How Populations Evolve Chapter 15What’s coming up:Populations, Genes, and Evolution Causes of Evolution How Natural Selection WorksEvolutionary changes occur _ from generation to generation , causing descendants to differ from their ancestorsEvolution is a _ property of populations (not individuals) All cells contain DNA A _ gene is a segment of DNA found at a specific place on a chromosome Here, there can be different sequences of nucleotides called alleles The _ specific alleles of an organism’s chromosomes (its genotype) interact with the environment to influence the development of its _physical and behavioral traits (its phenotype) Interaction between genotype and phenotype – illustrated by coat color in hamstersTwo alleles determine coat color in hamsters The dominant allele (B)- encodes for an enzyme that catalyzes black pigment formation The recessive allele (b)- encodes for an enzyme that catalyzes brown pigment So what is the genotype of a brown hamster?A black hamster?Alleles, Genotype, and Phenotype in Individuals – Fig. 15.1 A gene pool consists of _ all the alleles of all the genes in all individuals of a population _ - Population genetics deals with the frequency, distribution, and inheritance of alleles in populations -For a given _ gene , the proportion of times a _ certain allele occurs in a population relative to the occurrence of all the alleles for that gene is called its _ allele frequency _ A population of 25 hamsters contains 50 alleles of the coat color gene. Why?If 20 of those 50 alleles code for black coats, then the frequency of the black allele is 0.40, or 40% (20/50 = 0.40) A Gene Pool – Fig. 15.2Evolution _ is the change in allele frequencies within a population Some might define evolution on the basis of changes in the _ outward appearance or behaviors of the members of a population A population geneticist, however, defines evolution as _ the changes in allele frequencies that occur in a gene pool over time If _ allele frequencies change from one generation to the next , the population is evolving If allele frequencies do not change from generation to generation, the population is not evolving The equilibrium population is a hypothetical population in which _ evolution does not occur _ The _ Hardy-Weinberg principle is a mathematical model proposed independently in 1908 by Godfrey H. Hardy and Wilhelm Weinberg.The Hardy-Weinberg principle demonstrates that, under certain conditions, the _ frequencies of alleles and genotypes in a sexually reproducing population remain constant from one generation to the next An equilibrium population - allele frequencies do not change from generation to generationAn equilibrium can be maintained so long as five conditions are satisfied:1. _ No mutations must occur in the population 2. There must be _ no gene flow _ between populations 3. The population must be very _ large _ 4. Must be _ random mating 5. There can be no _ natural selection _ If any of these five conditions are violated, there may be changes in allele frequencies, and the population will evolve What Causes Evolution? From the conditions that disturb a Hardy-Weinberg equilibrium, we might predict five major causes ofevolutionary change:1. _ Mutation _ 2. _ Gene flow_ 3. _Small population size_ 4. _Non-random mating_ 5. _Natural selection_ Mutations are the original source of genetic variabilityThese are rare changes in the _base sequence of DNA in a gene They usually have little immediate effect and can be passed to offspring _only if they occur in cells that give rise to gametes_They are the source of new alleles and can be beneficial, harmful, or neutral_Mutations _arise spontaneously Mutations Occur Spontaneously Fig. 15.3_Gene flow between populations changes allele frequencies This is the movement of _ alleles from one population to another and can be caused by:-_ Movement of individuals between populations is a common cause of gene flow -Movement of _ pollen (sperm) and _seeds from flowering plants can move and distribute alleles. Fig. 15.4 Gene flow _increases the genetic similarity of different populations of a speciesMixing alleles _ prevents large differences in genetic compositions of populationsIf gene flow between populations is blocked, genetic differences may grow so large that one of the populations becomes a new species!In small populations, _ allele frequencies may drift _ _ Genetic drift - the random _ change in allele frequencies over time brought about by chance alone Has _ little effect in very large populationsOccurs more rapidly and has a bigger effect on small populations, where chance may cause the _ alleles of only a few individuals to be passed on Genetic Drift - –Fig. 15.5- Notice the allele frequencies based on the small number of reproducing individuals In very small populations, drift can result in the complete loss of an allele in only a few generations, even if it is the more frequent one The Effect of Population Size on Genetic Drift. Fig. 15.6 There are two causes of genetic drift 1. _ Population bottleneck _ 2. _ Founder effect _ Population bottleneck-a drastic reduction in population size brought about by a natural catastrophe oroverhunting -Can rapidly change allele frequencies and reduce genetic variationA bottleneck has been documented in the northern elephant seal -Hunted almost to extinction in the 1800s, the elephant seals had been reduced to only 20 individuals -A hunting ban allowed the population to increase to 30,000 -Present-day northern elephant seals are almost _ genetically identical If the environmental changes in a negative way for one seal, what does this mean for the others?? They are at risk of extinction despite their numbers.Population Bottlenecks Reduce Variation- Fig. 15.7The _ founder effect occurs when a small number of individuals leave a large population and establish a new isolated population By chance, the allele frequencies of founders may differ from those of the original population _ Over time, the new population may exhibit _ allele frequencies that differ from the original population_ A Human Example of the Founder Effect- Fig. 15.8 Mating within a population _ is almost never random _ Nonrandom mating alone will _ not change the overall frequency of alleles in a
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