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© 2014 Pearson Education, Inc. © 2014 Pearson Education, Inc. Ecology and Evolution Outcomes 1. Apply the Hardy-Weinberg principle to test for changes in allele frequency. 2. Apply logical deduction using the conditions of the H-W principle to determine the mechanism of evolutio n. 3. Distinguish between adaptive and non-adaptive mechanisms of evolution. © 2014 Pearson Education, Inc. Ecology and Evolution Why is evolution important to the discipline of ecology? © 2014 Pearson Education, Inc. Ecology and Evolution The oldfield mouse (Peromyscus polionotus) is widely distributed in the southeastern U.S. It is preyed upon by a variety of visually hunting predators such as hawks and owls. The mouse displays considerable variation in coat color both within and between populations across its range. © 2014 Pearson Education, Inc. Ecology and Evolution Figure 3.5 Coat color variation in mice. Two color variants of Peromyscus polionotus: (A) the darker inland form, and (B) the lighter beach-dwelling form. © 2014 Pearson Education, Inc. Ecology and Evolution Figure 3.6 Variation in coat color and genotypes at the Mc1R locus. Peromyscus polionotus exhibits extensive coat color variation across localities in Florida. Red areas indicate the distribution of beach populations; gray areas denote the distribution of inland populations. Characteristic phenotypes for each population are indicated by the coat coloration sketches, but coat color varies within populations as well. Adapted from Hoekstra et al. (2006). © 2014 Pearson Education, Inc. Ecology and Evolution Most populations of the mouse are dark colored, but populations on beaches and barrier islands have lighter colored coats. Does coat color affect the survival and ultimately reproduction (i.e. fitness) of oldfield mice? Two experiments suggest it does. © 2014 Pearson Education, Inc. Ecology and Evolution Kaufman (1974) carried out an experiment in which pairs of mice (one dark-coated, one light coated) along with an owl were placed in large cages located in habitats with different backgrounds (light or dark and with different vegetation densities). © 2014 Pearson Education, Inc. Ecology and Evolution In all cases mice that better matched the background survived better than mice that matched less well. © 2014 Pearson Education, Inc. Ecology and Evolution Kaufman et al. carried out a follow-up experiment in which they made silicone mouse models painted light or dark to mimic either the dark or light background. They placed the models in different habitats and measured from beak and claw marks how often the models were attacked. They found clear differences in attack rates. Models that matched their background were attacked much less. © 2014 Pearson Education, Inc. Ecology and Evolution Figure 3.10 Predation, coat color, and fitness in the oldfield mouse, using plastic models in the field. Hoekstra and colleagues placed light and dark silcone mouse models in light and dark environments to test predation rates. (A) The experimental sites: a light beach environment and a dark inland environment. (B) Proportion of attacks against light and dark mice in © 2014 Pearson Education, Inc. the light environment. (C) Proportion of attacks against light and dark mice in the dark environment. Adapted from Vignieri et al. © 2014 Pearson Education, Inc. Ecology and Evolution Observations made by Charles Darwin that were the building blocks of his theory on Evolution by Natural Selection 1) Individuals of a population are different from one another. Variation among individuals arises through: ►Mutation ►Sexual Reproduction (genetic recombination) 2) Many of these behavior, morphological or physiological differences, which we term traits, are inherited from the parents. 3) All plants and animals produce more offspring than are needed to simple replace the parents. 4) Among the different individuals in a population, some are better suited for survival than others. 5) Different ancestors leave different numbers of descendants. © 2014 Pearson Education, Inc. Ecology and Evolution Charles Darwin's Theory of Evolution by Natural Selection Natural Selection is the differential success in reproduction, and its product is adaptation of organisms to their environment. What this means: Organisms that possess the characteristics (traits) that are most helpful in a given environment are more likely to survive and increase their numbers. © 2014 Pearson Education, Inc. Why check for Hardy-Weinberg? © 2014 Pearson Education, Inc. Why check for Hardy-Weinberg? © 2014 Pearson Education, Inc. The Hardy-Weinberg equation can be used to test whether a population is evolving The Hardy-Weinberg Equilibrium Principle has two fundamental conclusions: 1. The allele frequencies in a population will not change, generation after generation. 