BIOL 121 Chapter Notes -Allele Frequency, Reproductive Isolation, Genetic Drift

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16 Aug 2013
UBC Bio 121: Evolution-James Cooke
Lena C
Evolution- Change in genetic characteristics-allele frequencies over time via natural
* Individuals do not evolve, selection acts on individuals;population is evolving when
ratio of genetic type is changing
Natural Selection- individuals with certain heritable traits tend to produce more offspring
than individuals without that trait, leads to change in genetic makeup.
* Natural Selection has 4 postulates:
1. Variation in population-Individual organisms that make up a population must vary
in traits they posses, ex: size & shape
2. Heritable variation-Some traits are heritable
3. Selection-In every generation more offspring are produced than survived;only
some individuals in the population survive long enough to produce offspring
4. Differential survival or reproduction- survival and reproduction are not random;
those with favorable variations survive an go on to reproduce
* Natural selection is not goal directed or progressive. It simply favors individuals better
adapted to the environment at the time
* Natural selection does not require competition, just a fitness difference
*Selection does not equal evolution; evolution is an outcome of the process of natural
Population- collection of individuals in a group
Darwinian Fitness- ability to produce more offspring, relative to that ability in other
individuals in a population
* Darwin referred to successful individuals as "more fit" than other individuals
Ex: Self sacrificing alleles: individuals with this alleles do not produce offspring. As a
result, selfish alleles increase frequency while self-sacrificing alleles decrease in
frequency, therefore it is not possible for individuals to sacrifice based on choice for the
good of the species.
Adaptation- increases individual's ability to create offspring; heritable trait that increases
the fitness of an individual in a particular environment than other individuals lacking the
Acclimation- changes in an individuals phenotype that occur in response to changes in
environmental conditions
Vestigial trait- structures with no function; ex: human tail bone
Genetic Correlation- genetic constraint; lack of genetic variation;selection on one trait
causes change in another trait.Occur because of pleitropy-single gene affect more than
one trait
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Fitness Trade-off- compromise between traits;
Balancing selection or heterozygote advantage- heterozygous individuals increase fitness
than homozygous= genetic variation maintained in population
Genetic variation- # and relative frequency of alleles that are present in a particular
*low genetic variation= bad for population when environment changes effects population
Directional selection- one extreme phenotype have higher fitness than the average or
other extreme=reduce genetic variation over time
Disruptive selection- only extreme phenotype on either range of variation have higher
fitness=genetic variation overtime
Stabilizing selection- only average phenotype have higher fitness=no genetic variation
over time
Genetic drift-any change in allele frequencies due to random events; causes allele
frequencies to change randomly. May cause increase or decrease fitness;drift up or down.
May cause loss or fixation of alleles (0=loss, 1=fix). Most pronounced in small
Genetic bottleneck-sudden reduction of the # of alleles in a population=change in allel
Founder Effect- change in allele frequencies that often occurs when a new population is
established from a small group of individuals due to sampling error
Gene flow- movement of alleles between populations; when individuals of one
population leaves and joins another then breed. Allele frequencies may change from new
alleles from immigrated population and the other remove alleles from their old
Mutation-introduce new alleles, can be advantageous or detrimental
Inbreeding-mating between relatives; mating wit closely related individuals
Sexual Selection-occurs when individuals within a population differ in their ability to
attract mates; females invest more in their offspring than males
Asymmetry of sex-1. Eggs are large and consume large energy, females produce few
young over course of lifetime. Female fitness is limited primarily by ability to gain
resources to produce more eggs. 2. Men can have limitless # of offspring, male fitness
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