Слайд 1Chapter 23
The Evolution of Populations
Слайд 2Overview: The Smallest Unit of Evolution
Natural selection acts on individuals,
but only populations evolve.
Genetic variations in populations contribute to evolution.
Microevolution
is a change in allele frequencies in a population over generations.
Two processes, mutation and sexual reproduction, produce the variation in gene pools that contributes to differences among individuals.
Слайд 4Population geneticists measure polymorphisms in a population by determining the
amount of heterozygosity at the gene and molecular levels.
Average heterozygosity
measures the average percent of loci that are heterozygous in a population.
Most species exhibit geographic variation, differences between gene pools of separate populations or population subgroups.
Some examples of geographic variation occur as a cline, which is a graded change in a trait along a geographic axis.
Слайд 5Cline
1.0
0.8
0.6
0.4
0.2
0
46
44
42
40
38
36
34
32
30
Georgia
Warm (21°C)
Latitude (°N)
Maine
Cold (6°C)
Ldh-B b allele frequency
Слайд 6Mutation
Mutations are changes in the nucleotide sequence of DNA.
Mutations cause
new genes and alleles to arise.
Only mutations in cells that
produce gametes can be passed to offspring.
A point mutation is a change in one base in a gene.
Слайд 7The effects of point mutations can vary:
Mutations in noncoding regions
of DNA are often harmless.
Mutations in a gene might not
affect protein production because of redundancy in the genetic code.
Mutations that result in a change in protein production are often harmful.
Mutations that result in a change in protein production can sometimes increase the fitness of the organism in its environment.
Слайд 8Mutations That Alter Gene / Chromosome Number or Sequence
Chromosomal mutations
that delete, disrupt, or rearrange many loci are typically harmful.
Mutation
rates are low in animals and plants.
Mutations rates are often lower in prokaryotes and higher in viruses.
Слайд 9Sexual Reproduction
Sexual reproduction can shuffle existing alleles into new combinations.
In
organisms that reproduce sexually, recombination of alleles is more important
than mutation in producing the genetic differences that make adaptation possible.
Слайд 10Hardy-Weinberg equation tests whether a sexually reproducing population is evolving
A
population is a localized group of individuals (a species in
an area) capable of interbreeding and producing fertile offspring.
A gene pool consists of all the alleles for all loci in a population.
A locus is fixed if all individuals in a population are homozygous for the same allele.
Слайд 11The frequency of an allele in a population can
be calculated.
If there are 2 alleles at a locus, p
and q are used to represent their frequencies.
The frequency of all alleles in a population will add up to 1:
p + q = 1
Hardy-Weinberg equations
Слайд 12The Hardy-Weinberg Principle: a Population
The Hardy-Weinberg principle describes an ideal
population that is not evolving.
The closer a population is to
the criteria of the Hardy-Weinberg principle, the more stable the population is likely to be.
Calculating Genotype Frequencies
p2 + 2pq + q2 = 1
where p2 and q2 represent the frequencies of the homozygous genotypes and 2pq represents the frequency of the heterozygous genotype.
Слайд 13The five conditions for nonevolving populations are rarely met in
nature:
No mutations
Random mating
No natural selection
Extremely large population
No
gene flow
Hardy-Weinberg Ideal Conditions
Слайд 14Applying the Hardy-Weinberg Principle
We can assume the locus that causes
phenylketonuria (PKU) is in Hardy-Weinberg equilibrium given that:
The PKU gene
mutation rate is low
Mate selection is random with respect to whether or not an individual is a carrier for the PKU allele
Natural selection can only act on rare homozygous individuals who do not follow dietary restrictions
The population is large
Migration has no effect as many other populations have similar allele frequencies
Слайд 15The occurrence of PKU is 1 per 10,000 births
q2 =
0.0001
q = 0.01
The frequency of normal alleles is
p = 1
– q = 1 – 0.01 = 0.99
The frequency of heterozygotes / carriers is
2pq = 2 x 0.99 x 0.01 = 0.0198
or approximately 2% of the U.S. population.
Слайд 16Three major factors alter allele frequencies and bring about most
evolutionary change:
Natural selection - nonrandom
Genetic drift - random
Gene flow
- random
Concept 23.3: Natural selection, genetic drift, and gene flow can alter allele frequencies in a population
Слайд 17Natural Selection and Genetic Drift
Natural Selection: Differential success in reproduction
results in certain alleles being passed to the next generation
in greater proportions by the more fit individuals.
Genetic drift: describes how allele frequencies fluctuate randomly from one generation to the next.
The smaller a sample, the greater the chance of deviation from a predicted result.
Genetic drift tends to reduce genetic variation through losses of alleles.
Слайд 18Genetic Drift
Generation 1
CW CW
CR CR
CR CW
CR CR
CR CR
CR
CR
CR CR
CR CW
CR CW
CR CW
p (frequency of CR) = 0.7
q
(frequency of CW ) = 0.3
Generation 2
CR CW
CR CW
CR CW
CR CW
CW CW
CW CW
CW CW
CR CR
CR CR
CR CR
p = 0.5
q = 0.5
Generation 3
p = 1.0
q = 0.0
CR CR
CR CR
CR CR
CR CR
CR CR
CR CR
CR CR
CR CR
CR CR
CR CR
Слайд 19Genetic Drift: The Founder Effect
The founder effect occurs when a
few individuals become isolated from a larger population.
