BMEN 515 Lecture Notes - Allele Frequency, Sexual Reproduction, Sexual Selection
SchoolUniversity of Calgary
Course CodeBMEN 515
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What Evolutionary Mechanisms Result in Adaptation?
Adaptation occurs when some individuals in a population contribute more offspring to the next generation that
others allele frequencies in the population change in a way that adapts individuals to the environment that influenced
such reproductive success (natural selection).
Natural selection acts on the phenotype.
The reproductive contribution of a phenotype to subsequent generations relative to the contributions of other
phenotypes is called its fitness.
Changes in absolute numbers of offspring are responsible for increases and decreases in the size of a population, but
only changes in the relative success of different phenotypes within a population lead to changes in allele frequencies
from one generation to another.
The fitness of a phenotype is determined by the average rates of survival and reproduction of individuals with that
Natural selection produces variable results
Natural selection can act on characters with quantitative variation in any one several different ways:
o Stabilizing selection—favouring average individuals
o Directional selection –favouring individuals that vary in one direction from the mean pop.
o Disruptive selection—favouring individuals that vary in opposite directions from the mean of the population.
Sexual selection influences reproductive success
Sexual selection is a special type of natural selection that acts on characteristics that determine the reproductive
Sexual selection may favour traits that enhance the bearer’s chances of reproduction, but reduce its chances of survival.
o Such costly traits reliably demonstrate the quality of their possessors as mates b/c they enable the choosing sex
(usually females) to distinguish between genuinely fit individuals and exaggerators.
Neutral mutations may accumulate within populations
An allele that does not affect the fitness of an organism is called a neutral allele.
Neutral alleles are unaffected by natural selection.
Neutral alleles tend to accumulate in a population over time, providing it with considerable genetic variation.
Sexual recombination amplifies the number of possible genotypes
Sexual recombination generates an endless variety of genotypic combinations that increases the evolutionary potential
of populations –a long-term advantage of sex.
Sex has at least three striking disadvantages in the short term:
o Recombination breaks up adaptive combinations of genes.
o Sex reduces the rate at which females pass genes on to their offspring.
o Diving offspring into separate genders greatly reduces the overall reproductive rate.
A number of hypotheses have be proposed for the existence of sex; one of them is that sexual recombination facilitates
repair of damaged DNA, because breaks and other errors in DNA one chromosome can be repaired by copying the intact
sequence from the homologous chromosome; another advantage is that it permits the elimination of deleterious
Sexual reproduction allows natural selection to eliminate deleterious mutations from the population over time and may
be especially valuable as a defense against pathogens and parasites.
Sexual recombination does not influence the frequencies of alleles; it generates new combinations of alleles on which
natural selection can act.
Frequency-dependent selection maintains genetic variation within populations.
Polymorphism may be maintained when the fitness of a genotype (or phenotype) depends on its frequency in a
population frequency-dependent selection.
Environmental variation favour genetic variation.
Much genetic variation is maintained in geographically distinct subpopulations
Much of the genetic variation in large populations is preserved as differences among members living in different places
Subpopulations often vary genetically because they are subjected to different selective pressures in different
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