Download Population Genetics

Evolution of Populations
Chapter 16
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 traits
Camouflage- organisms blendin with surrounding environment
Mimicry- species copy another
to insure their own survival
gene pool - all the
genes that exist
within a population
• gene flow - movement of genes into & out of
a gene pool
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
1.
2.
3.
4.
5.
The Hardy Weinberg Principle: Allele
frequencies will remain constant under five
conditions
Random Mating
Large Population
No movement (immigration or emigration)
No Mutations
No Natural Selection: equal change of
survival
I. Genes and Variation cont.
Sources of Mutation:
Mutations- change in DNA sequence
Gene Shuffling- random assortment of genes
during gamete production
Gene Expression Variation
Single-gene trait- controlled by one gene
Ex: Widow’s Peak
Polygenic trait- controlled by many genes
Skin color, eye color
II. Natural Selection as Genetic
Change
A. Natural Selection has 3
affects on phenotype
distribution
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
2.
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
generations.
Natural Selection has 3 affects on
phenotype distribution
3.
Disruptive SelectionIndividuals at upper and
lower ends are “better
fitted” the ones in the
middle
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.
IV. Process of Speciation
speciation - evolution of a new species;
happens when populations are isolated
Isolating Mechanisms
A.
B.
Reproductive Isolation: Two populations
cannot interbred and produce fertile
offspring
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
III. Evidence of Evolution
homologous structures - similar structures
found in related organisms that are
adapted for different purposes. Ex:
human arm and bat wing or whale flipper
analogous structures - structures found in
unrelated organisms that have a similar
function
Ex: bird wing and insect wing
Geographical Isolation????
http://www.pbs.org/wgbh/evolution/library/11/2/e
_s_4.html
Whale Evolution
http://www.pbs.org/wgbh/evolution/library/03/4/l_
034_05.html