Download III. A. Mechanisms of Evolution 1. Evolution occurs at the population

III.
A. Mechanisms of Evolution
1. Evolution occurs at the population level of organization….not organism!
2.
a. Mathematics
b. Discovered that evolution will NOT occur in a population unless
allelic frequencies are acted upon that cause change.
~ex. A change in the number of alleles for red in flowers in a
population over the number of recessive white alleles in a population.
c.
~when allelic frequencies remain constant, a population
is in genetic equilibrium (not evolving)
R-red, r-white
Red dominant to white
P=dominant allele, Q=recessive allele
p2=homo. Dominant
2pg=heterozygous
q2=homo. Recessive
p +q=1
square and you get
p2 + 2pq + q2 = 1
genetic equilibrium
Ex. p. 432 in text
100 humans, earlobe attachment
EE-40
(40x2=80) E alleles
Ee-40
(40-E, 40-e)
ee-20
(20x2=40) e alleles
E=120, e=80 total of 200 alleles
E frequency is .6, e frequency is .4
p + q = 1 , .6 + .4 = 1
p2 + q2 = 1, .36 + .48 + .16 = 1
If data is taken in another generation and the frequencies of the alleles have changed, then
evolution (change) has taken place
d.
-the population is very large
-no immigration/emigration
-mutations do NOT occur
-
e.
-any of the above not followed.
3.
a. any change in the gene (allelic) frequency in a population that results from
chance.
b.
c.
-when part of a large population is separated (isolated) from the
larger population.
-uncommon genes in the large population become more common in the
new population due to mating with those closer genetically than
from a larger population.
-Ex.
-Old order Amish have more polydactyle (6 fingers) than new Amish
d. Bottleneck (extreme genetic drift)
-Population declines drastically and then rebounds
-Gene Pool is similar to that of the population at its lowest level.
-rebound population has ____________________________
-ex. cheetah’s are very genetically similar.
4.
a. the movement of genes in and out of a population
b.
5. Nonrandom mating
a. Rarely is mating completely random
b.
c. Favors homozygous
6.
a. random change in genes
b.
-those mutations that benefit the survival of the organism.
-provides the variety needed for natural selection and evolution.
7.
a. selects the individuals that are best adapted for survival andreproduction.
b. acts on the PHENOTYPE and ultimately changes the GENOTYPE.
c.
STABILIZING SELECTION
~
~works on eliminating the extreme expressions of a trait
~average fitness of gene most common
~ex. birth weight of humans
DIRECTIONAL SELECTION
~natural selection favors one of the extremes in a population.
~ex.
DISRUPTIVE SELECTION
~natural selection favors both extremes
~
~ex. northern water snakes either white or gray skin.
SEXUAL SELECTION
~
~in populations where males and female differ a great deal
~ex. birds, Cardinals, Peacock, Lions
~
~Males may evolve threatening displays
B. Reproductive isolation
1. When a species can no longer mate and produce fertile offspring with
the original population.
2.
a. occurs BEFORE fertilization
b.
c. behavioral isolation
d.
3. POSTZYGOTIC ISOLATION
a. when fertilization has occurred by the offspring cannot develop
or are sterile.
b.
C.
1. A population must diverge and then be reproductively isolated from
other populations.
2.
a. a physical barrier divides one population into two or more populations.
(mountain ranges, wide rivers, lava flows, etc.)
-smaller populations are unable to breed with each other.
-smaller populations have a better chance of evolving
away from the main population
b. ex. i
3. SYMPATRIC SPECIATION
a. new species evolves without a physical barrier.
b. fairly frequent in plants (especially through polyploidy)
D.
1. Adaptive radiation Divergent evolution
a. occurs rather quickly when the opportunity of a new habitat or
niches open up.
b.
c. after a volcanic eruption
d.
e. ex. cichlids, bird beaks
2.
a. many species evolve together and depend on each other for
their existence.
b.
ex. comet orchid & moth
ex. plant and insect pest
-plant evolves chemical to ward off insect, insect evolves to
combat the chemical.
3.
a. unrelated species evolve similar traits even if they are in a
different part of the world.
-develop similarities in morphology (structure), behavior, and
physiology (body function)
b. ex. flying squirrel and flying phalanger
c. ex.
4. Rate of speciation (new species)
page 441 Figure 25
a.
b. GRADUALISM theory
-evolution takes a long time
-fossil record supports this theory
c.
-tries to explain abrupt transitions in the fossil record
-rapid spurts of genetic change causes species to diverge
quickly
-