2. If the allele frequencies in a population are given by p and q, then the genotype frequencies will be given by p2, 2pq q2. © 2014 Pearson Education, Inc. Activity - The Hardy-Weinberg Equilibrium Principle Consider a population of 500 flower plants. 320 CRCR 160 CWCW 20 CRCW © 2014 Pearson Education, Inc. Conditions for Hardy-Weinberg Equilibrium The Hardy-Weinberg theorem describes a hypothetical population that is not evolving. It is a model which has assumptions. 1. No mutations 2. Random mating 3. No natural selection 4. Extremely large population size 5. No gene flow If the population is not in H-W equilibrium, then one of the assumptions has been violated. © 2014 Pearson Education, Inc. Non-random mating alters genotype frequency, not allele frequency © 2014 Pearson Education, Inc. Non-random mating alters genotype frequency, not allele frequency Second assumption of H-W is violated: 2. If the allele frequencies in a population are given by p and q, then the genotype frequencies will be given by p2, 2pq q2. Allele frequencies of A1 = 0.5 ,A2 = 0.5 at the start and at generation 3. But! Genotype frequencies have changed. © 2014 Pearson Education, Inc. Natural selection, genetic drift, and gene flow can alter allele frequencies in a population Zimmer and Emlen 2013 © 2014 Pearson Education, Inc. Natural selection, genetic drift, and gene flow can alter allele frequencies in a population Three major factors alter allele frequencies and bring about most evolutionary change Natural selection Genetic drift Gene flow Why not mutation? Although mutation can change allele frequencies, even a beneficial mutation will not increase in frequency without a selection pressure. © 2014 Pearson Education, Inc. Violation of Hardy-Weinberg - Mutation Mutation is not a strong force for evolution. ∆32 is a 32 base pair deletion in the CCR5 gene. The gene encodes a cell surface protein called CCR-5. The 32 base pair deletion confers resistance to it carriers from HIV. But most carriers do not live in areas with high incidence of HIV. © 2014 Pearson Education, Inc. Violation of Hardy-Weinberg - Mutation But if an allele mutates from one form (A1) to another form (A2) at a constant rate and is not eliminated, it can change the allele frequency of a population – over a very long period of time. Bergstrom and Dugatkin © 2014 Pearson Education, Inc. 2012 Violation of Hardy-Weinberg - Genetic Drift The smaller a sample, the more likely it is that chance alone will cause deviation from a predicted result. Genetic drift describes how allele frequencies fluctuate unpredictably from one generation to the next. Genetic drift tends to reduce genetic variation through losses of alleles, especially in small populations. © 2014 Pearson Education, Inc. Violation of Hardy-Weinberg - Genetic Drift Ten simulations of random genetic drift using 3 different population sizes. The random loss of one allele in small populations leads to fixation of the other allele. Large populations do not exhibit dramatic effects of drift because it is unlikely that all of one allele would be lost due to chance. © 2014 Pearson Education, Inc. The Founder Effect The founder effect occurs when a few individuals become isolated from a larger population. Allele frequencies in the small founder population can be different from those in the larger parent population due to chance. Zimmer & Emlen 2013 © 2014 Pearson Education, Inc. Genetic Drift- The Bottleneck Effect The bottleneck effect can result from a drastic reduction in population size due to a sudden environmental change or other indiscriminate reduction in population size. By chance, the resulting gene pool may no longer be reflective of the original population’s gene pool. If the population remains small, it may be further affected by genetic drift. © 2014 Pearson Education, Inc. Case Study: Impact of Genetic Drift on the Greater Prairie Chicken Loss of prairie habitat caused a severe reduction in the population of greater prairie chickens in Illinois. The surviving birds had low levels of genetic variation, and only 50% of their eggs hatched. Prairie chicken video. http://www.youtube.com/watch?v=s2_wdMmEupQ © 2014 Pearson Education, Inc. Figure 21.11 Pre-bottleneck Post-bottleneck (Illinois, 1820) (Illinois, 1993) Greater prairie chicken Range of greater prairie chicken Population size Number of alleles per locus Percentage of eggs hatched 1,000–25,000 50 5.2 3.7 93 50 Kansas, 1998 (no bottleneck) 750,000 5.8 99 Nebraska, 1998 (no bottleneck) 75,000– 200,000 5.8 96 Location Illinois 1930–1960s 1993 http://www.youtube.com/watch?v=s2_wdMmEupQ#t=0 © 2014 Pearson Education, Inc. Effects of Genetic Drift: A Summary 1. Genetic drift is significant in small populations. 2. Genetic drift can cause allele frequencies to change at random. 3. Genetic drift can lead to a loss of genetic variation within populations. 