Download 15.2 Mechanisms of Evolution

15.2 Mechanisms of
Evolution
Can Individuals Evolve?
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Genes determine
most of an
individual’s features.
Individuals cannot
evolve a new
phenotype in
response to their
environment.
Populations, not
individuals, evolve.
Evolution of a Population
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Natural selection acts on the range of
phenotypes in a population.
Evolution occurs as a population’s
genes and their frequencies change
over time.
How can a population’s genes
change over time?
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All of the alleles of a population’s
genes together make up a gene pool.
Allelic frequency - % of any specific
allele in the gene pool.
Genetic equilibrium – a population in
which the frequency of alleles
remains the same over generations.
Changes in Genetic Equilibrium
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A population in genetic equilibrium is
not evolving.
Mutations are one cause of genetic
change.
Lethal mutations disappear quickly,
but mutations that cause a useful
variation become part of the gene
pool
Changes in Genetic Equilibrium
cont.
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Genetic drift – the alteration of allelic
frequencies by chance events.
Gene flow – transport of genes into or out
of a population by migrating individuals.
Genetic drift, gene flow, and mutations
can greatly affect small populations.
Natural selection is usually the most
significant cause of changes in any gene
pool.
Natural Selection Acts on
Variation
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Some variations increase or
decrease an organism’s chance of
survival in an environment.
Variations are controlled by alleles.
Allelic frequencies in a gene pool will
change due to natural selection of
variations.
Three Types of Natural
Selection
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Stabilizing Selection – favors average
individuals and reduces variation in a
population.
Three Types of Natural
Selection cont.
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Directional Selection – favors one of the
extreme variations of a trait and can lead
to rapid evolution of a population.
Three Types of Natural
Selection cont.
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Disruptive Selection – favors both extreme
variations of a trait, leading to the
evolution of two new species.
Evolution of Species
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Speciation – occurs when members
of a similar population no longer
interbreed to produce fertile
offspring.
There are three different ways that
organisms can become isolated from
each other to form a new species.
Geographic Isolation
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Physical barriers can break large
populations into smaller ones.
Reproductive Isolation
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Occurs when formerly
interbreeding organisms can no
longer mate and produce fertile
offspring.
May occur because genetic
material becomes so different that
fertilization cannot occur.
May be isolated by behavior, such
as mating at different times.
Change in Chromosome
Number
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Mistakes in mitosis or meiosis can lead to
polyploidy.
Polypolids within a population may
interbreed and form a new species.
Rate of Speciation – Two
Hypotheses
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Gradualism – species originate
through a gradual change of
adaptations.
Punctuated Equilibrium – speciation
occurs relatively quickly with long
periods of genetic equilibrium in
between.
Patterns of Evolution
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Adaptive Radiation – an ancestral species
evolves into an array of species to fit a
number of diverse habitats.
Patterns of Evolution cont.
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Divergent Evolution –
pattern in which
species that once
were similar to an
ancestral species
diverge.
Populations become
less alike as they
adapt, resulting in
new species.
Patterns of Evolution cont.
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Convergent
Evolution –
pattern in
which distantly
related
organisms
evolve similar
traits.