EVOLUTION: GENES AND
POPULATIONS
CH 23
http://evolution.berkeley.edu/evolibrary/
news/070401_lactose
Organisms DO NOT evolve. Natural
selection acts on individuals causing
populations to evolve
Microevolution: changes in allele
frequencies over time. Caused by:
 Natural selection
 Genetic drift
 Gene flow
Only natural selection causes adaptive
evolution
I.
Genetic Variation and Evolution
In order for evolution to occur,
there must be genetic variation in a
population
A. Where does genetic variation come
from?
 New genes and alleles come
from mutation and gene
duplication
 Gene recombination during
meiosis: reshuffles alleles during
sexual reproduction
 Mutation is most important in
asexually reproducing
organisms while gene
recombinations are more
important in sexually
reproducing organisms
B. Hardy Weinberg Equilibrium and
Allele Frequencies
1. The Hardy Weinberg equilibrium
Population: a group of actively
interbreeding members of a
species
Gene pool: all of the alleles
present in a population
The Hardy-Weinberg principle
states that allele frequencies in a
population will remain unaltered
from generation to generation IF
the following criteria are met:
 Extremely large population
 No mutation
 No selection
 No gene flow (migration)
 Mating is random
2. Using Hardy Weinberg to
calculate allele frequencies
p2 + 2pq +q2 = 1
p + q =1
Ex:
Albinism is a recessive disorder
and occurs with a frequency of
1/20000 people
What is the frequency of
individuals who are:
Dominant
Recessive
Heterozygous
II. Why Do allele Frequencies Change?
 In natural populations, allele
frequencies DO change over time
(this is the basis for evolution)
 Hardy Weinberg equilibrium
establishes baseline to measure
change
 Allele frequencies can change due
to mutation, gene flow, genetic
drift, and natural selection
A. Mutation: introduces new alleles
B. Gene flow (migration): individuals
and their alleles enter and leave the
population
gene flow limits variation and
reduces speciation
new genes can enter into the
population, which may have a
negative effect. This lowers
survivability
 mating causes gene flow between
central and eastern populations
 Immigration from mainland
introduces alleles that decrease fitness
o High immigration between
mainland and central region
o Low immigration between
mainland and eastern region
 Gene flow can also increase fitness of
population
o Insecticide resistant genes entering
a population can increase the
fitness of that population
C. Genetic drift: in small populations,
allele frequencies change
RANDOMLY from generation to
generation
https://highered.mcgrawhill.com/sites/dl/free/0072835125/126997/
animation45.html
 Bottleneck effect: a drastic change
in environment could significantly
reduce population size. New allele
frequency may not reflect
individual population
Shaking just a few marbles through the
narrow neck of a bottle is analogous to a
drastic reduction in the size of a population
after some environmental disaster. By chance,
blue marbles are over-represented in the new
population and gold marbles are absent.
Think of the effect of excessive hunting
on animal populations
 Founder effect: a few individuals
from a population get separated
from the rest
Genetic drift is:
 More significant in small
populations and RANDOMLY
causes allele frequencies to change
 Can reduce variation in a
population
 Can cause unfavorable alleles to
become fixed
D. Selection: environment or humans
select for favorable alleles which are
then passed on.
 Selection is the only mechanism of
adaptive evolution
 It is the fitness of individuals that
affects future generations
 Fitness is the ability to reproduce
and pass on alleles
 Natural selection acts on
organisms’ phenotype which thus
favors certain genotypes
 Types of natural selection:
o Selection to match
environment
o Selection to avoid predators:
Camouflage
Mimicry
Poison
o Selection for pesticide
resistance
o Sexual selection: natural
selection for mating
o Intrasexual selection: members of
the same sex compete to mate
o Intersexual selection: when females
choose their mate
 The three modes of selection
o Directional selection: favors
individuals at one end of
phenotypic range
o Disruptive selection: favors
individuals at both ends of
phenotypic range
o Stabilizing selection: acts
against extreme phenotypes
III. Maintaining Alleles in a population
Natural selection tends to remove
variation by favoring certain
alleles
Mechanisms to preserve variation:
o Diploidy: Recessive allele is
hidden in heterozygote. Explains
why Tay Sachs is still around
o Heterozygote advantage: in the
case of malaria, being
heterozygous for sickle cell
anemia protects you from
malaria and you don’t have
sickle cell anemia
o Frequency dependent selection:
fitness of a phenotype decreases
if it becomes too common in the
population