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Evolution of Populations
Chapter 11
I. Genes and Variation
Relative (allelic) frequency - the percentage of a
particular allele (trait) in a gene pool.
Natural Selection- In nature, unequal ability to
survive and reproduce
Artificial Selection- Mankind “selects” for desired
Camouflage- organisms blend-in
with surrounding environment
Mimicry- species copy another to
insure their own survival
I. Genes and Variation
gene pool - all the genes that
exist within a population
gene flow – movement of alleles
into or out of a population
B. Genetic drift - change
in allelic frequencies
by chance
Ex: sudden
extinction of a
dominant species;
small populations
most affected
C. Genetic equilibrium - when alleles stay the same
from generation to generation
The Hardy Weinberg Principle: Allele
frequencies will remain constant under five
Random Mating
Large Population
No movement (immigration or emigration)
No Mutations
No Natural Selection: equal change of
II. Natural Selection as Genetic
A. Natural Selection has 3
affects on phenotype
1. Directional SelectionIndividuals on one end of a
curve are “better fitted” than
the middle or other end
Peccaries naturally choose to consume those
cactus plants with the fewest spines As a result,
at flowering time there are more cacti with
higher spine numbers; thus, there are more of
their alleles going into pollen, eggs, and seeds
for the next generation.
Natural Selection has 3 affects on
phenotype distribution
Stabilizing SelectionIndividuals near center
of a curve are “better
fitted” than both ends
Peccaries are consuming the low-spine
number plants, and the insects are killing
the high-spine-number plants. As these
gene combinations are removed from the
cactus gene pool, there is less and less
variety possible in subsequent
Natural Selection has 3 affects on
phenotype distribution
Disruptive SelectionIndividuals at upper and
lower ends are “better
fitted” the ones in the
Years of collecting have left their toll on
the roadside cacti. In this environment, it is
maladaptive to be good looking and have
a reasonable number of spines. Low
spine-number plants are not picked
because they don't "look right", and high
spine-number varieties are left alone
because they are too hard to pick.
Gradually, the gene pool changes in favor
of the two extreme spine number types.
III. Process of Speciation
speciation - evolution of a new species
Isolating Mechanisms
Reproductive Isolation: Two populations
cannot interbred and produce fertile
Behavioral Isolation: Two populations
capable of breeding but cannot b/c of
courtship rituals
C. Geographic Isolation: Two populations are
separated by geographic barriers
Ex: Rivers, Oceans, Mountains
D. Temporal Isolation: Two or more populations
reproduce at different times
• Geographical Isolation
• Whale Evolution
Convergent Evolution
Process by which unrelated organisms
independently evolve similarities when adapting
to similar environments
Produces analogous structures
Process by which two species evolve in
response to changes in each other
Punctuated Equilibrium
Pattern of evolution in which long stable periods
are interrupted by brief periods of rapid change
Pattern of evolution categorized by slow and
constant change