PA Standard: 3.3D
Chapter 16-1
Genetic Equilibrium
Variations of Traits in a Population
Population genetics – study of evolution from genetic point of view.
POPULATION IS THE SMALLEST UNIT THAT CAN EVOLVE.
– Within a population, individuals vary in observable traits.
Examples: fish length, human height and weight, hair color…
Causes of Variation
 Variations are influenced by environment and heredity.

Variations in genotype arise in three main ways:
1. mutations – changes in DNA
2. recombination – independent assortment and crossing-over
3. random fertilization – which sperm cell will fertilize which egg is game
of chance.
Allele Frequencies and the Gene Pool
Gene pool – all the genes in a population at any one time.
Allele frequency – how common is an allele in a population.
 number of individuals with a particular phenotype divided by total number of
individuals.
Example: 50% “R” and 50% “r”.
Phenotypic frequency – how common is a phenotype in a population
 number of a certain allele divided by the total number of alleles of all types in
the population.
Example: 25% Red, 50% Pink Flowers, and 25% White Flowers
Hardy-Weinberg Genetic Equilibrium
Both Hardy and Weinberg showed that allele frequencies in a population tend to remain the
same from generation to generation unless influenced by the environment.
A population is in an equilibrium and, therefore, is not evolving if:
1.
2.
3.
4.
NO MUTATIONS - Allele frequencies are not changing because of mutations.
NO MIGRATION - Individuals neither enter nor leave the population.
LARGE POPULATION - The population is large.
RANDOM MATING - Individuals mate randomly, not only with those closest to them.
5. NO NATURAL SELECTION - Natural selection does not occur.
PA Standard: 3.3D
Chapter 16-1
Genetic Equilibrium
Variations of Traits in a Population
Population genetics – study of evolution from
genetic point of view.
POPULATION IS THE SMALLEST UNIT THAT CAN
EVOLVE.
– Within a population, individuals vary in
observable traits.
Examples: fish length, human height and weight,
hair color…
Causes of Variation
 Variations are influenced by environment
and heredity.
 Variations in genotype arise in three main
ways:
1. mutations – changes in DNA
2. recombination – independent
assortment and crossing-over
3. random fertilization – which sperm cell
will fertilize which egg is game of
chance
PA Standard: 3.3D
Allele Frequencies and the Gene Pool
Gene pool – all the genes in a population at any
one time.
Allele frequency – how common is an allele in a
population.
 number of individuals with a particular
phenotype divided by total number of
individuals.
Example: 50% “R” and 50% “r”.
Phenotypic frequency – how common is a
phenotype in a population
 number of a certain allele divided by the total
number of alleles of all types in the population.
Example: 25% Red, 50% Pink, & 25% White
Flowers
RR
rr
RR
Rr
Rr
rr
Rr
Rr
RR
RR
rr
rr
Freq. of allele R:
Freq. of allele r:
Freq. of red flowers:
Freq. of pink flowers:
Freq. of white flowers:
PA Standard: 3.3D
Hardy-Weinberg Genetic Equilibrium
Both Hardy and Weinberg showed that allele
frequencies in a population tend to remain the same
from generation to generation unless influenced by the
environment.
A population is in an equilibrium and, therefore, is not
evolving if:
1. NO MUTATIONS - Allele frequencies are not
changing because of mutations.
2. NO MIGRATION - Individuals neither enter nor
leave the population.
3. LARGE POPULATION - The population is large.
4. RANDOM MATING - Individuals mate randomly,
not only with those closest to them.
5. NO NATURAL SELECTION - Natural selection
does not occur.