How Evolution
Occurs
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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
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