Download The Evolution of Populations

Chapter 23
The Evolution of Populations
Overview: The Smallest Unit of Evolution
•
One misconception is that organisms evolve, in the Darwinian sense, during
their lifetimes
–
Natural selection does act on individuals in that each organism’s
combination of traits affects its survival and reproductive success
compared to other individuals
• Ex) Finches with larger, deeper beaks showed an increased rate of
survival during a long period of drought in the Galapagos
– The beaks of individual finches, however, did not grow during
their lifetime in response to the drought
–
The evolutionary impact of natural selection, therefore, is only apparent
in the changes in a population of organisms
over time
Fig. 23-1
• Ex) The average beak size in the next
generation of finches was greater than
in the pre-drought population
Overview: The Smallest Unit of Evolution
• The changes in these finch populations of the Galapagos is an
example of evolution on its smallest scale, or microevolution
– Microevolution: the change in allele frequencies in a
population over generations
– Microevolution can occur as a result of:
• Natural selection
• Genetic drift: chance events that alter allele frequencies
• Gene flow: the transfer of alleles between population
– Although each of these mechanisms affects genetic
composition of populations, only natural selection
consistently improves the match between organisms and
their environment (adaptive evolution)
Concept 23.1: Mutation and sexual reproduction produce
the genetic variation that makes evolution possible
• Darwin knew that natural selection could not cause
evolutionary change unless individuals differed in
their inherited characteristics
– Two processes produce the variation in gene
pools that contributes to differences among
individuals:
• Mutation
• Sexual reproduction
Genetic Variation
•
Variation in individual genotype leads to variation in individual phenotype
–
However, not all phenotypic variation is heritable
• Ex) The caterpillars of the moth N. arizonaria owe their different
appearances to chemicals in their diet, not genotype
– Caterpillars raised on a diet of oak flowers resemble the
flowers
– The siblings of these caterpillars that were raised on oak
leaves resemble oak twigs
–
•
Fig. 23-2
Phenotype can be the product of genetics, the environment, or a
combination of both
Natural selection can only act on variation with a genetic component
(a)
(b)
Variation Within a Population
• Characters that vary within a population maybe discrete or
quantitative
– Discrete characters can be classified on an either-or basis
• Ex) Purple or white flowers
– Many discrete characters are determined by a single
gene locus with different alleles that produce distinct
phenotypes
– Quantitative characters vary along a continuum within a
population
• Ex) Height
– Heritable quantitative variation usually results from
the influence of 2+ genes on a single phenotypic
character
•
Biologists can measure genetic variation in a population at both:
–
The whole-gene level (gene variability)
• Gene variability can be measured as the average heterozygosity,
the average percent of loci that are heterozygous in a population
– Ex) On average, a fruit fly is heterozygous for ~1920 (14%) loci
for its 13,700 genes
• Thus, the fruit fly has an average heterozygosity of 14%
– Average heterozygosity is usually estimated by examining the
protein products of genes using gel electrophoresis
–
The molecular level of DNA (nucleotide variability)
• Nucleotide variability is measured by comparing the DNA
sequences of 2 individuals in a population and then averaging the
data from many such comparisons
– Ex) The fruit fly has ~180 million nucleotides and 2 fruit flies
differ in this sequence by ~1.8 million (1%) of their nucleotides
• Thus the fruit fly population has a nucleotide variability of
~1%
Variation Between Populations
•
Most species also exhibit geographic variation, differences in the genetic
composition of separate populations
–
Some of this variation appears to have resulted from chance events
(genetic drift) rather than natural selection
• This type of variation leaves genes intact, meaning that the
phenotypic effects of these genetic differences seem to be neutral
–
Ex) Several population of house mice (M. musculus) have evolved in
isolation from one another, during which time many of the original
chromosomes have become fused
Fig. 23-3
• The patterns of fused chromosomes, however, differ from one
population to another
1
2.4
8.11
9.12
3.14
5.18
10.16 13.17
6
7.15
19
XX
1
2.19
3.8
4.16 5.14
9.10 11.12 13.17 15.18
6.7
XX
Other examples of geographic variations occur as a cline, a graded change
in a trait along a geographic axis
•
Some clines are produced by a graded change in an environmental
variable
• These types of variation are generally the result of natural selection
due to the close association between the environmental variable
and the frequency of particular alleles
•
Ex) The impact of temperature on the frequency of the cold-adaptive
allele in mummichog fish
1.0
Fig. 23-4
• Individuals with this allele
produce enzymes that are
better catalysts in cold water
than other versions of the
same enzyme, meaning they
can swim faster in cold water
Ldh-B b allele frequency
•
0.8
0.6
0.4
0.2
0
46
44
Maine
Cold (6°C)
42
40
38
36
Latitude
(°N)
34
32
30
Georgia
Warm
(21°C)
Mutation
• The ultimate source of new alleles is mutation,
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
• In animals, most mutations occur in somatic
cells and are thus lost when the individual
dies
Animation: Genetic Variation from Sexual Recombination
Point Mutations
•
A change in as little as one base in a gene (a point mutation) can have a
significant effect on phenotype
–
Because organisms reflect 1000s of generations of past selection, their
phenotypes generally provide a good match to their environment
• Thus, it is unlikely that a new mutation that alters a phenotype will
improve it
• In general, most such mutations are at the very least slightly
harmful
–
Because most of the DNA in eukaryotes does not code for protein
products that will influence phenotype, however, point mutations in
these regions are often harmless
• In addition, point mutations in coding portions of the genome will
not necessarily affect protein function due to the redundancy of the
genetic code
• Also, even if there is a change in amino acid composition, this may
not affect a protein’s shape and function
Mutations That Alter Gene Number or Sequence
•
Chromosomal mutations that delete, disrupt, or rearrange many loci are typically
harmful
–
The effects of such large-scale mutations may be neutral only if genes are left
intact
•
•
In rare cases, chromosomal rearrangements may even be beneficia
Duplication of large chromosome segments is usually harmful
–
Duplication of small pieces of DNA is sometimes less harmful, allowing them to
persist over several generations and accumulate mutations
•
–
The result in an expanded genome with new loci that may take on new
functions
These types of increases in gene number appear to have played a major role in
evolution
•
Ex) The dramatic increase in the number of olfactory genes allowed early
mammals to better detect faint odors and to distinguish between many
different smells
–
In more recent times, 60% of human olfactory receptor genes,
compared to only 20% in mice, have been inactivated by mutations,
demonstrating that a versatile sense of smell is more important in
mice than in humans
Mutation Rates
•
Mutation rates tend to be low in animals and plants, averaging ~1 in every
100,000 genes per generation
–
Though mutation rates are often lower in prokaryotes, mutations can
still quickly generate genetic variation in these populations because of
their short generation spans
–
This short generation span is also true of viruses, making it difficult to
create single-drug treatments against viral diseases
• Ex) The HIV virus have a very short generation time (2 days), and,
because it has an RNA genome, it also has a much higher mutation
rate than a typical viral DNA genome
– This is because host cells lack RNA repair mechanisms
• The most effective AIDS treatments to date have thus been drug
“cocktails” that combine several medications
– This is because it is less likely that multiple mutations
conferring resistance to many drugs will occur in a short time
period
Sexual Reproduction
•
In organisms that reproduce sexually, most of the genetic variation in a
population results from the unique combination of alleles that each individual
receives
–
Though all the differences in these alleles originated from past
mutations, sexual reproduction can shuffle existing alleles into new
combinations
• Three mechanisms contribute to this shuffling:
– Crossing over
– Independent assortment of chromosomes during meiosis
– Random fertilization
• The combined effects of these mechanisms ensures that sexual
reproduction rearranges existing alleles into new combinations,
thus providing much of the genetic variation that makes evolution
possible
Concept 23.2: The Hardy-Weinberg equation can
be used to test whether a population is evolving
• Though the individuals in a population must
differ genetically for evolution to occur, this
variation does not guarantee that a population
will evolve
– For evolution to occur, one of the factors
that cause evolution to occur must be at
work
• Natural selection
• Genetic drift
• Gene flow
Gene Pools and Allele Frequencies
The first step in testing whether evolution is occurring in a population is to
clarify what is meant by a population
A population is a localized group of individuals capable of
interbreeding and producing fertile offspring
• On average, members of a population are more closely related to
one another than to members of different populations of the same
species
Porcupine herd
• In other cases, even if 2
populations share the
same area, members of
either population are
more likely to breed with
members of their own
population
Beaufort Sea
Porcupine
herd range
T
ES S
HW RIE
RT ITO
NO RR
TE
• In some cases, this may
be due to geographic
isolation
MAP
AREA
Fortymile
herd range
Fortymile herd
CANADA
Fig. 23-5
ALASKA
•
ALASKA
YUKON
•
Gene Pools and Allele Frequencies
• A gene pool consists of all the alleles for all loci in
a population
– If only one allele exists for a specific locus in a
population, that allele is said to be fixed in the
gene pool
• Thus all individuals of that population will be
homozygous for that allele
– If there are 2+ alleles for a specific locus,
however, individuals of a population may be
either homozygous or heterozygous
•
The frequency of an allele in a population can be calculated
–
For diploid organisms, the total number of alleles at a locus is the total
number of individuals x 2
• Ex) In a population of 500 wildflower plants with 2 alleles for flower
pigment (CR and CW), there are a total number of 1000 (500 x 2)
copies of genes for flower color
–
The total number of dominant alleles at a locus is 2 alleles for each
homozygous dominant individual plus 1 allele for each heterozygous
individual
• Ex) In this same population of wildflowers, 320 have red (CRCR)
flowers, 160 have pink (CRCW) flowers, and 20 have white (CWCW)
flowers
– Thus the total number of dominant alleles in this population is
800 (320 x 2 for CRCR plants + 160 x 1 for CRCW plants)
–
The same logic applies for recessive alleles
• Ex) The total number of recessive alleles in this same population of
wildflowers is 200 (160 x 1 for CRCW plants + 20 x 2 for CWCW
plants)
• By convention, if there are 2 alleles at a locus, p and q are used to
represent their frequencies
–
p represent the frequency of the dominant allele
• Ex) p = frequency of the CR allele = 800/1000 (800 CR alleles
out of a total of 1000 alleles in the total population) = 0.8 =
80%
–
q represents the frequency of the recessive allele
• q = frequency of the CW allele = 200/1000 (200 CW alleles out
of a total of 1000 alleles in the total population) = 0.2 = 20%
• The frequency of all alleles in a population will add up to 1 (p + q = 1)
–
For loci that have more than 2 alleles, the sum of all allele
frequencies must still equal 1 (100%)
The Hardy-Weinberg Principle
• One way to assess if evolution is occurring at a
particular locus is to determine the genetic makeup
of a population that is NOT evolving at that locus
– This scenario can then be compared with data
from the real population
• If there are no differences, we can conclude
that the real population is NOT evolving
• If there are differences, we can conclude
that the real population IS evolving
– If the population is evolving, we can then try to
figure out why
The Hardy-Weinberg Principle
• One way to assess if evolution is occurring at a
particular locus is to determine the genetic makeup
of a population that is NOT evolving at that locus
– This scenario can then be compared with data
from the real population
