Natural Selection Lecture PDF

Evolution of Populations Chap. 23  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 –...

Evolution of Populations Chap. 23  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  Natural Selection Differential success in reproduction results in certain alleles being passed to the next generation in greater proportions  Genetic Drift The smaller a sample, the greater the chance of 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  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  The Bottleneck Effect The bottleneck effect is a sudden reduction in population size due to a change in the environment 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  Effects of Genetic Drift: A Summary 1. Genetic drift is significant in small populations 2. Genetic drift causes 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  Gene Flow (Migration) 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 differences between populations over time Gene flow is more likely than mutation to alter allele frequencies directly Fig. 23-11  Mutation A random permeant change in DNA sequence. Mutations can have positive and negative effects on an organism. Most mutations are silent.  Modes of natural Selection Three modes of selection: Stabilizing selection favors intermediate variants and acts against extreme phenotypes Directional selection favors individuals at one end of the phenotypic range Disruptive selection favors individuals at both extremes of the phenotypic range Fig. 23-13 Original population Phenotypes (fur color) Original Evolved population population (a) Directional selection (b) Disruptive selection (c) Stabilizing selection  Because the environment can change, adaptive evolution is a continuous process Genetic drift and gene flow do not consistently lead to adaptive evolution as they can increase or decrease the match between an organism and its environment  Heterozygote Advantage Heterozygote advantage occurs when heterozygotes have a higher fitness than do both homozygotes Natural selection will tend to maintain two or more alleles at that locus The sickle-cell allele causes mutations in hemoglobin but also confers malaria resistance Fig. 23-17 Frequencies of the sickle-cell allele 0–2.5% 2.5–5.0% 5.0–7.5% Distribution of malaria caused by 7.5–10.0% Plasmodium falciparum 10.0–12.5% (a parasitic unicellular eukaryote) >12.5%  Neutral Variation Neutral variation is genetic variation that appears to confer no selective advantage or disadvantage For example, –Variation in noncoding regions of DNA –Variation in proteins that have little effect on protein function or reproductive fitness 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 Fig. 23-19 Origin of Species Chap. 24  Speciation, the origin of new species, is at the focal point of evolutionary theory Evolutionary theory must explain how new species originate and how populations evolve Microevolution consists of adaptations that evolve within a population, confined to one gene pool Macroevolution refers to evolutionary change above the species level  The Biological Species Concept The biological species concept states that a species is a group of populations whose members have the potential to interbreed in nature and produce viable, fertile offspring; they do not breed successfully with other populations Gene flow between populations holds the phenotype of a population together  Reproductive Isolation Reproductive isolation is the existence of biological factors (barriers) that impede two species from producing viable, fertile offspring Hybrids are the offspring of crosses between different species Reproductive isolation can be classified by whether factors act before or after fertilization  Prezygotic barriers block fertilization from occurring by: –Impeding different species from attempting to mate –Preventing the successful completion of mating –Hindering fertilization if mating is successful  Habitat isolation: Two species encounter each other rarely, or not at all, because they occupy different habitats, even though not isolated by physical barriers Water-dwelling Thamnophis Terrestrial Thamnophis  Temporal isolation: Species that breed at different times of the day, different seasons, or different years cannot mix their gametes Eastern spotted skunk Western spotted skunk (Spilogale putorius) (Spilogale gracilis)  Behavioral isolation: Courtship rituals and other behaviors unique to a species are effective barriers playbutton playbutton playbutton  Mechanical isolation: Morphological differences can prevent successful mating  Gametic isolation: Sperm of one species may not be able to fertilize eggs of another species  Postzygotic barriers prevent the hybrid zygote from developing into a viable, fertile adult: –Reduced hybrid viability –Reduced hybrid fertility –Hybrid breakdown  Reduced hybrid viability: Genes of the different parent species may interact and impair the hybrid’s development  Reduced hybrid fertility: Even if hybrids are vigorous, they may be sterile  Hybrid breakdown: Some first-generation hybrids are fertile, but when they mate with another species or with either parent species, offspring of the next generation are feeble or sterile  Limitations of the Biological Species The biological species concept cannot be applied to fossils or asexual organisms (including all prokaryotes)  Other Definitions of Species Other species concepts