Transcript Chp.-23-Evolution-of-Populations

THE EVOLUTION OF
POPULATIONS
EVOLUTION AND VARIATION
Microevolution- small scale evolution; change in
allele frequencies in a population over
generations.
 Discrete Characters- classified on an either-or
basis
 Quantitative Characters- vary along a continuum
 Average Heterozygosity- (gene variability) the
average percent of loci that are heterozygous.
 Nucleotide Variability- comparing DNA
sequences of two individuals
 Geographic Variation- differences in genetic
composition of separate populations.

MUTATION
Mutation- the ultimate source of new alleles
 Point mutations- a change in one base in a gene
 Neutral and Beneficial Mutations
 Mutations Rates




Plants/Animals- 1/100,000 genes per generation
Prokaryotes- fewer mutations, shorter generation
span, more genetic variation
Viruses- more mutations, shorter generation span,
RNA genome with fewer repair mechanisms
GENE POOLS AND ALLELE FREQUENCY
Population- a group of individuals of the same
species that live in the same area and interbreed,
producing fertile offspring.
 Gene pool- all of the alleles for all the loci in all
individuals of the population.


Fixed- only one allele exists for a particular locus and
all individuals are homozygous for that allele
HARDY-WEINBERG PRINCIPLE
H-W Equilibrium describes a constant frequency
of alleles within a gene pool.
 p2 + 2pq + q2 = 1

 where
p2 and q2 represent the frequencies of the homozygous
genotypes and 2pq represents the frequency of the
heterozygous genotype
Alleles in the population
Frequencies of alleles
p = frequency of
CR allele
= 0.8
q = frequency of
CW allele
= 0.2
Gametes produced
Each egg:
Each sperm:
80%
20%
chance chance
80%
20%
chance chance
HARDY-WEINBERG ASSUMPTIONS
1. No
mutations
2. Random mating
3. No natural selection
4. Extremely large population size
5. No gene flow
*Departure from any of these conditions usually
results in evolutionary change.
PRACTICE HARDY-WEINBERG PROBLEM

For a locus with two alleles (A and a) in a
population at risk from an infections
neurodegenerative disease, 16 people had
genotype AA, 92 had genotype Aa, and 12 had
genotype aa. Use the Hardy-Weinberg equation
to determine whether this population appears to
be evolving.
MECHANISMS THAT ALTER ALLELE
FREQUENCY

Natural selection


Leads to adaptive radiation
Genetic drift
Founder Effect
 Bottleneck Effect


Gene flow
Fig. 23-8-3
CR CR
CR CR
CW CW
CR CW
CR CW
CR CR
CW CW
CR CR
CR CW
CR CR
CR CW
CR CW
Generation 1
p (frequency of CR) = 0.7
q (frequency of CW ) = 0.3
CW CW
CR CW
CR CR
CR CR
CR CR
CW CW
CR CR
CR CW
CR CR
CR CR
CR CR
CR CR
CR CR
CR CR
CR CR
CR CW
Generation 2
p = 0.5
q = 0.5
CR CR
CR CR
Generation 3
p = 1.0
q = 0.0
Fig. 23-9
Original
population
Bottlenecking
event
Surviving
population
EFFECTS OF GENETIC DRIFT
1.
2.
3.
4.
Genetic drift is significant in small populations
Genetic drift causes allele frequencies to
change at random
Genetic drift can lead to a loss of genetic
variation within populations
Genetic drift can cause harmful alleles to
become fixed
MECHANISMS THAT ALTER ALLELE
FREQUENCY

Natural selection


Leads to adaptive radiation
Genetic drift
Founder Effect
 Bottleneck Effect


Gene flow

the transfer of alleles into or out of a population due
to the movement of fertile individuals or their
gametes.
NATURAL SELECTION AND ADAPTIVE
EVOLUTION
Relative Fitness- the contribution an individual
makes to the gene pool of the next generation,
relative to the contributions of other individuals.
 Natural selection is the only evolutionary
mechanism that continually leads to adaptive
evolution.

DIRECTIONAL
SELECTION
Original population

Occurs when
conditions favor
individuals
exhibiting one
extreme of a
phenotypic range,
thereby shifting
the frequency
curve for the
phenotypic
character in one
direction or
another.
Phenotypes (fur color)
Original population
Evolved population
(a) Directional selection
Fig. 23-13b
DISRUPTIVE
SELECTION

Occurs when
conditions favor
individuals at both
extremes of a
phenotypic range
over individuals
with intermediate
phenotypes.
Original population
Phenotypes (fur color)
Evolved population
(b) Disruptive selection
Fig. 23-13c
STABILIZING
SELECTION

Acts against both
extreme phenotypes
and favors
intermediate
variants.
Original population
Phenotypes (fur color)
Evolved population
(c) Stabilizing selection

Sexual Selection


Sexual Dimorphism


marked differences between the two sexes in
secondary sexual characteristics, which are not
directly associated with reproduction or survival.
Intrasexual Selection


a form of natural selection in which individuals with
certain inherited characteristics are more likely than
other individuals to obtain mates.
Selection within the same sex. Individuals of one sex
compete directly for mates of the opposite sex.
Intersexual Selection

“mate choice”- individuals of one sex (usually
females) are choosy in selecting their mates from the
other sex.
PRESERVATION OF GENETIC VARIATION

Diploidy


Balancing Selection


Individuals who are heterozygous at a particular
locus have greater fitness than do both kinds of
homozygotes
Frequency-Dependent Selection


Occurs when natural selection maintains two or more
forms in a population.
Heterozygote Advantage


Hides genetic variation from selection in the form of
recessive alleles
The fitness of a phenotype declines if it becomes too
common in the population
Neutral Variation
WHY NATURAL SELECTION CANNOT
FASHION PERFECT ORGANISMS
Selection can only act on existing variations.
 Evolution is limited by historical constraints.
 Adaptations are often compromises
 Chance, natural selection, and the environment
interact.

EXIT SLIP
Of all the mutations that occur in a population,
why do only a small fraction become widespread
among the population’s members?
 If a population stopped reproducing sexually (but
still reproduced asexually), how would its genetic
variation be affected over time? Explain.
