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Lecture

BMEN 515 Lecture Notes - Allele Frequency, Genotype Frequency, Population Genetics


Department
Biomedical Engineering
Course Code
BMEN 515
Professor
William Huddleston

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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 conditions exist
If an allele is not advantageous, its frequency remains constant from generation to generation.
The Hardy-Weinberg equilibrium describes a model situation in which allele frequencies do not change across
generations and genotype frequencies can be predicted from allele frequencies (must apply to sexually reproducing
organisms). Several conditions must exist for a population to be at Hardy-Weinberg equilibrium:
o Mating is random.
o Population size is infinite.
o There is no gene flow.
o There is no mutation.
o Natural selection does not affect the survival of particular genotypes.
The Hardy-Weinberg equation: p2 + 2pq + q2 = 1.
Deviations from Hardy-Weinberg equilibrium show that evolution is occurring
The patterns of deviation from Hardy-Weinberg equilibrium can help us identify specific mechanisms of evolutionary
change.
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