Download Mutation

Mutation
• The ultimate source of genetic variation; however it
usually occurs at low enough frequency to have only
minor effects on gene frequency over short
evolutionary time spans
• Exception - non-coding regions of DNA can evolve
very rapidly due to a combination of relaxed
selection and high mutation rates
Genetic Drift
Alteration of gene frequencies due to
chance (stochastic) effects
• Most important in small populations
• An agent that tends to reduce genetic variation as
the result of extinction of alleles
• Generally does not produce a fit between
organism and the environment; can, in fact, result
in non-adaptive or maladaptive changes
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Genetic Drift: Founder Effects
• Hawaiian Drosophila: 400 species derived from one
or few colonist species
• Some evolutionary biologists argue that founder
effects and genetic bottlenecks are of major
importance in major evolutionary changes
Non-Random Mating
Changes genotype frequencies but not gene
frequencies
• Assortative mating: Like phenotypes mate
preferentially with one another (e.g., tall with tall);
increases homozygosity at some, but not all, loci
• Disassortative mating: dissimilar phenotypes mate
preferentially (e.g., short with tall); increases
heterozygosity; might be important at MHC loci
• Inbreeding: mating between close relatives which
produces a deficit of heterozygotes; inbreeding exposes
lethal recessive alleles and usually depresses fitness
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• http://www.pbs.org/wgbh/evolution/librar
y/01/6/l_016_08.html
Migration
Movement followed by breeding in a new
location
• Migration can reduce local adaptation,
particularly in species with broad geographic
ranges
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–
–
–
e.g., the white-tailed deer
mountain lion
mussels: salinity tolerance
heavy metal tolerance in plants in England
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Selection
• Natural selection occurs when attributes of the phenotype,
e.g., size, color, and enzymatic activity, vary with in a consistent
way (covary) with Darwinian fitness (the product of survival and
reproduction)
— Directional
— Stabilizing
— Disruptive
Sexual selection is the differential reproductive success resulting
from competition between members of one sex, usually males,
to achieve matings and/or fertilizations; can result in the
evolution of exaggerated male traits which may be detrimental
to survival
Natural Selection
• The superior survival and/or reproduction of
some phenotypic variants compared to others
under the environmental conditions that prevail at
the moment
• Natural selection occurs when attributes of the
phenotype, e.g., size, color, and enzymatic
activity, vary with in a consistent way (=covary)
with Darwinian fitness (the product of survival
and reproduction)
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What’s Different about NS?
• The other forces of evolution, i.e., genetic drift,
inbreeding and gene flow, tend to act at the same
rate on all loci
• Natural selection: causes allele frequency changes
that proceed independently at different loci
—Different characteristics of a population evolve at
different rates (mosaic evolution)
• NS produces adaptations: i.e., traits that increase
the ability of an individual to survive or
reproduce compared to individuals lacking the
trait
Natural Selection & Evolution
• Natural selection occurs when different
phenotypes vary in average reproductive
success
• However, natural selection is different from
evolution by natural selection
• Natural selection can have no evolutionary
impact if unless phenotypes differ in
genotype (variation must have a genetic
basis)
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Types of Natural Selection
Directional:
• One extreme phenotype is fittest
(e.g., small or large individuals)
• Acts to drive favored allele to
fixation
Stabilizing:
• The intermediate (average)
phenotype is fittest; probably the
most common form of selection
• e.g., heterozygote advantage
Disruptive:
• Two or more phenotypes are
fitter than the intermediates
between them
• e.g., heterozygote disadvantage
Directional Selection
•
•
Many examples of
strong directional
selection associated
with human-caused
environmental change,
e.g., industrial
melanism; insecticide
resistance in insects,
heavy metal tolerance
in plants, or with
Components of fitness,
such as survival after
food depletion in Cliff
Swallows
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Directional
selection: evolution
of insecticide
resistance
Directional Selection
• Many good examples of strong directional selection
associated with human-generated environmental change,
e.g., industrial melanism; insecticide resistance in insects;
heavy metal tolerance in plants
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Stabilizing Selection: Birth Weight
• The most
common form of
selection
• Results from
tradeoffs: an
inescapable
compromise
between one trait
and another
Disruptive Selection
• Bimodal
distribution of bill
size in Blackbellied seedcracker
• Birds can either
specialize on hard
seeds (wide bills)
or soft seeds
(narrow bills)
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Classic Example of Disruptive Selection:
Gamete Size in Animals
Negative Frequency Dependent Selection
• Fitness of a genotype
increases when it
becomes rare
• Fitness of a genotype
decreases when it
becomes common
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Negative Frequency Dependent Selection:
Sex Ratio
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