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David Sadava, David M. Hillis, H. Craig Heller, May R. Berenbaum La nuova biologia.blu Genetica, biologia molecolare ed evoluzione S Mechanisms of Evolution What Is the Relationship between Fact and Theory in Evolution? Evolution is the change in genetic composition of populations over time. Evolutionary change is observed in lab experiments, natural populations, and the fossil record. These genetic changes drive the origin and extinction of species and the diversification of life. Biologists have also accumulated evidence on how evolutionary changes occur. Evolutionary theory is the understanding of the mechanisms of evolutionary change. What Is the Relationship between Fact and Theory in Evolution? Even before Darwin, several biologists had suggested that species change over time. But no one had proposed a viable mechanism for evolution. The young Charles Darwin was passionately interested in geology and natural history. In 1831 he was recommended for a position on the HMS Beagle, for a five-year survey voyage around the world. What Is the Relationship between Fact and Theory in Evolution? Darwin published his book, The Origin of Species, in 1859. The book provided exhaustive evidence from many different fields to support evolution and natural selection. But the genetic basis of evolutionary change was not understood until after 1900. What Is the Relationship between Fact and Theory in Evolution? In the twentieth century, Gregor Mendel’s publications were rediscovered, the roles of chromosomes and mutations were discovered, and population genetics was established. A “modern synthesis” of genetics and evolution took place over the period 1936–1947. In 1953, James Watson and Francis Crick published their paper on the structure of DNA. What Is the Relationship between Fact and Theory in Evolution? Evolution is genetic change occurring in a population—a group of individuals of a single species that live and interbreed in a particular geographic area. Individuals do not evolve, populations do. The gene pool is the sum of all copies of all alleles at allcontains loci in a the population. It genetic variation that produces the phenotypic traits on which natural selection acts. How Does Natural Selection Result in Evolution? Much of evolution occurs through gradual changes in the relative frequencies of alleles in a population from one generation to the next. Allele frequencies (p) are estimated by counting alleles in a sample of individuals: How Does Natural Selection Result in Evolution? If a locus has two alleles, A and a, there could be three genotypes: AA, Aa, and aa. The population is polymorphic at that locus. If p is the frequency of allele A, and q is the frequency of allele a, p+q=1 q=1–p If there is only one allele at a locus, its frequency = 1. The population is monomorphic at that locus; the allele is said to be fixed. How Does Natural Selection Result in Evolution? If certain conditions are met, the genetic structure of a population does not change over time. The Hardy–Weinberg equilibrium describes a model situation in which allele frequencies do not change. Genotype frequencies can be predicted from allele frequencies. How Does Natural Selection Result in Evolution? Conditions that must be met for Hardy–Weinberg equilibrium: • No mutation • No selection among genotypes • No gene flow • Population size is infinite (no genetic drift) • Mating is random How Does Natural Selection Result in Evolution? If these conditions hold: • Allele frequencies remain constant • After one generation, genotype frequencies occur in these proportions: Genotype AA Aa aa Frequency p2 2pq q2 How Does Natural Selection Result in Evolution? There are two ways of producing a heterozygote: p q or q p, or 2pq The Hardy–Weinberg equation: p2 + 2pq + q2 = 1 One Generation of Random Mating Restores Hardy–Weinberg Equilibrium How Does Natural Selection Result in Evolution? Populations in nature never fit the conditions for Hardy–Weinberg equilibrium. But it is useful for predicting genotype frequencies from allele frequencies. Also, patterns of deviation from the model help identify mechanisms of evolutionary change. What Are the Mechanisms of Evolutionary Change? In addition to natural selection there are four other mechanisms of evolution: • Mutation • Gene flow • Genetic drift • Nonrandom mating What Are the Mechanisms of Evolutionary Change? Mutation is the origin of genetic variation. Mutation is any change in the nucleotide sequences of DNA. Mutations are random with respect to the needs of an organism; selection acting on the random variation results in adaptation. Most mutations are harmful or neutral; a few are beneficial. Mutations (even low rates) can also restore genetic variation that other processes have removed. What Are the Mechanisms of Evolutionary Change? Gene flow is a result of the migration of individuals and movement of gametes between populations. New individuals can add alleles to the gene pool or change allele frequencies. What Are the Mechanisms of Evolutionary Change? Genetic drift results from random changes in allele frequencies. Harmful alleles may increase in frequency, and rare advantageous alleles may be lost. In large populations, genetic drift can influence frequencies of alleles that do not affect survival and reproduction. What Are the Mechanisms of Evolutionary Change? In small populations, genetic drift can be significant. Population bottleneck: environmental conditions result in survival of only a few individuals. Genetic drift can