4. Genetic drift can cause harmful alleles to become fixed. © 2014 Pearson Education, Inc. Violation of Hardy-Weinberg - Gene Flow Gene flow consists of the movement of alleles among populations. Alleles can be transferred through the movement of fertile individuals or gametes (for example, pollen). Gene flow tends to reduce genetic variation among populations over time Freeman & Herron 2007 © 2014 Pearson Education, Inc. Violation of Hardy-Weinberg - Gene Flow Gene flow can decrease the fitness of a population Consider, for example, the great tit (Parus major) on the Dutch 13% first time breeders 43% first time breeders island of Vlieland Immigration of birds from the mainland introduces alleles that decrease fitness in island populations. Natural selection reduces the frequency of these alleles in the eastern population where immigration from the mainland is low. In the central population, high immigration from the mainland overwhelms the effects of selection. © 2014 Pearson Education, Inc. Directional, Disruptive, and Stabilizing Selection There are three modes of natural selection Directional selection favors individuals at one end of the phenotypic range Disruptive selection favors individuals at both extremes of the phenotypic range Stabilizing selection favors intermediate variants and acts against extreme phenotypes © 2014 Pearson Education, Inc. Frequency of individuals Figure 21.13 Original population Original Evolved population population Phenotypes (fur color) (a) Directional selection (b) Disruptive selection © 2014 Pearson Education, Inc. (c) Stabilizing selection Directional Selection © 2014 Pearson Education, Inc. Disruptive Selection © 2014 Pearson Education, Inc. Stabilizing Selection © 2014 Pearson Education, Inc. Overview: The Smallest Unit of Evolution • Question: Do organisms evolve during their lifetimes? Answer: No • Question: What is the smallest unit of evolution? Answer: The population Natural selection acts on individuals, but only populations evolve. © 2014 Pearson Education, Inc. Natural selection is the only mechanism that consistently causes adaptive evolution • Evolution by natural selection involves both chance and “sorting” – New genetic variations arise by chance – Beneficial alleles are “sorted” and favored by natural selection • Only natural selection consistently results in adaptive evolution, an increase in the frequency of alleles that improve fitness © 2014 Pearson Education, Inc. Relative Fitness The phrases “struggle for existence” and “survival of the fittest” are misleading as they imply direct competition among individuals. Reproductive success is generally more subtle and depends on many factors. Relative fitness is the contribution an individual makes to the gene pool of the next generation, relative to the contributions of other individuals. Selection indirectly favors certain genotypes by acting directly on phenotypes. © 2014 Pearson Education, Inc. Natural Selection does not predict perfection There is a temptation to regard the match between organism and environment as an example of evolved perfection. But, there is nothing in the process of evolution by natural selection that implies perfection. Natural selection can only favor those that are fittest from among the range of genetic variants available. But, no population contains the full range of genetic variation that might exist, so the range of genetic variants might be limited. Organisms come to match their environments by being ‘the fittest available’ or ‘the fittest yet’; they are not ‘the best imaginable’. © 2014 Pearson Education, Inc. Proximate and Ultimate explanations A proximate explanation is the immediate cause of a phenomenon. It explains how a trait operates or is regulated. An ultimate explanation provides more general reasons, involving evolutionary arguments and adaptations. Ultimate explanations are remote in time and space. It explains why a trait exists. © 2014 Pearson Education, Inc. Why Natural Selection Cannot Fashion Perfect Organisms 1. Selection can act only on existing variations. 2. Evolution is limited by historical constraints. 3. Adaptations are often compromises. 4. Chance, natural selection, and the environment interact. Natural selection operates on a “better than” basis. © 2014 Pearson Education, Inc. Why Natural Selection Cannot Fashion Perfect Organisms – Historical Factors Organisms live where they do for reasons that are often, at least in part, accidents of history. Moreover, they have lived in environments that were quite different from the present: some of the features that they acquired in other environments now hang like millstones around their necks. These characters that they have acquired limit and constrain where they can now live and what they do. These characters can also constrain how these organisms respond to additional changes in their environment. © 2014 Pearson Education, Inc. Why Natural Selection Cannot Fashion Perfect Organisms – Historical Factors © 2014 Pearson Education, Inc. Activity - Hardy-Weinberg Peppered moth, Biston betularia (B. Kettlewell, 1959) © 2014 Pearson Education, Inc.