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Transcript
Natural Selection In the evolutionary struggle for existence, entire organisms, not individual genes, either survive and reproduce or do not. Natural selection can operate only on the phenotypic variation among individuals. An organism's phenotype includes all the physical and behavioral characteristics produced by the interaction of genotype and environment. Evolution as Genetic Change A species is defined as a group of similarlooking organisms that breed with one another and produce fertile offspring in the natural environment. Evolutionary biologists study groups of organisms called populations. A population is a collection of individuals of the same species in a given area at the same time. Evolution as Genetic Change Because all members of a population can interbreed, they and their offspring share a common group of genes called a gene pool. Each gene pool contains a number of alleles for each inheritable trait. The number of times an allele occurs in a gene pool compared with the number of times other alleles for the same gene occur is called the relative frequency of the allele. Evolution as Genetic Change Sexual reproduction alone does not change the relative frequency of alleles in a population. It just reshuffles the already existing alleles. Evolution, therefore, can be viewed another way. That is, evolutionary change involves a change in the relative frequencies of alleles in the gene pool of a population. Fitness and Adaptation Fitness is defined as the success an organism has in passing on its genes to the next generation. An adaptation is any genetically controlled characteristic of an organism that increases its fitness. The Niche The combination of an organism's "profession" and the place that it lives is called its niche. No two species can occupy the same niche in the same location for a long period of time. The more efficient species will survive and drive the less efficient species to extinction. If two species occupy different niches their chance of survival is greatly increased. The Process of Speciation A common way in which new species form is when populations are separated. The separation of populations that do not interbreed is called reproductive isolation. When a species is divided into two isolated populations, natural selection can work differently on each group. The Process of Speciation Because their gene pools are separate, adaptations that appear in one group are not passed to the other group. With enough time, this may result in two distinct species.