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Greater Prairie Chicken•Greater Prairie Chickens once lived throughout Canadian and US prairies •Loss of prairie habitat caused a severe reduction in the population of greater prairie chickens •Surviving birds had low levels of genetic variation, and only 50% of their eggs hatchedGenetic drift and loss of genetic variation•Greater Prairie Chicken wiped out in Canada •Some populations remain through US prairie states
Genetic drift and loss of genetic variation•Researchers used DNA from museum specimens comparing genetic variation in population before and after bottleneck•Results showed loss of alleles at several loci•Researchers introduced greater prairie chickens from populations in other states •Successful introducing new alleles and increasing egg hatch rate to 90%Effects of Genetic Drift: A Summary1.Genetic drift is significant in small populations 2.Genetic drift causes allele frequencies to change at random 3.Genetic drift can lead to a loss of genetic variation within populations 4.Genetic drift can cause harmful alleles to become fixedGene Flow•Gene flow consists of movement of alleles among populations •Alleles can be transferred through movement of fertile individuals or gametes (for example, pollen) •Gene flow tends to reduce variation among populations over timeGene flow and local adaptation•Gene flow can decrease the fitness of a population •Consider, Parus majorpopulations on Dutch island of Vlieland •Mating causes gene flow between central and eastern populations •Immigration from mainland introduces alleles that decrease fitness •Natural selection selects for alleles that increase fitnessGene flow and local adaptation•Birds in central region with high immigration have a lower fitness; birds in east with low immigration have a higher fitness Gene flow and local adaptation•Gene flow can increase fitness of a population•Consider spread of alleles for resistance to insecticides•Insecticides used to target mosquitoes carrying West Nile virus and malaria •Alleles evolved in some populations conferring insecticide resistance •Flow of insecticide resistance alleles into a population causes an increase in fitness
Gene flow•Gene flow important agent of evolutionary change in human populationsConcept 23.4: Natural selection is mechanism that consistently causes adaptive evolution•Evolution by natural selection involves both chance and “sorting” •New genetic variations arise by chance•Beneficial alleles are “sorted” and favoured by natural selection•Only natural selection consistently results in adaptive evolutionA Closer Look at Natural Selection•Natural selection brings about adaptive evolution by acting on an organism’s phenotype Relative Fitness•“Struggle for existence” and “survival of the fittest” misleading as they imply direct competition among individuals •Reproductive success is generally more subtle and depends on many factorsRelative Fitness•Relative fitness- contribution