How Evolution
Occurs
{
The selection process
Natural Selection is a mixture of both
Chance and necessity
Natural Selection is not goal directed. It does
not have a long term goal.
What acts as a selection pressure
on a population?
• Competition for food
• Competition for a mate
• Changes in the environment
• Predators
• Parasites
Main Types of Selection Pressures
Directional Selection
 Natural selection favors one extreme of the
population for that trait
 often happens when environment changes in a
consistent way- e.g.climate gets colder.
 Disruptive Selection
 Natural selection favors both extremes selected
 Causes species to diverge
 Stabilizing Selection
 Natural selection favors the average for population
selected
 Sexual Selection

Directional
Selection
Stabilizing
Selection

When the extremes of
the trait aren’t as well
suited
Disruptive
Selection
Causes divergence within
the species
 Occurs when two different
types of resources in one
area
 Results in specialization for
each branched group
 May lead to formation of
new species
 E.g. Darwin’s Finches

Sexual selection
Certain traits increase mating
success
 Intrasexual selection



Competition between males; whoever
wins gets the female
Intersexual selection

Males display traits that attract the
female
Examples of selection
pressures...

Predators - variants with adaptations allowing them to
escape predators have more offspring

Prey/Food - variants with adaptations allowing them to
obtain food have more offspring

Climate - those who can survive new climate best have
more kids

Mates - variants with adaptations allowing them to
attract a mate to have offspring
Gene flow
Movement of alleles from one
population to another
 Increases genetic variation
 Between neighboring populations it
keeps gene pools similar
 Less gene flow between two
populations creates more
differences
 Lack of gene flow increases chance
of evolution

Emigration
 To leave population and go to another one
 Immigration
 To enter a new population and go to
another one

Genetic drift
Changes in allele frequencies that
are due to chance
 Causes a loss of genetic diversity in
a population

Bottleneck effect
Genetic drift that occurs after an
event greatly reduces the size of a
population
 Destructive event leaves only a few
survivors in a population

Founder effect

Genetic drift that occurs after a
small number of individuals
colonize a new area
Effects of Genetic drift

Population loses genetic variation


Less likely to have individuals that
will be able to adapt to a changing
environment
Alleles that are lethal in
homozygous individuals may be
carried by heterozygous
individuals and become more
common in the gene pool
Hardy-Weinberg Equilibrium
Describes populations that are not
evolving
 5 conditions needed to stay in
equilibrium and not evolve

Very large population
 No emigration or immigration
 No mutations
 Random mating
 No natural selection

5 conditions

Very large population


No emigration or immigration




No new alleles being added
Random mating


Emigration = leaving
Immigration = entering
No gene flow can occur
No mutations


No genetic drift will occur
No sexual selection will occur
No natural selection

All traits will equally aid in survival
Hardy-Weinberg equation
p2 + 2 pq + q2 = 1
 Compare predicted genotype
frequencies with actual frequencies
 If the same, population is in
equilibrium
 If different, it is evolving

Five factors leading to
evolution

Genetic drift


Gene flow


New alleles can form through mutation. Mutations create the
genetic variation needed for evolution
Sexual selection


Movement of alleles from one population to another changes the
allele frequencies in each population
Mutation


Allele frequencies can change due to chance alone
Certain traits may improve mating success. Alleles for these traits
increase in frequency
Natural selection

Certain traits might be at an advantage for survival. Alleles for these
traits increase in frequency
Speciation through
isolation

Reproductive isolation

Members of different populations can
no longer mate successfully with one
another


Not physically able to
No viable offspring (offspring cannot mate)
Speciation through
isolation

Behavioral barriers

Differences in courtship or mating behaviors
Speciation through
isolation

Geographic isolation

Involves physical barriers that divide a
population into two or more groups
Speciation through
isolation

Temporal isolation

Timing prevents reproduction between
populations
Patterns in evolution

Convergent Evolution
Evolution toward similar
characteristics in unrelated species
 Example: Analogous structures

Patterns in evolution

Divergent evolution

Closely related species evolve in
different directions
Beneficial relationships
through coevolution

Coevolution

Process in which two or more species
evolve in response to changes in each
other
Extinction
Elimination of a species
 Background extinctions



Occur continuously but at a very low rate
Mass extinction



Rarer
More intense
Five mass extinctions in the last 600 million years
Speciation often occurs in
patterns

Punctuated equilibrium


Lots of speciation and then very little
speciation
Adaptive radiation

One ancestor into many different descendants
which are adapted to many different
environments
Cladogram