Chapter 13
Section 3 and more!
Objectives
▪ Describe an allele.
▪ Identify the five principles of natural selection.
▪ Describe how changes in allele frequencies are due
to chance events,
▪ Summarize speciation.
What is an allele?
▪ It is a version or
“flavor” of a
gene that one
can receive from
their parents.
Natural Selection Principles
1. Organisms produce
more offspring than
their environments
can support. Not all
individuals in a
population survive.
Natural Selection Principles
2. Offspring vary in
physical appearance.
Some offspring are
better able to survive in
their environment than
others.
Natural Selection Principles
3. Much of this
variation in physical
appearance is
caused by
differences in the
alleles inherited
from the individual’s
parents.
Natural Selection Principles
4. The inheritance of
alleles determines
how likely an
organism is to
survive and
reproduce.
Natural Selection Principles
5. The ability of some
organisms with certain
alleles, to survive and
produce more offspring
will lead to a change in
allele frequency in the
population. Over time,
more individuals will
have the “helpful”
alleles.
Other Mechanisms of Evolution
▪ As in the peppered moth’s population, natural
selection can change the frequencies of alleles
in a population.
▪ However, there are other ways in which allele
frequencies may change.
Other Mechanisms of Evolution
▪ Alternate means of
allele frequency
changing:
–Genetic drift:
changes in allele
frequencies that
are due to chance
events.
▪ Key to Genetic Drift:
changes allele frequency
unrelated to an allele’s
impact on survival and
reproduction.
Other Mechanisms of Evolution
▪ Alternate means of allele
frequency changing:
–Migration: the
movement of
individuals into or out
of a population.
▪ These individuals bring
or take their alleles with
them.
Two Types of Migration
1. Emigration – organisms leaving an ecosystem
2. Immigration - organisms moving into an ecosystem
Other Mechanisms of Evolution
▪ Alternate means of
allele frequency
changing:
–Founder Effect:
Decreases the
genetic variation in
the new population
by reducing the
number of different
alleles.
▪ Sometimes, members of a population
migrate to another ecosystem.
▪ The new ecosystem has never been
colonized by this organism.
▪ These migrants are the founders of a
new population
Other Mechanisms of Evolution
▪ Alternate means
of allele
frequency
changing:
–Bottleneck
Effect: Occurs
when an entire
population is
reduced to just a
few individuals.
▪ The population that survives after a
bottleneck disaster has less genetic
variation.
▪ Even when the population increases,
genetic variation will remain low!
Other Mechanisms of Evolution
▪ Alternate means of allele frequency
changing:
–Mutations: produce new alleles.
▪ These new alleles are caused by
changes in DNA sequences.
Speciation
▪ Evolution does not occur
only to change existing
species.
▪ Entirely new species also
evolve.
▪ Speciation is the formation
of new, genetically distinct
species from populations of
existing species.
Speciation
▪ New species arise when a
population splits or two
populations of the same
species are under different
selective pressures (ex:
predators, food sources,
diseases, human
interference) .
Types of Speciation
1. Geographical Isolation
Example: A new highway is built
that divides a single population
of beetles into two populations.
After many generations, the
beetles will develop differences
in their gene pool, thus possibly
resulting in two new species.
Types of Speciation
2. Reproductive Isolation:
Prevents members of two
populations of the same species
from mating, even if they
inhabit the same area.
Reasons: ill-attraction, sexual
structures don’t match correctly,
periods of sexual maturity don’t
match
Speciation
As separate populations evolve along separate paths, they
become increasingly different.
Eventually they will become so different that these
organisms will form their own subspecies, which is the
first step toward speciation.