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BIOA02H3 Study Guide - Rhagoletis, Dont, Eusociality

Biological Sciences
Course Code
Kamini Persaud

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Chp. 22 The Mechanisms of Evolution
Charles Darwin partook in a 5 year long voyage in the 19th century on the
H.M.S. Beagle which traveled around the world
oThe Archipelago Galapagos Islands was where he gathered his most
groundbreaking evidence of natural selection FINCHES
oHe postulated that some animals had come to the archipelago from
mainland South America and then undergone different changes on
each of the islands
Darwins theory for evolutionary change:
oSpecies are not immutable; they change over time
oThe process that produces these changes is natural selection
Darwin, with the influence of economist Thomas Malthus, postulated that
populations of all species have the potential for rapid increase in numbers
Without high death rates, even the most slowly reproducing species would
quickly reach enormous population sizes
Darwin suggested that slight variations among individuals affect the chance
that a given individual will survive and reproduce
Natural selection (formally defined): the differential contribution of offspring
to the next generation by various genetic types belonging to the same
oIndividuals do not evolve, POPULATIONS do
oPopulation: a group of individuals of a single species that live and
interbreed in a particular geographic area at the same time
Adaption refers both to the processes by which characteristics that appear to
be useful to their bearers evolve and to the characteristics themselves
For a population to evolve, its members must possess heritable genetic

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oCharacter: a particular
feature of the individuals
oTrait: a specific form of a
oGenotype: the genetic
coding for that trait
oPhenotype: physical
A population evolves when individuals with different genotypes survive or
reproduce at different rates
If there were NO evolution in a population: the Hardy-Weinberg equation
shows that allele frequencies will remain the same from generation to
generation (genotypic frequencies can be predicted from allele frequencies)
unless some mechanism acts to change them
oThe models conditions are never met = population deviate from Hardy-
Weinberg equilibrium
oTherefore, there are mechanisms acting to change allele frequencies,
and these mechanisms drive evolution
Known evolutionary mechanisms include:
oGene flow
oGenetic drift
oNon-random mating
oNatural selection
oOrigin of genetic variation
oChange in an organisms DNA and appear to be random
oBoth create and help maintain genetic variation within populations
oRates are low with only 1 mutation/locus in a million zygotes; but are
sufficient to create considerable variation because each of a large
number of genes may mutate
oMinor deviation from H-W equilibrium

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Gene Flow
oMigration of individuals and movements of gametes between
oCommon because few populations are completely isolated from others
oAdd new alleles to gene pool of population or may change the
frequencies of alleles already present
Genetic Drift
oRandom changes in allele frequencies from one generation to the next
oGreat affect on small populations where genetic variation can be
reduced and harmful alleles may increase
oMay also affect large populations but it wont alleles influencing
survival and reproduction
oPopulation bottleneck: large populations are reduced to small ones and
genetic variation is thus decreased
oFounder effect: where few pioneering individuals colonize a new region
and because of the colonized populations small size, genetic variation
is reduced
Non-random Mating
oMating patterns may alter genotype frequencies if individuals in a
population choose other individuals of particular genotypes as mates
oSelf-fertilization (selfing) is common among many groups of organisms,
especially plants; reduces the frequency of heterozygous individuals
from H-W equil. And increases the frequency of homozygous
individuals from H-W equil.; BUT does NOT change allele frequencies
and thus does not result in adaptation
oSexual selection DOES change allele frequencies and often results in
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