1. Chapter 23: The Evolution of Populations

2. MAIN IDEAS OF CHAPTER 23  WHAT IS POPULATION GENETICS  CAUSES OF MICROEVOLUTION  HOW GENETIC VARIATION IN A POPULATION ARISES  THE IMPACT OF SELECTIVE FORCES ON POPULATIONS

3. Population genetics Population: a localized group of individuals belonging to the same species Species: a group of populations whose individuals have the potential to interbreed and produce fertile offspring Gene pool: the total aggregate of genes in a population at any one time Population genetics: the study of genetic changes in populations “Individuals are selected, but populations evolve.”

4. Microevolution: Evolution on the small scale A change in the allele frequencies in a population. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 23.1

5.  Increasing proportion of heavy-metal tolerant plants over successive generations  This adaptation is not result of plants acquiring tolerant genes.  Inherent variation/reproductive success Microevolution example the bent grass (Argrostis tenuis).

6. Acceptance of natural selection as the main mechanism of evolution more difficult to accept. HHow does chance variations arise in a population? LLack of understanding of the precise transmission of these variations from parents to offspring. AAlthough Gregor Mendel and Charles Darwin were contemporaries, Mendel’s discoveries were unappreciated at the time. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

7. When Mendel’s research was rediscovered in the early twentieth century, many geneticists believed that the laws of inheritance conflicted with Darwin’s theory of natural selection.  Darwin emphasized quantitative characters, those that vary along a continuum. These characters are influenced by multiple loci.  Mendel and later geneticists investigated discrete “either-or” traits. The modern evolutionary synthesis Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

8. A comprehensive theory of evolution, the modern synthesis, took form in the early 1940s. integrated Darwinian selection and Mendelian inheritance IIt integrates discoveries and ideas from Paleontology Taxonomy Biogeography Population genetics.

9. The modern synthesis emphasizes: (1) populations as the units of evolution. (2) the central role of natural selection as the most important mechanism of evolution. (3) gradualism explains how large changes can evolve as an accumulation of small changes over long periods of time. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

11. Hardy-Weinberg Theorem Serves as a model for the genetic structure of a nonevolving population (equilibrium) 5 conditions: 1- Very large population size; 2- No migration; 3- No net mutations; 4- Random mating; 5- No natural selection

12. Hardy-Weinberg Equation p = frequency of one allele (A); q = frequency of the other allele (a); p+q = 1.0 (p=1-q & q=1-p) P2=frequency of AA genotype; 2pq=frequency of Aa plus aA genotype; q2=frequency of aa genotype; p2 + 2pq + q2 = 1.0

13. Microevolution A change in the gene pool of a population over a succession of generations

14. How do changes in the gene pool occur?  1. genetic drift The Bottleneck Effect The Founder effect  2. gene flow  3. mutation  4. natural selection

15. 1. Genetic drift: Changes in the gene pool of a small population due to chance. (usually reduces genetic variability)

17. Genetic Drift The Bottleneck Effect: genetic drift resulting from a reduction of a population (natural disaster). such that the surviving population is no longer genetically representative of the original population

18. Bottlenecking is an important concept in conservation biology of endangered species.  Populations that have suffered bottleneck incidents have lost at least some alleles from the gene pool.  This reduces individual variation and adaptability.  For example, the genetic variation in the three small surviving wild populations of cheetahs is very low when compared to other mammals. Their genetic uniformity is similar to highly inbred lab mice! Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 23.5x

19. Understanding the bottleneck effect  Can increase understanding of how human activity affects other species Figure 23.8 B (b) Similarly, bottlenecking a population of organisms tends to reduce genetic variation, as in these northern elephant seals in California that were once hunted nearly to extinction.

