Transcript Mechanisms of Evolution - APBiology2010-2011

Mechanisms of Evolution
How does evolution happen?
Mutation
The ultimate source of genetic variation.
• Mutations are changes in the nucleotide
sequence of DNA
• Mutations cause new genes and alleles to
arise
• Ex. Could be as simple as a mistake
during meiosis! (duplication, single base
change, frameshift)
Natural Selection
• Environmental conditions
determine which
individuals in a
population survive and
produce the most
offspring.
• Natural selection is the
differential success in
reproduction.
• Its product is the
adaptation of organisms
to their environment.
Natural Selection: Directional
Selection
Occurs when conditions
favor individuals
exhibiting one extreme of
the phenotypic range.
Caused by environmental
change & migration of
populations to new
habitat
Natural Selection: Directional Selection
Natural Selection: Disruptive Selection
• Conditions favor individuals at both
extremes of a phenotypic range over
individuals at an intermediate phenotypic
range.
• Example: Black bellied seedcracker
finches in Cameroon exhibit small billed
birds and large billed birds.
Natural Selection: Disruptive Selection
Natural Selection: Stabilizing Selection
• Selection acts against both phenotypic
extremes and favors the intermediate. This
reduces variation.
• Example: Human baby birth weight tends
to fall within the range of 6.6-8.8 lbs.
Natural Selection: Stabilizing Selection
Sexual Selection
Differential reproduction based on selection
of the most desirable secondary sex
characteristics.
Fig. 23-16
EXPERIMENT
Females prefer to mate with
males that give long mating
calls. This experiment tested
whether the genetic makeup
of LC males is superior to SC
males. They fertilized ½ the
eggs of each female with
sperm from an LC male and
the other half with sperm
from an SC male.
Female gray
tree frog
SC male gray
tree frog
LC male gray
tree frog
SC sperm  Eggs  LC sperm
Offspring of Offspring of
SC father
LC father
Fitness of these half-sibling offspring compared
They then compared fitness
of offspring.
RESULTS
Fitness Measure
Mating call indicated male’s
overall genetic quality.
1995
1996
Larval growth
NSD
LC better
Larval survival
LC better
NSD
Time to metamorphosis
LC better
(shorter)
LC better
(shorter)
NSD = no significant difference; LC better = offspring of LC males
superior to offspring of SC males.
Genetic Drift
• Random change in
allele frequency
resulting in the spread
of neutral or
disadvantageous
traits.
• Results in loss of
genetic variation.
• Occurs in small
populations.
Genetic Drift: The Founder Effect
• A few individuals become isolated from a
population and establish a population
where the gene pool differs from the
source population.
• Example: Inherited disorders among small,
isolated human populations (blue people)
Genetic Drift: The Bottleneck Effect
• This occurs when a
population
experiences a sudden
change in the
environment that
drastically reduces
the population (fire,
flood, over hunting
etc).
In the 1890s hunters reduced the
population of northern elephant
seals to 20 individuals. Now there
are 30,000 with no genetic
variation.
Gene Flow
• The transfer of alleles into or out of a
population due to the movement of fertile
individuals or their gametes.
• Ex. Migration of people throughout the
world has increased gene flow between
populations that were once isolated from
one another.
Symbiogenesis (Endosymbiosis)
One species is
incorporated by a
second species
creating a new
species.
Symbiogenesis Example
Lichen formed from a
fungus incorporating
an algae creating a
whole new life form
that is neither fungus
nor algae.
Symbiogenesis Example
• Eukaryotic Cells:
Chloroplasts and
mitochondria were
once “free-living”
bacteria, that became
engulfed by other
bacteria.
How does evolution happen?
• Microevolution: change in allele
frequencies in a population over
generations.
Main mechanisms
1. Natural Selection
2. Genetic Drift
3. Gene Flow
How does evolution happen?
• Macroevolution:
broad pattern of
evolutionary change
over long time spans.
– Ex. Origin of new
groups of species i.e.
mammals or flowering
plants