Allele frequencies in
the small founder population can be different from those in the larger parent population.
Слайд 20Genetic Drift: The Bottleneck Effect
The bottleneck effect is a sudden
reduction in population size due to a change in the
environment, such as a natural disaster.
The resulting gene pool may no longer be reflective of the original population’s gene pool.
If the population remains small, it may be further affected by genetic drift.
Слайд 21Genetic Drift: The BottleNeck Effect
Original
population
Bottlenecking
event
Surviving
population
Слайд 22Effects of Genetic Drift: A Summary
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.
Слайд 23 Gene Flow: Immigration & Emmigration
Gene flow consists of the
movement of alleles among populations.
Alleles can be transferred through the
movement of fertile individuals or gametes (for example, pollen).
Gene flow tends to reduce differences between populations over time.
Gene flow is more likely than mutation to alter allele frequencies directly.
Слайд 25Only natural selection consistently results in adaptive evolution.
Natural selection brings
about adaptive evolution by acting on an organism’s phenotype.
Concept 23.4:
Natural selection is the only mechanism that consistently causes adaptive evolution
Слайд 26Natural Selection: Relative Fitness
The natural selection phrases “struggle for existence”
and “survival of the fittest” are misleading as they imply
direct competition among individuals.
Reproductive success is generally more subtle and depends on many factors.
Relative fitness is the contribution an individual makes to the gene pool of the next generation, relative to the contributions of other individuals.
Selection favors certain genotypes by acting on the phenotypes of certain organisms.
Слайд 27Directional, Disruptive, and Stabilizing Selection
Three modes of natural selection:
Directional selection
favors individuals at one end of the phenotypic range.
Disruptive selection
favors individuals at both extremes of the phenotypic range.
Stabilizing selection favors intermediate variants and acts against extreme phenotypes.
Слайд 28Natural Selection
Original population
(c) Stabilizing selection
(b) Disruptive selection
(a) Directional selection
Phenotypes (fur
color)
Frequency of individuals
Original
population
Evolved
population
Слайд 29The Key Role of Natural Selection in Adaptive Evolution
Natural selection
increases the frequencies of alleles that enhance survival and reproduction.
Adaptive
evolution = the match between an organism and its environment.
Слайд 30Natural Selection - Adaptive Evolution
(a) Color-changing ability in cuttlefish
(b) Movable
jaw
bones in
snakes
Movable bones
Слайд 31Because environments change, adaptive evolution is a continuous process.
Genetic drift
and gene flow are random and so do not consistently
lead to adaptive evolution as they can increase or decrease the match between an organism and its environment.
Слайд 32Sexual Selection
Sexual selection is natural selection for mating success.
It can
result in sexual dimorphism, marked differences between the sexes in
secondary sexual characteristics.
Male showiness due to mate choice can increase a male’s chances of attracting a female, while decreasing his chances of survival.
Слайд 34How do female preferences evolve?
The good genes hypothesis suggests that
if a trait is related to male health, both the
male trait and female preference for that trait should be selected for.
Слайд 35The Preservation of Genetic Variation
Various mechanisms help to preserve genetic
variation in a population:
Diploidy maintains genetic variation in the form
of hidden recessive alleles.
Heterozygote advantage occurs when heterozygotes have a higher fitness than do both homozygotes. Natural selection will tend to maintain two or more alleles at that locus.
The sickle-cell allele causes mutations in hemoglobin but also confers malaria resistance.
Слайд 36Heterozygote Advantage
0–2.5%
Distribution of
malaria caused by
Plasmodium falciparum
(a parasitic unicellular eukaryote)
Frequencies of
the
sickle-cell allele
2.5–5.0%
7.5–10.0%
5.0–7.5%
>12.5%
10.0–12.5%
Слайд 37In frequency-dependent selection, the fitness of a phenotype declines if
it becomes too common in the population.
Selection favors whichever phenotype
is less common in a population.
Frequency-Dependent Selection
Слайд 38Frequency Dependent Selection
“Right-mouthed”
1981
“Left-mouthed”
Frequency of
“left-mouthed” individuals
Sample year
1.0
0.5
0
’82
’83
’84
’85
’86
’87
’88
’89
’90
Слайд 39Neutral Variation
Neutral variation is genetic variation that appears to confer
no selective advantage or disadvantage.
For example,
Variation in noncoding regions
of DNA
Variation in proteins that have little effect on protein function or reproductive fitness.
Слайд 40Why Natural Selection Cannot Fashion Perfect Organisms
Selection can act only
on existing variations.
Evolution is limited by historical constraints.
Adaptations are often
compromises.
Chance, natural selection, and the environment interact.
Слайд 41You should now be able to:
Explain why the majority of
point mutations are harmless.
Explain how sexual recombination generates genetic variability.
Define
the terms population, species, gene pool, relative fitness, and neutral variation.
List the five conditions of Hardy-Weinberg equilibrium.
Слайд 42Apply the Hardy-Weinberg equation to a population genetics problem.
Explain why
natural selection is the only mechanism that consistently produces adaptive
change.
Explain the role of population size in genetic drift.
Слайд 43Distinguish among the following sets of terms: directional, disruptive, and
stabilizing selection; intrasexual and intersexual selection.
List four reasons why natural
selection cannot produce perfect organisms.
