Download Natural Selection

AP Biology
Enduring Understanding 1A
Change in genetic makeup of
a population over time is
evolution.
Natural Selection is a major
mechanism of evolution
Natural Selection – in a population, organisms
with a genetic trait that increases the chance of
having offspring will pass on their genes to the
next generation more than those without.
Reproductive Fitness – relative ability to survive
and leave offspring
Requirements
1. More offspring born than can survive to
adulthood
2. Competition for resources
3. Genetic variation/diversity within the
population
4. Some varieties have an increased chance to
survive long enough to reproduce.
Genetic variation and mutation play roles in
natural selection. A diverse gene pool is
important for the survival of a species in a
changing environment.
Environments can be more or less stable or
fluctuating, and this affects evolutionary rate
and direction; different genetic variations can be
selected in each generation.
An adaptation is a genetic variation that is
favored by selection and is manifested as a trait
that provides an advantage to an organism in a
particular environment.
In addition to natural selection, chance and
random events can influence the evolutionary
process, especially for small populations.
Mathematical approaches are used to calculate
changes in allele frequency, providing evidence
for the occurrence of evolution in a population.
This can be done using the Hardy-Weinberg
Principle
The Hardy-Weinberg Principle
The Hardy-Weinberg law is an algebraic formula
to estimate the frequency of a dominant or
recessive gene in a population based on the
frequency with which the trait or condition is
found in a population.
A population with:
1. A large number of individuals
2. No migration
3. No mutations
4. Random mating
5. No natural selection
Is said to be in Hardy-Weinberg equilibrium and
no evolution can occur.
The equation is p2 + 2pq + q2 = 1 where:
p = the frequency of the dominant allele
q = the frequency of the recessive allele
p2 = the frequency of the homozygous dominant
genotype
2pq = the frequency of the heterozygous
genotype.
p2 = the frequency of the homozygous recessive
genotype.
Cystic fibrosis is a genetic disorder in
homozygous recessives that causes death during
the teenage years. If 4 in 10,000 newborn babies
have the disease, what are the expected
frequencies of the three genotypes in newborns,
assuming the population is at HardyWeinberg equilibrium? Why is this assumption
not strictly correct?
The allele y occurs with a frequency of 0.8 in a
population of clams. Give the frequency of
genotypes YY, Yy, and yy. Show your work!
In the year 2374, humans finally developed the technology necessary for
time travels. You are a scientist interested in the population genetics of
extinct animals. Taking advantage of this technological advance, you
decide to go to the past 8 million years to conduct a field work in
Venezuela to study a population of Phoberomys pattersoni*, the world’s
largest extinct rodent weighing approximately 700 kg (1500 lb) and
looking vaguely like a giant guinea pig. The coat color of this rodent varies
between tan (dominant) and brown (recessive). Assume the population is
in Hardy-Weinberg equilibrium. You observed 336 tan Phoberomys and 64
brown Phoberomys during your study.
a) What is the frequency of the homozygous
recessive genotype?
b) What is the allelic frequency of the dominant
(tan) allele in the population?
c) Of the animals you observed, how many were
heterozygous?
You make another trip to Venezuela and this time
you observe 650 animals.
d) How many of the 650 animals would you
expect to be tan, assuming the population is
still in Hardy-Weinberg equilibrium?
e) How many of these tan animals are
homozygous for the dominant allele?
f) How many of these 650 animals would you
expect to be brown, assuming the population is
still in Hardy-Weinberg equilibrium?
Key Vocabulary
1. Biological Evolution
2. Natural Selection
3. Reproductive Fitness
4. Competition
5. Variation
6. Adaptation
7. Hardy-Weinberg Principle
8. Genetic equilibrium
9. Hardy-Weinberg Conditions
10. Hardy-Weinberg Equation