20. Genetic Drift Founder effect  the new population could have a much different genetic ratio than the original one.

21. Founder Effect: genetic drift attributable to colonization by a limited number of individuals from a parent population

22. Microevolution: 2. Gene Flow G enetic exchange due to the migration of fertile individuals or gametes between populations. (reduces differences between populations) Pollen carried on the wind

23. Microevolution: 3. Mutations a change in an organism’s DNA. Mutation in gamete can immediately change gene pool. original source of genetic variation (raw material for natural selection)

24. Microevolution: 4- Nonrandom mating: inbreeding and assortive mating (both shift frequencies of different genotypes)

25. Microevolution: 4- Natural Selection Differential success in reproduction. The only form of microevolution that adapts a population to its environment

26. Genetic Variation within and between populations: Polymorphism: coexistence of 2 or more distinct forms of individuals (morphs) within the same population Geographical variation: differences in genetic structure between populations (cline)

27. Variation preservation Prevention of natural selection’s reduction of variation Diploidy 2nd set of chromosomes hides variation in the heterozygote Balanced polymorphism 1- heterozygote advantage (hybrid vigor; i.e., malaria/sickle-cell anemia); 2- frequency dependent selection (survival & reproduction of any 1 morph declines if it becomes too common; i.e., parasite/host)

28. The effect of selection on a varying characteristic: Selection Trends. 3 types 1. Directional Selection :  favors variants at one extreme

29. 2. Diversifying Selection  favors variants of opposite extremes

30. 3. Stabilizing Selection  Acts against extreme phenotypes  Favors more common intermediate  Reduces variation & maintains status quo.

31. Sexual selection-may lead to pronounced differences between the sexes. Sexual dimorphism: secondary sex characteristic distinction A product of sexual selection Sexual selection: A) intrasexual selection (within a sex) Rams butting horns B) intersexual selection  Mate choice

32. 1. Evolution is limited by historical constraints.  Evolution does not scrap ancestral anatomy and build from scratch.  Evolution co-opts existing structures and adapts them to new situations. Natural selection cannot fashion perfect organisms Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

33. 2. Adaptations are often compromises.  Organisms are often faced with conflicting situations that prevent an organism from perfecting any one feature for a particular situation. For example, because the flippers of a seal must not only allow it to walk on land, but also swim efficiently, their design is a compromise between these environments. Similarly, human limbs are flexible and allow versatile movements, but at the cost of injuries, such as sprains, torn ligaments, and dislocations.  Better structural reinforcement would compromise agility. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

34. 3. Not all evolution is adaptive.  Chance affects the genetic structure of populations to a greater extent than was once believed. For example, founders of new populations may not necessarily be the best individuals, but rather those individuals that are carried into the open habitat by chance. 4. Selection can only edit existing variations.  Selection favors only the fittest variations from those phenotypes that are available.  New alleles do not arise on demand. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

35. Natural Selection maintains sexual reproduction

36. Sex is an evolutionary enigma. It is far inferior to asexual reproduction as measured by reproductive output.  If a population consisted of half sexual females and half asexual females, the asexual condition would increase. All offspring of asexual females would be reproductive daughters. Only half of the offspring of sexual females would be daughters; the other half would be necessary males. 3. Natural selection maintains sexual reproduction Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

37. Theoretically, sex has a “two-fold disadvantage.”  A female producing two offspring per generation would generate a population of eight females after four generations if reproducing asexually, but only one female if reproducing sexually. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 23.15

38. Sex must confer some selective advantage to compensate for the costs of diminished reproductive output.  Otherwise, a migration of asexual individuals or mutation permitting asexual reproduction would outcompete sexual individuals and the alleles favoring sex. In fact, most eukaryotes maintain sex, even in those species that can also reproduce asexually. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

39. The “textbook” explanation for the maintenance of sex is that the process of meiosis and fertilization generate genetic variation on which natural selection can act.  However, this hypothesis that sex is maintained in spite of its disadvantages because it produces future adaptation in a variable world is difficult to defend.  Natural selection acts on the present, favoring individuals here and now that best fit the current, local environment. A stronger hypothesis would present advantages to sex that place a value on genetic variation on a generation-to- generation time scale. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings