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Transcript
Population Genetics • The science of genetic change in population. • Population – all the members of a species that occupy a particular area at the same time • Gene Pool – all the genes in all the members of a population Genes and Variation • Genetics Joins Evolutionary Theory – Variation is the raw material for natural selection – Gene pool – consists of all the genes, including all the different alleles for each gene, that are present in a population – Relative frequency – the number of times that the allele occurs in a gene pool, compared with the number of times other alleles for the same gene occur – Therefore – evolution is any change in the relative frequency of alleles in the gene pool of a population over time • 3 Sources of Genetic Variation – Mutations – Genetic Recombination in Sexual Reproduction (Ind assortment and crossing over) – Lateral Gene Transfer (conjugation) • Single-Gene (2 pheno) vs. Polygenic Traits (many pheno/bell curve) ***Natural Selection acts directly on PHENOTYPES not actual alleles*** some phenotypes are better suited to an environment than others and they will survive, reproduce and pass on their genes. Evolution as Genetic Change in Populations How Natural Selection Works – 3 Types Stabilizing Selection Individuals with the average form of a trait have the highest fitness Represents the optimum for most traits Results in a similar morphology between most members of the species Directional Selection Individuals that display a more extreme form of a trait have greater fitness than individuals with an average form of the trait A shift in one direction Peppered moth Disruptive Selection Individuals with either extreme variation of a trait have greater fitness than individuals with the average form of the trait A shift in both direction, away from the center Shell color (dark rocks and light sand) Genetic Drift • Random changes in the frequency of a gene in the absence of natural selection occurs because of CHANCE • Occurs efficiently in small populations because small changes affect more members • Two examples: a. Bottleneck effect b. Founder effect a. Bottleneck Effect • Genetic drift (reduction of alleles in a population) resulting from a disaster that drastically reduces population size. – Examples: 1. 2. Earthquakes Volcano’s b. Founder Effect • Genetic drift resulting from the colonization of a new location by a small number of individuals. • Results in random change of the gene pool. • Example: 1. Islands (first Darwin finch) Hardy-Weinberg Principle • Genetic Equilibrium – situation in which allele frequencies in the gene pool of a population remain constant • The concept that the shuffling of genes that occurs during sexual reproduction, by itself, cannot change the overall genetic makeup of a population. • Shows mathematically and theoretically that there are situations where evolution DOES NOT OCCUR – Seldom achieved in nature Hardy-Weinberg Principle • This principle will be maintained in nature only if ALL five of the following conditions are met: 1. Very large population 2. Isolation from other populations 3. No net mutations 4. Random mating 5. No natural selection Hardy-Weinberg Principle Species • A group of populations whose individuals have the potential to interbreed and produce viable offspring. Speciation • The evolution of new species. Species that occupy an otherwise unoccupied niche face no competition, they will therefore have a 100% success rate Reproductive Isolation • Any mechanism that impedes two species from producing fertile and/or viable hybrid offspring -factor necessary for the formation of a new species. • Barriers: 1. Geographic (rivers, mountains) 2. Behavorial - differences in courtship behavior 3. Temporal - fertile periods (time) Interpretations of Speciation • Two theories: 1. Gradualist Model (Neo-Darwinian): Slow changes in species overtime. 2. Punctuated Equilibrium: Evolution occurs in spurts of relatively rapid change. Macroevolution • The origin of taxonomic groups higher than the species level. Adaptive Radiation aka Divergent Evolution • Emergence of numerous species from a common ancestor introduced to new and diverse environments. • Example: Darwin’s Finches Darwin’s Finches an example of Adaptive Radiation Convergent Evolution • Species from different evolutionary branches may come to resemble one another if they live in very similar environments. • Example: 1. Ostrich (Africa) and Emu (Australia). 2. Sidewinder (Mojave Desert) and Horned Viper (Middle East Desert) 3. Shark and Dolphin Coevolution • Evolutionary change, in which one species act as a selective force on a second species, inducing adaptations that in turn act as selective force on the first species. • Example: 1. Acacia ants and acacia trees 2. Humming birds and plants with flowers with long tubes