emphasize the unity within a species rather than the separateness of different species The morphological species concept defines a species by structural features – It applies to sexual and asexual species but relies on subjective criteria  The ecological species concept views a species in terms of its ecological niche – It applies to sexual and asexual species and emphasizes the role of disruptive selection The phylogenetic species concept: defines a species as the smallest group of individuals on a phylogenetic tree – It applies to sexual and asexual species, but it can be difficult to determine the degree of difference required for separate species  Speciation can take place with or without geographic separation Speciation can occur in two ways: –Allopatric speciation –Sympatric speciation Fig. 24-5 (a) Allopatric speciation (b) Sympatric speciation  Allopatric (“Other Country”) Speciation In allopatric speciation, gene flow is interrupted or reduced when a population is divided into geographically isolated subpopulations  The Process of Allopatric Speciation The definition of barrier depends on the ability of a population to disperse Separate populations may evolve independently through mutation, natural selection, and genetic drift Fig. 24-6 A. harrisi A. leucurus  Evidence of Allopatric Speciation Regions with many geographic barriers typically have more species than do regions with fewer barriers  Sympatric (“Same Country”) Speciation In sympatric speciation, speciation takes place in geographically overlapping populations  Habitat Differentiation Sympatric speciation can also result from the appearance of new ecological niches For example, the North American maggot fly can live on native hawthorn trees as well as more recently introduced apple trees  Sexual Selection Sexual selection can drive sympatric speciation Sexual selection for mates of different colors has likely contributed to the speciation in cichlid fish in Lake Victoria  In sympatric speciation, a reproductive barrier isolates a subset of a population without geographic separation from the parent species Sympatric speciation can result from polyploidy, natural selection, or sexual selection Speciation can occur rapidly or slowly and can result from changes in few or many genes Many questions remain concerning how long it takes for new species to form, or how many genes need to differ between species Broad patterns in speciation can be studied using the fossil record, morphological data, or molecular data  Patterns in the Fossil Record The fossil record includes examples of species that appear suddenly, persist essentially unchanged for some time, and then apparently disappear Niles Eldredge and Stephen Jay Gould coined the term punctuated equilibrium to describe periods of apparent stasis punctuated by sudden change The punctuated equilibrium model contrasts with a model of gradual change in a species’ existence Fig. 24-17 (a) Punctuated pattern Time (b) Gradual pattern  Speciation Rates The punctuated pattern in the fossil record and evidence from lab studies suggests that speciation can be rapid The interval between speciation events can range from 4,000 years (some cichlids) to 40,000,000 years (some beetles), with an average of 6,500,000 years  From Speciation to Macroevolution Macroevolution is the cumulative effect of many speciation and extinction events  The Hardy-Weinberg equation can be used to test whether a population is evolving  The Hardy-Weinberg Principle The Hardy-Weinberg principle describes a population that is not evolving If a population does not meet the criteria of the Hardy-Weinberg principle, it can be concluded that the population is evolving  Hardy-Weinberg Equilibrium The Hardy-Weinberg principle states that frequencies of alleles and genotypes in a population remain constant from generation to generation In a given population where gametes contribute to the next generation randomly, allele frequencies will not change Mendelian inheritance preserves genetic variation in a population Fig. 23-6 Alleles in the population Frequencies of alleles Gametes produced p = frequency of Each egg: Each sperm: CR allele = 0.8 q = frequency of CW allele = 0.2 80% 20% 80% 20% chance chance chance chance  Hardy-Weinberg equilibrium describes the constant frequency of alleles in such a gene pool If p and q represent the relative frequencies of the only two possible alleles in a population at a particular locus, then – p2 + 2pq + q2 = 1 – where p2 and q2 represent the frequencies of the homozygous genotypes and 2pq represents the frequency of the heterozygous genotype Fig. 23-7-1 80% CR ( p = 0.8) 20% CW (q = 0.2) Sperm CR CW (80%) (20%) 64% ( p2) 16% ( pq) CRCR CRCW 16% (qp) 4% (q2) CRCW CW CW Fig. 23-7-4 80% CR ( p = 0.8) 20% CW (q = 0.2) Sperm CR CW (80%) (20%) 64% ( p2) 16% ( pq) CR CR CR CW 16% (qp) 4% (q2) CR CW CW CW 64% CR CR, 32% CR CW, and 4% CW CW Gametes of this generation: 64% CR + 16% CR = 80% CR = 0.8 = p 4% CW + 16% CW = 20% CW = 0.2 = q Genotypes in the next generation: 64% CR CR, 32% CR CW, and 4% CW CW plants  Conditions for Hardy-Weinberg Equilibrium The Hardy-Weinberg theorem describes a hypothetical population In real populations, allele and genotype frequencies do change over time  The five conditions for nonevolving populations are rarely met in nature: –No mutations –Random mating –No natural selection –Extremely large population size –No gene flow