Darwin developed the major features of an explanatory theory for evolutionary change based on two major propositions.docx

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Biomedical Engineering
BMEN 515
William Huddleston

Darwin developed the major features of an explanatory theory for evolutionary change based on two major propositions: o Species are not immutable; they change over time. o The process that produces these changes is natural selection.  Darwin also observed that, although offspring ten to resemble their parents, the offspring of most organisms are not identical to one another or to their parents.  He suggests that slight variations among individuals affect the chances that a given individual will survive and reproduce  natural selection: differential contribution of offspring to the next generation by various genetic types belonging to the same population.  Individuals do no evolve; populations do.  A population is a group of individuals of a single species that live and interbreed in a particular geographic area at the same time. Adaptation has two meanings  Refers both to the processes by which characteristic that appear to be useful to their bearers evolve and to the characteristics themselves.  With respect to characteristics, an adaptation is a phenotypic characteristic that has help an organism adjust to conditions in its environment. Population genetics provides an underpinning for Darwin’s Theory  We cannot directly observe the genetic composition of organisms; what we do see in nature are phenotypes, the physical expression of organism’ genes.  The features of a genotype are its characters (e.g. eye colour).  The specific form of a character (e.g. brown eyes) is a trait.  A heritable trait is a characteristic of an organism that is at least partly determined by its genes.  A population evolves when its individuals with different genotypes survive or reproduce at different rates.  Population genetics has three main goals: o To explain the origin and maintenance of genetic variation o To explain the patterns and organization of genetic variation o To understand the mechanism that cause changes in allele frequencies in populations  Different forms of a gene, alleles, may exist at a particular locus.  At any particular locus, a single individual has only some of the alleles found in the population to which it belongs.  The sum of all copies of all alleles at all loci found in a population constitutes its gene pool.  The gene pool contains the genetic variations that produce the phenotypic traits on which natural selection acts. Most populations are genetically variable  Nearly all populations have genetic variation for many characters. Evolutionary change can be measure by allele and genotype frequencies  Allele frequencies are usually estimated in locally interbreeding groups, Mendelian populations, within a geographic population of a species.  Allele frequency = p = (number of copies of the allele in the pop. / sum of alleles in the pop.)  If only two alleles for a given locus are found among the members of a diploid population, they may combine to form three different genotypes  polymorphic (more than one allele).  The frequencies of different alleles at each locus and the frequencies of different genotypes in a Mendelian population describe that population’s genetic structure. The genetic structure of a population does not change over time if certain cond
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