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Mechanisms of
Chapter 17
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
 Evolution occurs as the
frequency of genes in a
population changes.
Evolution-What Happens?
See page 404, Figure 5.
Snapdragon Flowers
Evolution occurs when
there is a change in the
gene pool-specifically a
change in allele frequency.
 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
 Evolution results when there are Forces that
change allelic frequencies and disrupts
genetic equilibrium.
 Forces that cause Evolution:
1. Gene flow: Transport of genes by migrating
2. Nonrandom Mating: Mating based on
preferences example: a female may choose
a mate based on male size, color, or ability
to gather food
Forces of Evolution Continued
3. Mutation: Change in DNA
4. Genetic Drift: chance event
changes allelic frequencies –
Greatly affect small populations
such as the animals of the
Galapagos Islands or Amish.
Causes of Genetic Drift
Random mating over a long time
 No immigration of males
 No emigration of females
Forces of Evolution Continued
5. The most significant factor causing
changes in the gene pool (evolution) is
natural selection.
Overproduction of offspring
Some variations increase or decrease
chances of survival. These variations can
be inherited.
So, frequency of alleles changes over
generations due to natural selection.
Adaptation: over time, those traits that
improve survival and reproduction become
more common.
3 Types of Natural Selection
 Stabilizing selection – favors average
 Directional selection – favors one of
the extreme variations of a trait
 Disruptive selection – favors
individuals with either extreme of a trait
(eliminates intermediate phenotypes)
 Stabilizing selection favors average
 Directional selection favors one of the
extreme variations
 Disruptive selection or Diversifying
selection favors both extreme variations of a
What is a Species?
 A population or
group of
populations whose
members have the
ability to breed with
one another and
produce fertile
Evolution of Species
 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
Why Don’t the Populations
 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
 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.