The Mechanisms of Evolution
Essential Question: How does evolution
occur beyond Natural Selection?
Populations and Their Gene Pools
• Gene Pool: consists of all the alleles in all the
individuals that make up a population. This is
where genetic variation is stored.
• You can think of a gene pool as the hat from
which the next generation draws its genes.
A population is the
smallest level at which
evolution can occur. A
common but mistaken
belief is that individual
organisms evolve.
• Genetic Drift: a change in gene pool of a population
due to chance not natural selection.
• 2 types: The Bottleneck Effect & Founder Effect
– All populations are subject to some genetic drift.
– The smaller the population = the more impact
genetic drift has on that population.
– Genetic drift can have major effects on a population.
• Example Natural Disaster Scenario
• Gene Flow: exchange of genes with another
population.
• This takes place when fertile individuals or their
gametes migrate between populations.
• Gene flow tends to reduce genetic differences
between populations.
Sexual Selection (Nonrandom mating)
 operates in populations where males and
females differ significantly in appearance.
 Qualities of sexual attractiveness
appear to be the opposite of qualities
that might enhance survival.
Artificial Selection
• Artificial Selection: selective
breeding of domesticated
plants and animals to
produce offspring with
genetic traits that humans
value
– Humans have been modifying
species for thousands of
years. Such as in dogs.
– Darwin observed that artificial
selection could create change
in a species over a short
period of time.
Evidence of speciation is visible in the 3 main
patterns of evolutions:
1. Adaptive
Radiation
(Divergent
Evolution)
2. Coevolution
3. Convergent
Evolution
Adaptive Radiation, also known as divergent
evolution, occurs when many species diverge from
one single ancestor.
 Can occur in a relatively short time when one species
gives rise to
many different
species in
response to the
creation of new
habitat or some
other ecological
opportunity
 Follows large-scale extinction events
 Coevolution occurs when
the relationship between
two species might be so
close that the evolution of
one species affects the
evolution of the other
species.
• Examples include
Mutualism, parasitism,
and predator-prey
relationships.
• Convergent Evolution
occurs when unrelated
species evolve similar
traits even though they
live in different parts of
the world.
• The species have
analogous structures.
Evolution
 Evolution proceeds
in small, gradual
steps according to a
theory called
gradualism.
Evolution
 Punctuated equilibrium
explains rapid spurts of
genetic change causing
species to diverge quickly.
How are the two models similar?
Different?