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Speciation
Gene Pool
1. Gene Pool: The total number of genes (alleles) in an
interbreeding population
* the larger the gene pool, the greater the genetic
diversity of the population
* a small gene pool indicates little genetic diversity
Genetic Drift
1. Genetic drift —is one of the basic mechanisms of
evolution.
2. In each generation, some individuals leave behind a few
more descendents (and genes) than other individuals.
3. The genes of the next generation will be the genes of the
“lucky” individuals, not necessarily the healthier or “better”
individuals.
4. It happens to ALL populations.
Bottleneck Effect
1. Bottleneck effect: severe reduction in population
size due to intense selective pressure or a natural
calamity which alters the allele frequency
2. Because genetic drift acts more quickly to reduce
genetic variation in small populations, undergoing a
bottleneck can reduce a population’s genetic
variation by a lot.
Founder Effect
1. Founder effect:- when a new colony is started by a few
members of the original population.
2. This small population size means that the colony may
have:
* reduced genetic variation from the original population
* a non-random sample of the genes in the original
population
If the red “star” is the person who has the
gene for Huntington’s, then we can see how
this one gene can be traced back to the
original founder.
Slide # 13
Speciation: Often Begins with a Physical Separation From
the Original Population
Grand Canyon is NOT a physical
barrier for all species. What kind
of organisms would be able to
cross the canyon?
1. Geographic isolation: a barrier that physically separates
members of a species into two or more groups
a. EX: Mountains, large body of water (lake or ocean),
volcanic eruption, canyon, road
b. A barrier may physically separate some species, but not
others
Slide # 14
Reproductive Isolation Leads to Speciation
1. Mutations cause changes in chromosome number
– Humans are the only primates that have 46 chromosomes
2. Members of an original species can no longer breed together to
produce fertile offspring. – Cannot interbreed.
Three species of Flycatcher. Reproductive Isolation keeps
these as three separate species. Why?
Females only respond to mating call of males that are like them.
Slide # 15
Temporal Isolation
Temporal isolation: differences in mating times /
seasons
Because they mate at different times, the wood
frog and the leopard frog remain separate species
and hybrids are not produced
Species are Not Fixed
Time
1. Not all organisms have been on Earth for the same
amount of time
2. Graph below shows when different groups appeared and
how numerous they were during different time periods
Slide # 17
Effects of Natural Selection: Directional Selection
1. Directional selection: members at
one end of distribution curve have a
higher fitness than those in the
middle or at the other end of the
curve.
2. Can lead one population evolving
into a new species.
Food becomes scarce.
Notice the shift in beak size
Slide # 18
Effects of Natural Selection: Stabilizing Selection
1. Stabilizing selection: members in
the center of the distribution curve
have a higher fitness than those at
each end.
2. This type of selection favors average
individuals.
3. Reduces variation in a population;
evolution is not likely to happen.
4. Ex: babies that are between 6 & 8
pounds at birth have a better chance
of surviving 1st year of life.
Slide # 20
Effects of Natural Selection: Disruptive Selection
Number of Birds
in Population
1. Disruptive selection: when members
at BOTH ends of the distribution curve
have a higher fitness than those in the
middle
2. In some cases, there are no
intermediate forms.
3. This can lead to the evolution of two
new species.
Population splits into two
subgroups specializing in
different seeds.
Beak Size
Cladograms and Phylogenetic Trees
Show Relatedness
Remember: Individuals Do not Change to Fit the Environment