Transcript Mechanisms of Evolution

Mechanisms of
Evolution
Section 15.2
How Does Evolution Work?
 Individual organisms cannot
evolve. Each individual’s traits
are determined by its genes.
 Natural selection acts on the
range of phenotypes in a
population.
 Evolution occurs as the
frequency of genes in a
population changes.
Definitions
 Gene pool: All alleles of the
population’s genes.
 Allelic frequency: % of a specific
allele in the gene pool.
 Genetic Equilibrium: This exists
when the frequency of alleles remains
the same over generations. The
population is not evolving.
When Evolution Occurs
 Anything that changes allelic frequencies and
disrupts genetic equilibrium results in evolution.
 Examples:
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Mutation
Genetic Drift: chance event changes allelic
frequencies – Greatly affect small populations such
as the animals of the Galapagos Islands or Amish.
Gene flow: Transport of genes by migrating
individuals.
Natural Selection
 The most significant factor causing
changes in the gene pool (evolution) is
natural selection.
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Some variations increase or decrease
chances of survival.
These variations can be inherited; they are
controlled by genes.
So, frequency of alleles changes over
generations due to natural selection.
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3 Types of Natural Selection
 Stabilizing selection – favors average
individuals
 Directional selection – favors one of
the extreme variations of a trait
 Disruptive selection – favors
individuals with either extreme of a trait
(eliminates intermediate phenotypes)
Evolution of Species
(Speciation)
 Significant changes in the gene pool can
lead to evolution of a new species over
time.
 Speciation occurs when members of
similar populations no longer interbreed
to produce fertile offspring within their
natural environment.
Why Don’t the Populations
Interbreed?
 Geographic isolation – physical barrier
divides a population.
 Reproductive isolation – formerly
interbreeding organisms can no
longer mate to produce offspring..
 Polyploidy – results in immediate
reproductive isolation. Very common
in plants.
Speciation can occur quickly
or slowly
 Gradualism – idea that species originate
through a gradual accumulation of
adaptations.
 Punctuated equilibrium – hypothesis that
speciation occurs relatively quickly, in
rapid bursts, with long periods of genetic
equilibrium in between.
Patterns of Evolution
 Adaptive Radiation – ancestral species
evolves into an array of species to fit diverse
habitats. This is a type of divergent evolution
where species diverge or become less and less
alike as they adapt to different environments.
 Convergent Evolution – Unrelated species
occupy similar environments in different parts of
the world. Similar pressures of natural selection
lead to similar adaptations.