NOTES – CH 23: Population Genetics
• POPULATION: a localized group of individuals
• SPECIES: a group of populations whose individuals have the potential to
• GENE POOL:
individuals in a population
The Hardy-Weinberg Theorem:
● a tool that describes a gene pool of a non-evolving population
● states that allele frequencies and genotypes in a population’s gene pool
over the
generations
● for Hardy-Weinberg equilibrium to occur, the following conditions must be met:
1)
2)
3)
(no immigration or emigration)
4)
(no mating preference for particular phenotype)
5)
(all genotypes have an = chance of surviving & reproducing)
HOWEVER, in nature:
1)
& may mate with one another
2) there are always mutations (chance with every DNA replication)
3) gene flow often occurs between populations
4)
5) natural selection is always occurring
**Therefore, in nature there will always be changes in populations (“microevolution”)
**So why study population genetics? Why use the H-W Theorem?
1) shows how
is related to
;
2) provides a benchmark genetic equilibrium against which change can be noted;
3) permits an
; especially useful in estimating the number
of carriers of lethal alleles in human populations.
Hardy-Weinberg Equation:
Draw Punnett Square below:
● p = frequency of dominant allele (A)
● q = frequency of recessive allele (a)
●p+q=1
● frequency of possible diploid combinations (AA, Aa, aa):
p2
(AA)
+
2pq
(Aa)
+
q2
=
1
(aa)
Example: If the frequency of a recessive allele is 35% in a population of 1500 people, how many people would you predict would
be carriers of this allele, but would not express the recessive phenotype?
Example: In a population with 2 alleles for a particular locus, B and b, the allele frequency of B is 0.78. If the population consists of
172 individuals, how many individuals are heterozygous? How many will show the recessive phenotype?
DEFINITIONS:
 Microevolution = studies how pop’s of organisms change from generation to generation;
 Macroevolution = studies changes in groups of related species over long periods of geologic time; determines
Causes of Microevolution:
1) Natural Selection
2) Genetic Drift (changes in the gene pool of a small population
)
Examples:
-Bottleneck Effect: results from
-Founder Effect: few individuals in a population
3) Gene Flow (
between populations)
4) Mutation (introduces
into a population)
5) Nonrandom Mating (individuals
based upon their traits)
Ways Natural Selection Acts on a Population:
1) Stabilizing Selection: eliminates individuals with
; existing
population frequencies of common traits are maintained
Example of Stabilizing Selection in humans:
*human babies most commonly weigh 3-4 kg; babies much smaller or larger have higher infant mortality rates.
2) Directional Selection:
of a range of traits; common during
periods of environmental change
Examples:
-
-
Peppered Moth example:
 100 years after the first dark moth was discovered in 1848,
;
 the light variety continued to dominate in unpolluted areas outside of London.
3) Diversifying (a.k.a. Disruptive) Selection: occurs when environment
while selecting against common traits
4) Sexual Selection: differential mating of males in a population;
-females tend to increase their fitness by increasing the quality of their offspring by choosing superior male mates (and
are therefore “choosier” or more selective when finding a mate)
-males increase their fitness by maximizing the quantity of offspring produced
**as a result, in vertebrate species, the male is typically the “showier” sex
-
-
-
● INTRASEXUAL SELECTION = direct competition among individuals of one sex (
, etc.)
● INTERSEXUAL SELECTION = “
”; individuals of one sex are choosy (usually the females)