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Transcript
21 Evidence and Mechanisms of Evolution Evolution Evolutionary theory is the understanding and application of the mechanisms of evolutionary change to biological problems. • Random mutations lead to traits that make individuals more adapted to dealing with these biological problems • Gradual, random Evolutionary theory has many useful applications: Development of vaccines; understanding and treating diseases Developing better agricultural crops and industrial processes Understanding the diversification of life and how species interact Using the term “theory” • In everyday speech, “theory” means an untested hypothesis, or a guess. • In science, “theory” refers to the entire body of work on the understanding and application of a field of knowledge. • Large amount of data, supportive evidence Darwin • The young Charles Darwin was passionately interested in geology and natural science. • In 1831 • • • 5 year voyage on the HMS Beagle Often went ashore to collect rocks and specimens, make observations In the Galápagos Islands he observed that species were similar to, but not the same as, species on the mainland of South America. He also realized that species varied from island to island. • This variation due to isolation from mainland and changes that occurred • He wondered what the mechanism for this change was – transmission of genetics was not yet known Darwin These observations, and many others, led Darwin to propose an explanatory theory for evolutionary change based on three propositions: Species change over time Divergent species share a common ancestor The mechanism that produces the change is natural selection Darwin 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. Darwin Darwin observed that, though offspring tended to resemble their parents, they are not identical. He suggested that slight variations among individuals affect the chances of surviving and producing offspring Natural selection. • Humans have been artificially selecting for thousands of years • Breeders • Domestication of animals and crops • Individuals do not evolve; populations do. • Population: A group of individuals of the same species that live and interbreed in a particular geographic area. • A population evolves when individuals with different genotypes survive or reproduce at different rates. Population Genetics Population genetics has three goals: Explain the patterns and organization of genetic variation Different forms of a gene are known as alleles A single individual only has some of the alleles found in a population Explain the origin and maintenance of genetic variation “Gene Pool” - all the alleles of the population How they come together in one individual causes variation Understand mechanisms that cause changes in allele frequencies Natural Selection Artificial selection for different characters in a single species of wild mustard produced many crop plants. Allele Frequencies Locally interbreeding groups are called Mendelian populations. Allele frequencies, or their proportion in the gene pool, are estimated by counting alleles in a sample of individuals. Allele Frequencies Allele frequency: number of copies of the allele in the population p sum of alleles in the population 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. Figure 21.6 Calculating Allele Frequencies Allele Frequencies 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 (not changing) Genotype frequencies may not be the same as allele frequencies. Genotype frequencies deal with how many individuals have certain genotype Frequencies of different alleles at each locus and frequencies of genotypes in a Mendelian population make up the genetic structure of the population. How genetic structure of a population changes over time is a measure of evolutionary change. Hardy-Weinberg • It would be thought that 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. • If your data varies from Hardy-Weinberg, then evolutionary mechanisms are at play with your population • Genotype frequencies can be predicted from allele frequencies. Hardy-Weinberg Conditions that must be met for Hardy–Weinberg equilibrium: Mating is random Population size is infinite Large populations aren’t affected by genetic drift (random fluctuation in allele frequency) No gene flow—no migration into or out of the population No mutation Natural selection does not affect survival of any genotypes Hardy-Weinberg 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 Example showing the Hardy-Weinberg to be true Hardy-Weinberg For generation 1, probability of two A alleles coming together is: p p p (0.55) 0.3025 2 2 Probability of two a alleles: q q q (0.45) 0.2025 2 2 Hardy-Weinberg There are two ways of producing a heterozygote: p q or q p , or 2 pq The Hardy-Weinberg equation: p 2 pq q 1 2 2 in butterfly example: .3025 + 2(.45)(.55) + .2025 = 1 Hardy-Weinberg Populations in nature never fit the conditions for Hardy-Weinberg equilibrium. However, it is useful for predicting genotype frequencies from allele frequencies. Also, because the model describes conditions that would result in no evolution, patterns of deviation from the model help identify mechanisms of evolution. Hardy-Weinberg Hardy-Weinberg equilibrium is a null hypothesis that assumes evolutionary forces are absent. Deviations from Hardy-Weinberg show that evolution is occurring Null hypothesis is opposite of your hypothesis (alternative hypothesis,what you would expect) Do experimentation that results in either retaining null OR rejecting it in favor of your alternative hypothesis Known evolutionary mechanisms: Mutation Gene flow Genetic drift Nonrandom mating Natural selection Evolutionary Mechanisms Mutation • Mutation is the origin of genetic variation. • Random • Most mutations are harmful or neutral • Few are beneficial • Mutation rates are low • Other mechanisms for evolution are at play if large deviations from Hardy-Weinberg are seen Evolutionary Mechanisms Gene Flow • Gene flow is a result of migration • Moving to new locations Evolutionary Mechanisms Genetic Drift • Happens in small populations • Population bottleneck • Loss of genetic variation • Genetic drift results from random changes in allele frequencies. • Harmful alleles may increase in frequency, and rare advantageous alleles may be lost. Evolutionary Mechanisms Nonrandom mating • Nonrandom mating occurs when individuals choose mates with particular phenotypes. • If individuals choose the same genotype as themselves, homozygote frequencies will increase. • Sexual selection Evolutionary Mechanisms Natural Selection 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. Individual fitness is based on survival (in that particular environment) to reproductive success Maintaining Genetic Variation • Neutral allele • Mutation that does not affect fitness • These will accumulate in population • Molecular techniques allow neutral alleles to be identified and used to study divergence of populations and species. • Sexual reproduction • Recombining of existing genes Maintaining Genetic Variation The variety of genetic combinations possible in sexually reproducing species may be especially valuable in defense against pathogens and parasites 21.4 How Is Genetic Variation Maintained within Populations? Subpopulations in different geographic regions maintain genetic variation. The subpopulations may be subjected to different environmental conditions and selective pressures. Constraints Evolution is constrained in many ways. • Lack of genetic variation can prevent evolution of potentially favorable traits • If the allele for a given trait does not exist in a population, that trait cannot evolve, even if it would be favored by natural selection • Evolution must work within the boundaries of universal constraints such as: Cell size, constrained by surface area-to-volume ratios Protein folding, constrained by the types of bonding that can occur Laws of thermodynamics that constrain energy transfers Constraints Adaptations involve both fitness costs and benefits. Benefit must outweigh cost if an adaptation is to evolve; the trade-off must be worthwhile.