Chapter 23: Population
Genetics
HARDY-WEINBERG THEOREM
Microevolution
 Measure of how allele frequencies change over time
 Allele – different forms of a gene

Ex: beetle color – green or brown
23.1
GENETIC VARIATION MAKES EVOLUTION
POSSIBLE
Genetic Variation
 Differences in genes among individuals
Sources of Genetic Variation
 Mutations


Change in DNA sequence
Occur more frequently in
asexually reproducing organisms

Rapid reproduction = gets
sloppy
 Sexual reproduction


Crossing over
Independent assortment


Only half of genes are passed on;
this is random
Random fertilization
23.2
THE HARDY-WEINBERG EQUATION CAN BE
USED TO TEST WHETHER A POPULATION IS
EVOLVING
Population
 Localized group of individuals belonging to the same
species
 Species = group of populations whose individuals
have potential to interbreed
Gene Pool
 Total aggregate of genes in
a population at any one
time
 All alleles at all loci in all
individuals
 Example: flower
population with white and
pink flowers

Population of 500 individuals
20 white (rr)
 320 homozygous pink (RR)
 160 heterozygous pink (Rr)


So, 1000 alleles:

800 R alleles, 200 r alleles
Hardy-Weinberg Theorem
 Frequency of alleles and genotypes in a population’s
gene pool remain constant over generations UNLESS
acted upon by agents other than sexual
recombination (chance)
Hardy-Weinberg Equilibrium
 Allele frequency is constant from generation to
generation
Required Conditions for H-W Equilibrium
 1. Very large population
Required Conditions for H-W Equilibrium
 2. Random mating
Required Conditions for H-W Equilibrium
 3. Isolation from other populations
Required Conditions for H-W Equilibrium
 4. No natural selection acting on the populatiom
Required Conditions for H-W Equilibrium
 5. No net mutations (changes to the DNA code)
Hardy-Weinberg Equation
 p+q=1
 p = frequency of dominant allele
 q = frequency of recessive allele
 p2 + 2pq + q2 = 1
 p2 = frequency of homozygous dominant genotype (AA)
 2pq = frequency of heterozygous genotype (Aa)
 q2 = frequency of homozygous recessive genotype (aa)
23.3
NATURAL SELECTION, GENETIC DRIFT, AND
GENE FLOW CAN ALTER ALLELE
FREQUENCIES IN A POPULATION
Natural Selection & Gene Frequencies
 If an allele gives the organism an advantage, it will
more likely be passed on and it’s frequency will
increase over time
Genetic Drift & Gene Frequencies
 Chance events can cause allele frequencies to
fluctuate, especially in small populations
The Founder Effect & Gene Frequency
 Few individuals isolated from a population start a
new population with a different allele frequency than
the original population
The Bottleneck Effect & Gene Frequency
 Sudden change in the environment reduces the size
of the population
 By chance alone, certain alleles may be over- or
underrepresented or absent in survivors
Gene Flow
 Transfer of alleles
into or out of a
population due to
immigration and
emigration
23.4
NATURAL SELECTION IS THE ONLY
MECHANISM THAT CONSISTENTLY CAUSES
ADAPTIVE EVOLUTION
Relative Fitness
 The contribution an individual
makes to the gene pool of the next
generation relative to the
contributions of other individuals

A genotype's fitness depends on the
environment in which the organism
lives.


The fittest genotype during an ice age, for
example, is probably not the fittest
genotype once the ice age is over.
Fitness lumps everything that matters to
natural selection (survival, mate-finding,
reproduction) into one idea.


The fittest individual is not necessarily the
strongest, fastest, or biggest.
A genotype's fitness includes its ability to
survive, find a mate, produce offspring —
and ultimately leave its genes in the next
generation.
The brown beetles have a
greater fitness relative to
the green beetles.
Types of Selection
 Stabilizing
 Favors intermediates
 Directional
 Favors one extreme
 Disruptive (diversifying)
 Favors both extremes
Sexual Selection
 Individuals with certain inherited traits are more
likely to obtain mates than others
Natural Selection Isn’t Perfection!
Only acts upon existing variations
2. Limited by historical constraints (acts on existing
structures and adaptations)
3. Adaptations are often compromises
4. Chance, natural selection, and the environment
interact
1.