reduce genetic variation in the population. What Are the Mechanisms of Evolutionary Change? Genetic drift also affects small populations that colonize a new region. The colonizing population is unlikely to have all the alleles present in the whole population. This is called a founder effect (equivalent to a population bottleneck). What Are the Mechanisms of Evolutionary Change? Non-random mating occurs when individuals choose mates with particular phenotypes. Example: self-fertilization is common, especially in plants. If individuals choose the same genotype as themselves, homozygote frequencies will increase. How Does Natural Selection Result in Evolution? Natural selection acts on the phenotype rather than directly on the genotype. The reproductive contribution of a phenotype to subsequent generations relative to other phenotypes is called its fitness. Only changes in the relative success of different phenotypes lead to change in allele frequencies. How Does Natural Selection Result in Evolution? Natural selection can act on traits with quantitative variation in three ways: • Stabilizing selection preserves average phenotype. • Directional selection favors individuals that vary in one direction. • Disruptive selection favors individuals that vary in both directions from the mean. What Are the Mechanisms of Evolutionary Change? Sexual selection is a form of non-random mating that favors traits that increase the chances of reproduction. Sexual selection may favor traits that enhance an individual’s chances of reproduction but reduce its chances of survival. How Is Genetic Variation Maintained within Populations? Many mutations do not affect the function of the resulting proteins. An allele that does not affect fitness is a neutral allele. They tend to accumulate in a population. Sexual reproduction results in new combinations of genes through the combination of gametes, crossing over, and independent assortment. Sexual recombination produces genetic variety that increases evolutionary potential. How Is Genetic Variation Maintained within Populations? Frequency-dependent selection: a polymorphism can be maintained when fitness depends on its frequency in the population. Example: A scale-eating fish in Lake Tanganyika. “Left-mouthed” and “right-mouthed” individuals are both favored; the host fish can be attacked from either side. How Is Genetic Variation Maintained within Populations? Much genetic variation is maintained in subpopulations of a species in different geographic regions. The subpopulations may be subjected to different environmental conditions and selective pressures. Example: Populations of white clover that produce cyanide are more likely to be killed by frost. In Europe, there is gradual change in phenotype—a clinal variation. How Is Genetic Variation Maintained within Populations? Environmental variation also helps to preserve genetic variation. Example: Colias butterflies live in an environment with temperature extremes. The population is polymorphic for an enzyme (PGI) that influences flight at different temperatures. Heterozygotes are favored because they can fly over a larger temperature range. What Are the Constraints on Evolution? Adaptation: trait that increases the chance that a given individual will survive and reproduce, increasing the frequency of the trait in the next generation. All evolutionary innovations are modifications of previously existing structures. Example: Two lines of bottom-dwelling fishes. • Skates and rays • Sole and flounder What Are Species? Biological species concept: groups of actually or potentially interbreeding populations that are reproductively isolated from other such groups. Species are the result of speciation: the divergence of biological lineages and emergence of reproductive isolation. Reproductive isolation, when groups can no longer exchange genes, is a key factor in the divergence of sexually reproducing organisms. What Is the Genetic Basis of Speciation? Not all evolutionary change results in new species. Speciation usually requires the evolution of reproductive isolation. How can one lineage ever split into two reproductively isolated species? What Barriers to Gene Flow Result in Speciation? Allopatric speciation occurs when populations are separated by a physical barrier. Thought to be the dominant mode of speciation in most groups. The environments in which the isolated populations live are different, and so the populations evolve differently. What Barriers to Gene Flow Result in Speciation? Sympatric speciation is speciation without physical isolation. This can occur with disruptive selection if certain genotypes have a preference for distinct microhabitats where mating takes place. Sympatric speciation most commonly occurs by polyploidy—duplication of whole sets of chromosomes. Chromosome duplication in a single species is autopolyploidy; combining of chromosomes from two species is allopolyploidy. What Happens when Newly Formed Species Come into Contact? Prezygotic isolating mechanisms prevent hybridization from occurring. • Mechanical • Temporal • Behavioral • Habitat • Gametic What Happens when Newly Formed Species Come into Contact? Postzygotic isolating mechanisms reduce the fitness of hybrid offspring. • Low hybrid zygote viability—zygotes fail to mature or have severe abnormalities. • Low hybrid adult viability—offspring have lower survival rates. • Hybrid infertility—offspring are infertile (e.g., mules). Adapted from Life: The Science of Biology, Tenth Edition, Sinauer Associates, Sunderland, MA, 2014 Inc. All rights reserved