Download Lecture Ch 23 The evolution of populations

CH 23: The Evolution of Populations
I.
Population Genetics
A. Relates genetics to evolution in various populations populations
Modern Synthesis- importance of populations as units of evolution with natural selection as the
main mechanism. Emphasizes gradualism to explain how large changes occurred by smaller
changes over time but that idea is continually being challenged.
B. Populations
1. Population- localized group of individuals belonging to the same species
2. species-individuals belong to the same species when they have the potential to interbreed and
produce fertile offspring in nature
3. gene pool-total aggregate of genes in a population at a given time/this is what natural selection
acts on at that time
A. 4. microevolution – a generation to generation change in a population’s frequencies al alleles
or genotypes-a change in a population’s genetic structure
II.
Hardy-Weinberg Theorem (nonevolving)
states that the frequencies of alleles and genotypes in a population’s gene pool remain constant
over generations unless acted on by agents other than sexual recombination (meiosis/random
fertilization)
A. Conditions needed to maintain equilibrium
a.
the population size must be relatively large-if not, genetic drift will occur
b.
the population must be isolated from other populations-if not gene flow will begin
c.
there must be no mutations of genes
d.
mating must be random-if not, certain traits will be passes vs others
e.
natural selection must not be present
2. Vocabulary and Equations
1.
p = frequency of dominant alleles q = frequency of recessive alleles
p2 + 2pq + q2 = 1
2
2.
p (AA) both alleles are homozygous dominant. (Remember you get two alleles- one
from mom and one from dad)
3.
2pq (Aa or aA)– one allele is dominant and one is recessive. (Multiply by 2 because
you could get p from mom and q from dad or p from dad and q from mom so two
variations)
4.
q2 (aa) both alleles are homozygous recessive
5.
PRACTICE YOUR H-W PROBLEMS
II. Microevolution
A. Causes of microevolution
1. genetic drift- changes in the gene pool of a small population due to chance
if you flip a coin 1000 times and 700=heads, 300=tails, suspicious
if flip a coin 10 times and 7=heads, 3=tails, think chance
a small number of samples (trials) allows chance to play a more important role
Therefore, small populations offset H-W equilibrium ( large populations are best to show if
evolution is occurring based on the H-W equilibirum)
2. bottleneck effect- can cause genetic drift by some disaster (natural or human-caused)
unselectively killing individuals The surviving population is not representative of the
original population, therefore it’s like genetic drift.
3. founder effect- can cause genetic drift by a few individuals colonizing a new isolated
island, bringing in small amounts of genetic variation
4. gene flow- genetic exchange due to the migration of fertile individuals or gametes between
populations. Tends to reduce differences between populations due to genetic drift or natural
selection and can make them one population again with a common genetic make-up
5. Mutations- mutations of genes may affect p and q frequencies
Although mutations are a rare event, if the mutant form is being passed on more frequently,
genetic frequencies will move away from the H-W equilibrium. Mutations usually harm an
organism, but can be beneficial when the environment is changing. Mutations’ effects can be
seen in faster reproducing species (bacteria, insects)
6. Nonrandom mating
a.
Inbreeding-mating between closely-related partners
b.
assortative mating-individuals select partners that are like themselves and will
cause genetic frequency to move away from the H-W equilibirium
7. Natural selection- H-W equilibrium requires that all individuals within a population have
an equal chance of passing on genes to offspring, but natural selection doesn’t make this so
III.
Genetic variation-raw material for natural selection to work on which then causes evolution
A. polymorphism-when a population has two or more distinct morphs (discrete traits like red
and white flower petals) at equally high frequencies to be noticeable Ex. spotted and striped
snakes
B. geographical variation-variation between populations due to differences in the environment
a.
cline-a graded change of some trait along a geographical axis ex. the average
body size of several North American bird and mammal species increase
gradually with increasing latitude
C. Mutation and sexual recombination- Sexual recombination is more important than
mutation on generation to generation variation which then makes evolution occur.
Recombination occurs during meiosis.
D. Diploidy and balanced polymorphism makes sure variation lives on
1. diploid-each individual have two types of alleles for a gene (one from each parent)
2.
ensure that even recessive alleles are maintained within the gene pool
E. Balanced polymorphism-ability of natural selection to maintain genetic variability
a.
heterozygote advantage-if heterozygotes are most successful, two or more
alleles will remain in the gene pool Ex. sickle-cell anemia, if homozygous
recessive, organisms die due to lack of O2, if homozygous dominant, they
die if bit by mosquitoes carrying malaria, if heterozygous, organisms live
since they have only a few abnormal red blood cells and have immunity to
malaria
IV.
Natural Selection as a Mechanism
A. Evolutionary Fitness- the relative contribution an individual makes to the gene pool of the
next generation based on what genotypes are. The relative fitness of an individual is based not
only on one allele, but how that alleles works with all the alleles in the individual Ex an allele for
high tree trunk growth rate doesn’t help if the alleles for root support are lacking
B. Modes of natural selection - natural selection can affect phenotype frequency in 3 ways
1.
stabilizing selection-acts against extreme phenotypes Ex. human baby weights
2.
directional selection-shifts the frequency curve toward a certain phenotype due to
environment at that time
3.
disruptive selection-favors extreme phenotypes
C. sexual dimorphism-Ex male peacocks very beautiful while females are less colorfulmales and females look very different. Sexual selection leads to sexual dimorphism where
females choose the more ornamental males or the ones with the best nests
D. Natural Selection doesn’t produce “perfect” creatures
1. organisms are locked into historical constraints- adaptations are usually compromiseswhile some adaptations may be beneficial, they may take away from other adaptations
that are also. Ex. seals would move better on rocks with legs, but less efficient in water
due to lack of flippers
2. not all evolution is adaptive-genetic drift will cause evolution due to chance not
adaptation
3. selection can only edit variations that exist now, not what might happen in the future.