How Populations Evolve Chapter 15 What s coming up Populations Genes and Evolution Causes of Evolution How Natural Selection Works Evolutionary changes occur from generation to generation ancestors causing descendants to differ from their Evolution 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 influence the development of its physical and behavioral traits its phenotype of an organism s chromosomes its genotype interact with the environment to Interaction between genotype and phenotype illustrated by coat color in hamsters Two 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 occurrence of all the alleles for that gene is called its allele frequency the proportion of times a certain allele occurs in a population relative to the 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 2 Evolution is the change in allele frequencies within a population Some might define evolution on the basis of changes in the outward appearance or behaviors members of a population of the A population geneticist however defines evolution as a gene pool over time the changes in allele frequencies that occur in 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 H Hardy and Wilhelm Weinberg is a mathematical model proposed independently in 1908 by Godfrey 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 generation An 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 of evolutionary change 1 Mutation 2 Gene flow 3 Small population size 4 Non random mating 5 Natural selection Mutations are the original source of genetic variability These 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 15 4 sperm and seeds from flowering plants can move and distribute alleles Fig Gene flow increases the genetic similarity of different populations of a species Mixing alleles prevents large differences in genetic compositions of populations If 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 populations Occurs 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 or overhunting Can rapidly change allele frequencies and reduce genetic variation A 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 7 The founder effect a new isolated population occurs when a small number of individuals leave a large population and establish 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 population But it will change the distribution of genotypes and therefore of phenotypes in a population Individuals may have preferences for their choice of mates If they prefer mates that are similar it is Snow geese favor mates of the same color known as assortative mating Many organisms have limited mobility remain near their place of birth hatching or germination Nonrandom mating may lead to inbreeding with two
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