no matter how many times you shuffle them, the types of cards and their frequencies remain the same.

18. 5 Reasons Hardy-Weinberg Doesn’t Hold True: • Departure from these 5 conditions results in evolution. • Extremely large population size. • No gene flow. • No mutations. • Random mating. • No natural selection.

19. Mutation and Sexual Recombination • These provide variety within gene pools. • Mutations are changes in the nucleotide sequences that give rise to new genes and new alleles. Sometimes they’re good, usually they are not. • Most mutations occur in somatic cells and are never passed on. • Only a small percentage of gametes ever get into the populations, so any mutation occurring in the gametes likely won’t get passed on.

20. Mutation and Sexual Recombination • Mutation rates in general are low. The larger the organism the less likely a mutation will occur and vice-versa. • For example: Plants and animals with long generation times are relatively large and have a much lower frequency of mutations than do microorganism and viruses.

21. Mutation and Sexual Recombination • Sexual recombination is the best way to produce variation within a population on a generation to generation time scale. Movie

22. Mutation and Sexual Recombination • There are 3 factors which cause the most evolutionary change by altering allele frequencies: • Natural selection • Genetic drift • Gene flow

23. 1. Natural Selection • As you know, when organisms are more fit to survive, they are more likely to pass on the traits that make them better suited for survival and this often changes the allele frequency within a population.

24. 2. Genetic Drift • Genetic drift is an unexpected fluctuation in allele frequency from one generation to the next. This is often due to a chance event where a large proportion of the population is wiped out.

25. 2. Genetic Drift • There are two situations which increase the likelihood of genetic drift that have a large impact on a population: • A. The bottle neck effect • B. The founder effect

26. A. The Bottle Neck Effect • A sudden change in the environment which drastically changes a population can have a profound impact on the genetic makeup of the population. • It may change the population in such a way that the survivors no longer represent the original population. • The survivors are said to have gone through a “bottleneck.”

27. B. The Founder Effect • When a few organisms become isolated from a large population and establish a new population whose gene pool is not reflective of the new population, we say the “founder effect” has occurred. • These founders pass through an isolation bottleneck and represent a gene pool with altered allele frequencies.

28. 3. Gene Flow • Gene flow occurs when populations gain or lose alleles as organisms come and go within a population. Gene flow tends to reduce differences between populations.

29. Variation • Variations are heritable differences within a population and comprise the raw material for diversity and natural selection. • Only the genetic component of variation can have evolutionary consequences as a result of natural selection.

30. Variation • Variation within a population comes from either discrete characters or quantitative characters: • Discrete-an either or basis determined from a single locus. • Quantitative-comes from 2 or more loci that determine the phenotype.

31. Fitness • The adaptive advantage of an organism which allows it to make a genetic contribution to the gene pool of the next generation.

32. An Animation • This animation illustrates how changes affect allele frequencies within a population. Movie

33. Modes of Selection • Natural selection alters the frequency distribution of heritable traits in three ways: • 1. Directional selection. • 2. Disruptive selection. • 3. Stabilizing selection.

34. Directional Selection • This is most common when a population’s environment changes or when members of a population migrate to a new habitat with different environmental conditions.

35. Disruptive Selection • This occurs when conditions favor individuals in both extremes over those of normal average phenotypes. It can be important in the early stages of speciation.

36. Stabilizing Selection • Acts against extreme phenotypes, it favors the intermediates. It reduces variation and maintains the status quo of a given phenotype.

37. Selection, In General • Regardless of the mode of selection, selection works to favor certain heritable traits through differential success. • Disruptive and stabilizing selection tend to reduce variation, but there are methods nature uses to preserve it.

38. Methods Nature Uses: • 1. Diploidy • 2. Balancing Selection • A. Heterozygous advantage • B. Frequency dependent selection • 3. Neutral Variation

39. Diploidy • Many eukaryotes are diploid and this hides a lot of variation from selection. • Recessiveness can be transferred from generation to generation even if they are harmful because they only cause harm when inherited from both parents when the zygote is formed.

40. Balancing Selection • Occurs when natural selection maintains stable frequencies of two or more phenotypic forms in a population. • Heterozygous advantage--acts in a way that is favored by natural selection over either homozygous form. • Frequency dependent selection--the fitness of any one morph declines if it becomes too common in a population.

41. Heterozygous Advantage

42. Neutral Variation • Some of the genetic variation has little or no effect on reproductive success. Much of the difference we see is found in untranslated parts of the genome. • Confers no advantage--are called pseudogenes. • Genetic drift can increase or decrease the frequency of pseudogenes. Difficult to measure--very debatable.

43. Sexual Selection • Sexual selection is natural selection for mating success. It can result in sexual dimorphism--differences between the sexes in secondary characteristics. • There are two types of sexual selection: • Intrasexual selection. • Intersexual selection. • Males are usually the showier sex.

45. Intrasexual Selection • In this, we have direct competition of one sex for mates of the opposite sex. A male often patrols a group of females and prevents other males from mating with her. He is often the psychological winner via a ritual that discourages competitors. This prevents harm to him and increases his own fitness.

46. Intersexual Selection • Individuals of one sex are choosy in selecting mates from the other sex. In most cases, a female’s choice depends on the showiness of a male. • Example: Peacocks display sexual dimorphism and both inter- and intra- sexual selection.

48. An Interesting Aside • Regarding showiness, the most intriguing thing is that it is often a hindrance to their survival. The benefits, however, seem to outweigh the costs. When a female chooses a showier male, she is often choosing the healthiest mate with the best genes. • This allows the male to pass his genes on to his offspring.

49. Natural Selection • It doesn’t fashion perfect organisms: • Evolution is limited by historical constraints. • Adaptations are often compromises. • Change and natural selection interact. • Selection can edit only existing variation.

50. 1. Evolution is Limited By Historical Constraints. • Each species comes from a long line of ancestral forms. • Ancestral anatomy isn’t scrapped by a new form, it’s a slow change. • This helps to explain why you don’t see an example of every species that has ever lived preserved in the fossil record.