Transcript Genetic Variation & Evolution

Genetic Variation &
Evolution
Chapter 23
What you need to know!
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How mutation and sexual reproduction
each produce genetic variation
Variation
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Variations exist between individuals in
a given population. The following are
heritable:
Mutation: mostly point mutations
(other mutations are possible)
Sexual Reproduction: crossing over,
independent assortment
Diploidy: recessive alleles can be
hidden (from natural selection) but
stay around
Variation
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Outbreeding: mating with non-related
partners increases new allele combinations
– Behavioral incest is blocked in most higher order
species
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Balanced polymorphism: different
phenotypes in a single population propagate
natural selection
Heterozygous advantage: advantage to
heterozygous alleles despite deadly
recessive alleles (ex. Sickle cell anemia)
Variation
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Hybrid Vigor: superior quality of offspring
when crossing highly inbred strains
Frequency dependent selection: predators form
“search images” fitting most common prey
phenotypes
– http://evolution.berkeley.edu/evolibrary/article/happ
yface_11
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Neutral Variation: no selection pressure for
certain phenotypes
Silent Mutations: mutations that do not impact
phenotypes (no selection pressure)
Natural Selection
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Differential reproductive success of
different phenotypes as a result of the
interaction with the environment
Due to genetic variations certain
phenotypes are better or worse adapted
Survival depends on the adaptations
(fitness)
Reproduction depends on survival
Microevolution
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Def: Natural selection causes changes in
relative frequencies of alleles in the gene
pool (Hardy Weinberg)
Adaptation means genes are selected to…
– Increase (good)
– Decrease (bad)
– Stay the same (neutral)
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Evolution is the accumulation of
advantageous traits
Stabilizing Selection
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Extreme phenotypes are bad
adapatations
Example primate birth weight:
– Too small and the baby dies
– Too large and the mother dies
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Draw…
Directional Selection
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A single extreme adaptation is good
Example: peppered moths
Draw…
Diversifying Selection
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Selection against the common
phenotype
Selection for both extremes
Example: weeds are either very tall
(difficult to uproot) or very short
(survive lawn mowers)
Sexual Selection
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Selection for sexual phenotypes:
Male competition: fittest male mates
most (antlers, horns, body size)
Female choice: attractiveness and
mating behavior
Sexual dimorphism: diifferent
appearance of males and females in
certain species
Artificial Selection
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Directional selection carried out by
humans
Humans select a desireable trait and
only breed animals with the selected
trait (dogs, cats, sheep, horses, cows,
etc.)
Imperfection
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Natural selection does not fashion the
perfect organism because:
Selection only edits existing variation
Evolution is limited by historical
constraints
Adaptations are often compromises
Change, natural selection and the
environment interact
Homework
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Concept Checks 23.1 (pg. 458):
What did Mendel’s findings about genetics
add to Darwin’s theory of evolution by
natural selection?
Suppose a population or organisms with
500 gene loci is fixed at half of these loci.
How many alleles are found in its gene
pool? Explain.
Which parts of the Hardy-Weinberg
equation (p2 +2pq + q2 = 1) correspond
to the frequency of individuals that have
at least one PKU allele?
Homework
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Concept Checks 23.2 (pg. 460):
Of all the mutations that occur, why
do only a small fraction become
widespread in a gene pool?
How does sexual recombination
produce variation?
Homework
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Concept Checks 23.3 (pg. 462):
In what sense is natural selection
more “predictable” than genetic
drift?
Distinguish genetic drift and gene
flow in terms of (a) how they occur
and (b) their implications for future
genetic variation in a population.
Homework
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Concept Checks 23.4 (pg. 470):
Does nucleotide variability in a population
always correspond to phenotypic
polymorphism? Why or why not?
What is the relative fitness of a sterile
mule? Explain.
How does sexual selection lead to sexual
dimorphism?
Explain what is meant by the
“reproductive handicap” of sex.