Download File - Lucinda Supernavage

AP
Biology
4/12/2016
• Today:
– Review Hardy-Weinberg ?s
– Microevolution Lectures
• Practice Exam this Saturday with Ms. Morgan at 8:30 –
12:30
• Practice Exam on April 30th with me???????
• Teacher-made final exam on April 26 & 27
• Bio AP Exam Monday, May 9 @ 8 am
• Cell Division Packets due FRIDAY!
• March Root Word Quiz Friday!
• Hardy Weinberg and Microevolution Quiz Tuesday
• MAKE-UPS BY THURSDAY: Chi Square & Genetics Quiz
Population Genetics
• Combines Darwinian selection and Mendelian
inheritance
• The study of genetic variation within a
population.
• Emphasis on quantitative characters.
• Population - a localized group of individuals
of the same species
• A population’s gene pool is the total of all
genes in the population at any one time.
• If all members of a population are
homozygous for a particular allele, then the
allele is FIXED in the gene pool.
Allele frequencies define gene pools
500 flowering plants
480 red flowers
320 RR
160 Rr
20 white flowers
20 rr
THERE ARE 1000 COPIES OF THE GENE
FOR COLOR IN THIS POPULATION! Why???
The gene frequencies are as follows:
320 x 2 (RR) + 160 x 1 (Rr) = 800 R
800/1000 = 0.8 (80%) R
160 x 1 (Rr) + 20 x 2 (rr) = 200 r
200/1000 = 0.2 (20%) r
The Hardy-Weinberg Theorem
• Used to describe a non-evolving population.
• Shuffling of alleles by meiosis and random
fertilization have no effect on the overall
gene pool.
• Natural populations are actually not
expected to actually be in Hardy-Weinberg
equilibrium.
• Deviation from H-W equilibrium usually
results in evolution.
• Understanding a non-evolving population,
helps us to understand how evolution occurs.
Assumptions of the H-W Theorem:
- Large population size: small populations can have chance
fluctuations in allele frequencies (e.g., fire, storm).
- No migration: immigrants can change the frequency of an
allele by bringing in new alleles to a population.
- No net mutations: if alleles change from one to another,
this will change the frequency of those alleles.
- Random mating: if certain traits are more desirable, then
individuals with those traits will be selected and this will not
allow for random mixing of alleles.
- No natural selection: if some individuals survive and
reproduce at a higher rate than others, then their offspring
will carry those genes and the frequency will change for the
next generation.
Hardy-Weinberg Equilibrium
The gene pool of a non-evolving population
remains constant over multiple generations; i.e., the
allele frequency does not change over generations
of time.
The Hardy-Weinberg Equation:
1.0 = p2 + 2pq + q2
where p2 = frequency of AA genotype; 2pq =
frequency of Aa plus aA genotype; q2 = frequency
of aa genotype
But we know that evolution does occur within populations.
Evolution within a species/population = microevolution.
Microevolution refers to changes in allele frequencies in
a gene pool from generation to generation.
Represents a gradual change in a population.
Causes of microevolution:
1) Genetic drift
2) Natural selection (1 & 2 are most important)
3) Gene flow
4) Mutation
1) Genetic drift
the alteration of the gene pool of a small population due
to CHANCE
Two factors may cause genetic drift:
a)Bottleneck effect may lead to reduced genetic variability
following some large disturbance that removes a large portion
of the population.
b) Founder effect may lead to reduced variability when a few
individuals from a large population colonize an isolated
habitat.
2) Natural selection
differential success in reproduction based on heritable
traits results in selected alleles being passed to
relatively more offspring (Darwinian inheritance).
 The only agent that results in adaptation to
environment.
 Survival of the fittest
 Increases the frequency of certain alleles in gene
pools
3) Gene flow
genetic exchange due to the migration of fertile
individuals or gametes between populations.
 Migration
 Pollen
 Moving to new countries
4) Mutation: change in an organism’s DNA and is
represented by changing alleles.
Mutations can be transmitted in gametes to
offspring, and immediately affect the composition
of the gene pool.
The original source of variation
New alleles originate only by mutations
Genetic (heritable) variation within and between
populations:
 exists both as what we can see (e.g., eye color)
and what we cannot see (e.g., blood type).
Not all variation is heritable…
 environment also can alter an individual’s
phenotype
 Hydrangeas (pH of soil)
Butterflies
color changes
are due to
seasonal
temperatures
difference in
hormones of the
endocrine
system
Polyphenism
 Geographic isolation leads to differences in gene pools
 Sexual recombination is the source of most genetic
differences between individuals in a population.
 Diploidy often hides genetic variation from selection in
the form of recessive alleles.
 Dominant alleles “hide” recessive alleles in
heterozygotes
 Neutral variation: genetic variation that results in no
competitive advantage (human fingerprints)
 Balanced polymorphism is the ability of natural selection
to maintain stable frequencies of at least two phenotypes
 Heterozygote advantage is one example of a
balanced polymorphism, where the heterozygote
has greater survival and reproductive success than
either homozygote
 Example: Sickle cell anemia where heterozygotes
are resistant to malaria
 Frequency-dependent selection = survival of one
phenotype declines if that form becomes too
common.
 Directional selection favors individuals at one end of
the phenotypic range.
 Diversifying selection favors BOTH extremes over
intermediate phenotypes.
Also called
DISRUPTIVE SELECTION
 Stabilizing selection natural selection favors an
average phenotype and selects against extreme
variations
Sexual dimorphism is the difference in appearance
between males and females of a species.
 Intrasexual selection is the direct competition between
members of the same sex for mates of the opposite
sex.
 This gives rise to males most often having
secondary sexual equipment that are used in
competing for females
 Antlers, manes, etc.
 Intersexual selection (mate choice), one sex is choosy
when selecting a mate of the opposite sex.
 This gives rise to often amazingly sophisticated
secondary sexual characteristics
 Peacock feathers
Natural selection does NOT produce perfect
organisms
a. Evolution is limited by historical constraints
o humans have back problems because our
ancestors were 4-legged.
b. Adaptations are compromises.
o Humans are athletic due to flexible limbs,
which often dislocate or suffer torn ligaments.
c. Not all evolution is adaptive. Chance probably
plays a huge role in evolution and not all changes are
for the best.
d. Selection edits existing variations. New alleles
cannot arise as needed, but most develop from what
already is present.