BIOL 107 Lecture Notes - Chromosome, Homologous Chromosome, Allele Frequency

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25 Jan 2013
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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 phenotype.
Natural selection produces variable results
Natural selection can act on characters with quantitative variation in any one several different
ways:
o Stabilizing selectionfavouring average individuals
o Directional selection favouring individuals that vary in one direction from the mean
pop.
o Disruptive selectionfavouring 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 success.
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.
How is Genetic Variation Maintained within Populations?
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 mutations.
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