Download Mechanism of Evolution

Population Genetics and
Evolution
AP Biology
Population Genetics

Macroevolution: evolution on a grade scale


Ex. Evolution of a new species - modern camel
Microevolution: change in the gene frequency
within a population over time

Ex. Peppered moths during the Industrial
Revolution
Genetics Language Review

Alleles - alternate forms of a gene

Ex. Height in plants - T = tall and t = short




Alleles can be dominant or recessive
Locus (pl. loci) - location of a gene on a
chromosome
Homozygous - two alleles are identical (tt)
Heterozygous - two alleles are different (Tt)
Genetics Language Review cont…



Genotype - genetic make-up of an individual
Phenotype - physical and physiological traits
of an individual
Incomplete Dominance - neither of the alleles
are dominant and blending occurs


red, white and pink flowers
Co-dominant - both alleles for the trait are
dominant

black, white and black-white feathers in chickens
Genetic structure of a parent population of field mice
Phenotype
Black Mice
Black Mice
White Mice
Genotype
AA
Aa
aa
Number of
Mice
196
168
36
(total # of mice = 400)
Genotype
Frequencies
Number of
Alleles in
Gene Pool
(total = 800)
Allele
Frequencies
XXXXXXXX
XXXXXXXX
Phenotype
Purple
Purple
White
Genotype
AA
Aa
aa
Number of
Flowers
320
160
20
(total # of flowers = 500)
Genotype
Frequencies
Number of
Alleles in
Gene Pool
(total = 1000)
Allele
Frequencies
XXXXXXXX
XXXXXXXX
The Hardy-Weinberg Principle

A way to determine how real populations
change over time by developing a model of a
population that does not change genetically
from one generation to the next


Actual populations can be compared to the
hypothetical model
If proportions of genotypes do not change from
generation to generation, the genotypes are said
to be in Hardy-Weinberg Equilibrium
The Hardy-Weinberg
equation:
2
p
+ 2pq
2
+q
=1
Where:





p = frequency of dominant allele (ex. A)
q = frequency of recessive allele (ex. a)
p2 = frequency of individuals
homozygous for allele A
2pq = frequency of individuals
heterozygous for alleles A and a
q2 = frequency of individuals
homozygous for allele a
Example: The dominant allele in a
population has the frequency of 0.6

A) Find the frequency of the recessive
allele


NOTE: p + q = 1 (allele frequency)
B) Find the frequencies of the
genotypes in this population.

NOTE: p2 + 2pq +q2 = 1 (genotypic
frequency)
Solution:
Conditions for HardyWeinberg
1.
2.
3.
4.
5.
The population is large (unaffected by
random gene changes)
Mating is random
Isolation (no gene flow between
populations)
No mutations
No natural selection (no genotype has a
reproductive advantage over another)