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Evolution Versus Genetic
Equilibrium
 If a population is not evolving and
allele frequencies do not change, the
population is in genetic equilibrium.
 When allele frequency does change,
the population is evolving.
5 conditions that can lead to
Evolution (disturb genetic
equilibrium)

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Nonrandom mating
Small population size (Genetic Drift)
Immigration or emmigration
Mutations
Natural Selection
Genetic Drift due to
Founder Effect
Sample of
Original Population
Founding Population A
Descendants
Founding Population B
Genetic Drift: rapid changes in gene frequency of a
small, isolated population
Genetic Drift due toFounder
Effect
Sample of
Original Population
Descendants
Founding Population A
Founding Population B
Natural Selection
 If different genotypes have
different fitness, genetic equilibrium
will be disrupted and evolution will
occur.
TYPES OF EVOLUTION
 Convergent Evolution – when species that are
not closely related evolve similar traits (two
species look like they are closely related and
they are not)
• Example: dolphins (mammals) and sharks
(fish)
 Divergent Evolution – one species evolves into
two or more species with different
characteristics
 Example: lions and tigers evolved from a
common ancestor
The Process of Speciation
Speciation
 Speciation –evolution of two different
species from one common ancestor;
formation of a new species
 Species- defined as a population or group of
populations whose members can interbreed
and produce FERTILE offspring
 What keeps two species apart so that they
evolve differently?

Isolating Mechanisms – keep groups apart
Isolating Mechanisms
 Reproductive Isolation- when two
populations no longer interbreed and
changes in one gene pool cannot
spread to the other, (gene pool splits)
 Can happen by behavorial isolation,
geographic isolation, and temporal
isolation
Behavioral Isolation
 Two populations who are capable of
interbreeding develop differences in
behaviors (behaviors don’t match)
 Example: Eastern and Western Meadowlarks do not mate with each
other because they use different
songs to attract mates.
-Certain species of lightning bugs use
different blinking patterns to attract
mates.
Geographic Isolation
 When two populations are separated
by geographic barriers such as rivers,
mountains, or bodies of water.
 Examples: squirrels separated by the
Grand Canyon
Squirrels in the Grand Canyon
Figure 15–14: Geographic Distribution
of Living Species
Beaver
NORTH
AMERICA
Muskrat
Muskrat
Beaver and
Muskrat
Coypu
Capybara
Capybara
SOUTH
AMERICA
Coypu
Coypu and
Capybara
Temporal Isolation
 When two or more species reproduce
at different times
 Example: 3 species of orchids have
flowers that last only one day and
must pollinate on that day. The
species bloom on different days and
they don’t cross pollinate one
another.
-Different species of skunks mate at
different times of the year