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Transcript
Chapter 16 Evolution of Population and Speciation 16-1 Genetic Equilibrium Population Genetics -The study of evolution from a genetic point of view. A population is the smallest unit in which evolution can occur. Mircroevolution- change in the collective genetic material of a population. Variations -Individuals in a population have variations. -ex. Size in field mouse—small to large Most Variations follow a Bell Curve -most organisms fit in the average form. What causes genes to vary? 1) Mutations 2) Recombination of genes 3) Random Pairing of Gametes Gene Pool -The total genetic information available in a population. (All the genes that are available). Allele Frequency -Divide the total number of a certain allele by the total number of alleles in the population. 1 Example: Flowers (w/incomplete dominance) RR = Red, rr = white, Rr = pink In a population, there are 2 red (RR), 1 white (rr), and 2 pink (Rr) Frequency of R = 6/10 = .60 Frequency of r = 4/10 = .40 Phenotype Frequency -The number of individuals with a certain phenotype, out of the total number of individuals. Ex. 2 red, 1 white, 2 pink Red = .4 (2/5) White = .2 (1/5) Pink = .4 (2/5) To predict the phenotypes of the next generation, multiply the allele frequency. Ex. RR = .6 x .6 = .36 rr = .4 x .4 = .16 Rr = 1 -.36 - .16 = .48 For you people that enjoy equations, The equation to determine allele frequencies: Remember that p + q = 1 p2 + 2pq + q2 = 1 (RR + 2Rr + rr = 1) Example: Tongue Rolling Can roll= dominant (T), Cannot roll = recessive (t) In a class of 32 students, 28 students can roll their tongue, and 4 students cannot roll their tongue. Using the equation p2 + 2pq + q2 = 1, T =p, t =q. q2 (tt) = 4/32, = .125 q = √.125 = .35 because, p + q = 1, p + .35 =1, p = 1-.35, p = .65 Fill in the equation p2(.65 x.65)+2pq(2[.65x.35])+q2(.35x.35)=1 p2(.42) +2pq (.46) + q2(.12) = 1 Hardy-Weinberg Genetic Equilibrium 2 For genetic equilibrium 1) no mutations occur 2) individuals do not enter or leave a population 3) population is large 4) random mating 5) selection does not occur 16-2 Disrupting Equilibrium Mutations -spontaneous mutations occur at relatively low rates -exposure to mutagens can increase rates of mutations Migration -Immigration- individuals move in. -Emigration- individuals move out Ex. Male lions that take over the pride. Ensures Gene Flow. Gene Flow -Genes moving from one population to another -gene flow increases variation Genetic Drift -allele frequencies in a population change -small populations are affected more by gene drift than large populations. Page 305 Ex. Elephant seals and Cheetahs- both species are homozygous for almost all traits Nonrandom Mating -Mate selection is influenced by: -geographic proximity- can result in kinship -similar physical traits (assortative mating) the mates would probably have similar genes because they have similar physical traits. Sexual Selection -Females choose mates based on certain traits that indicate reproductive fitness Ex. Birds- colors, dances, nest building ability Elk/goats- strength of males Stabilizing Selection The average form of the trait has the highest fitness. Ex. Geckos in a forest of medium green leaves- the medium green gecko has the advantage 3 Directional Selection -A more extreme form of the trait is favored. Ex. Flamingoes with longer legs and necks. Disruptive Selection -The extreme forms are favored Ex. Limpets in light and dark colors have an advantage over medium colored limpets. 16-3 Formation of Species 4 Speciation -species formation -forms many related populations of organisms Morphological Species Concept -using internal and external appearance and structures -easy to observe - phenotype variations can make identification difficult Biological Species Concept Ernst Mayr (1904-2005) -Species- a group of organisms that can successfully interbreed, but not breed with other groups. -does not work for extinct species -does not work for asexual reproduction -The modern definition of a species includes both the morphological and biological species concept. -A species is a single type of organism. Members of a species are morphologically similar and can interbreed to produce fertile offspring. Speciation began with isolation Geographic isolation - a physical separation of a population - canyons, rivers, islands, separated ponds... Ex. Squirrels in the Grand Canyon Allopatric speciation Species arise as a result of geographic isolation Reproductive Isolation -Barriers to successful breeding A) Prezygotic -before fertilization -different breeding times, different courtship behaviors -will not waste gametes B) Postzygotic Behavior 5 -after fertilization -offspring die or are sterile -donkeys and horses mules -wastes gametes Sympatric speciation Two subpopulations become reproductively isolated within the same geographic area. They use slightly different niches. Rates of Speciation a) Gradual- slowly evolving b) Punctuated Equilibrium- sudden changes, then long periods without change -may be a combination of both. 6