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Evolution
 Evolution is the change in organisms over time.
 Specifically the proportion of individuals in a
population that differ in one or more inherited
traits.
 Evolution has been taking place from the moment
living organisms started passing on genetic
information.
 When did that start?
 Where did life come from?
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Drift and Selection
 The Hardy – Weinberg principle
Genotype proportions in a population will remain
constant if:
The population is very large
Random mating occurs
No mutation takes palce
No genes are input from other sources (migration)
No selection occurs
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Drift and Selection
 But the frequency of alleles is changing in
natural populations.
 Five factors affect the proportion of
homozygotes and heterozygotes enough to
move away from the constant.
 mutation
 migration (immigration/emigration)
 genetic drift
 non random mating
 selection
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Genetic Drift
 Evolution can occur through the random process of
genetic drift.
 Genetic drift has the greatest effect on small
populations.
 It is a random change of the allele frequency due to
only a few individuals gametes forming the next
generation.
 The two causes of genetic drift are:
 Founder effect – a few individuals are dispersed and
form an isolated population. They have limited
genetic material, rare alleles will have a higher
frequency than in the original population.
 Bottle neck effect – depletion of genetic diversity by
a dramatic reduction in the original population. All
subsequent members in the population arise from a
few individuals.
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Selection
 Evolution can occur through the non random processes
of natural selection and sexual selection.
Natural selection
as proposed by Darwin.
Organisms reproduce more offspring than are necessary
for the species to survive.
Variation exists in the offspring.
Selection pressures have an effect on individuals.
Those best able to survive are more likely to pass on their
genetic information to their offspring.
The frequency of beneficial characteristics (alleles) in a
species increases so the population changes over time.
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Sexual Selection
 Sexual selection is a "special case" of
natural selection. Sexual selection acts on
an organism's ability to obtain or
successfully copulate with a mate.
 Sexual selection is often powerful enough
to produce features that are harmful to
the individual's survival
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Sexual selection
 Female Choice:
Intersexual selection, in which females choose males
based upon elaborate ornamentation or male
behaviours.
 Male Competition:
Intrasexual selection, in which males compete for
territory or access to females, or areas on mating
grounds where displays take place. Male-male
competition can lead to intense battles for access to
females where males use elaborate armaments
(e.g., horns of many ungulates).
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New Variation
 New genetic information is produced by
mutations.
 Mutations can add, delete or otherwise alter the
sequence of DNA in an individual.
 However most mutations are harmful or neutral
in effect. So they do not provide an advantage
that will be passed on.
 In rare cases the mutation will give benefit and
improve the ‘fitness’ of an individual and be
passed on.
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Fitness
absolute versus related fitness
 Absolute fitness
is the ratio of frequencies of a particular
genotype from one generation to the next.
 Relative fitness
is the ratio of surviving offspring of one
genotype compared with other genotypes.
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Just to clarify if needed:
Those offspring that best fit the environment
because of their variation are most likely to
survive and breed.
Those inherited traits (alleles) are likely to
become more frequent in future generations
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Rate of Evolution
 2 theories on how evolution progresses:
punctuated equilibrium and gradualism.
 The rate of evolution is influenced by selection
pressures.
 Evolution rate will be more rapid if selection
pressures are high.
 Other factors will also speed up the rate of
evolution such as shorter generation times,
warmer environments, sharing of beneficial DNA
sequences between different lineages through
sexual reproduction and horizontal gene transfer.
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Co-evolution and the Red Queen hypothesis
 Species that interact closely or frequently can
show co-evolution.
 This can be seen between:
 predator and prey
 parasites and their host
 herbivores and plants
 pollinators and plants
A change in a trait in one species can have an
influence (selection pressure) on the other species
so co-evolution occurs.
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Red Queen hypothesis
The “Red Queen” hypothesis in evolution is
related to the co-evolution of species.
It states that species must constantly adapt
and evolve to pass on genes to the next
generation and also to keep from going
extinct when other species within a symbiotic
relationship are evolving.
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Red Queen hypothesis illustrated by
parasite and host relationship.
 Hosts better able to resist and/or tolerate
parasites show greater fitness.
 Parasites better able to feed, reproduce
and find a new host show greater fitness
 As either host or parasite evolves each will
have an effect on the evolution of the
other.
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