• If there are no differences, we can conclude
that the real population is NOT evolving
• If there are differences, we can conclude
that the real population IS evolving
– If the population is evolving, we can then try to
figure out why
Hardy-Weinberg Equilibrium
•
The gene pool of a population that is not evolving can be described by the
Hardy-Weinberg principle
–
The Hardy-Weinberg principle states that frequencies of alleles and
genotypes in a population remain constant from generation to
generation unless some force besides Mendelian segregation and
recombination of alleles is at work
• Such a population is said to be in Hardy-Weinberg equilibrium
Fig. 23-6
•
If p and q represent the relative frequencies of the only two possible alleles
in a population at a particular locus, then p2 + 2pq + q2 = 1
–
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 equilibrium describes the
constant frequency of alleles in such a gene
pool
• If p and q represent the relative frequencies of
the only two possible alleles in a population at
a particular locus, then
– p2 + 2pq + q2 = 1
– where p2 and q2 represent the frequencies of
the homozygous genotypes and 2pq
represents the frequency of the heterozygous
genotype
Fig. 23-7-4
20% CW (q = 0.2)
80% CR ( p = 0.8)
Sperm
(80%)
CW
(20%)
64% ( p2)
CR CR
16% ( pq)
CR CW
CR
16% (qp)
CR CW
4% (q2)
CW CW
64% CR CR, 32% CR CW, and 4% CW CW
Gametes of this generation:
64% CR + 16% CR
= 80% CR = 0.8 = p
4% CW
= 20% CW = 0.2 = q
+ 16% CW
Genotypes in the next generation:
64% CR CR, 32% CR CW, and 4% CW CW plants
Conditions for Hardy-Weinberg Equilibrium
•
The Hardy-Weinberg theorem describes a hypothetical population that is not
evolving
–
In real populations, however, allele and genotype frequencies do change over
time
–
Such changes can occur when at least one of the following 5 conditions of
Hardy-Weinberg equilibrium is not met:
•
1) No mutations – by altering alleles or deleting/duplicating entire genes,
mutations modify the gene pool
•
2) Random mating – if individuals mate preferentially with a subset of a
population, random mixing of gametes does not occur, causing genotype
frequencies to change
•
3) No natural selection – differences in the survival and reproductive
success of individuals carrying different genotypes can alter allele
frequencies
•
4) Extremely large population size – the smaller the population size, the
more likely it is that allele frequencies will fluctuate by chance from one
generation to the next (genetic drift)
•
5) No gene flow – by moving alleles into or out of the population, gene flow
can alter allele frequencies
• Although evolution is common in natural
populations, it is also common for these same
populations to be in Hardy-Weinberg equilibrium
for specific genes
– This occurs because a population can be
evolving at some loci, yet also be in HardyWeinberg equilibrium at other loci
– In addition, some populations evolve so slowly
that changes in their allele and genotype
frequencies are difficult to distinguish from
those predicted for a non-evolving population
Applying the Hardy-Weinberg Principle
• The Hardy-Weinberg equation also has medical applications, such as
estimating the percentage of a population carrying the allele for an
inherited disease
–
Ex) Phenylketonuria (PKU), a metabolic disorder that results
from homozygosity for a recessive allele, occurs in ~1 out of
every 10,000 babies born in the US
• We can assume the locus that causes phenylketonuria (PKU) is in
Hardy-Weinberg equilibrium given that:
–
The PKU gene mutation rate is low (condition 1)
–
Mate selection is random with respect to whether or not an
individual is a carrier for the PKU allele (condition 2)
–
Natural selection can only act on rare homozygous individuals
who do not follow dietary restrictions (condition 3)
–
The US population is large (condition 4)
–
Migration has no effect as many other populations have similar
allele frequencies (condition 5)
• The 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 carriers is
– 2pq = 2 x 0.99 x 0.01 = 0.0198
– or approximately 2% of the U.S. population
Concept 23.3: Natural selection, genetic drift, and
gene flow can alter allele frequencies in a
population
• Three major factors alter allele frequencies and
bring about most evolutionary change:
– Natural selection
– Genetic drift
– Gene flow
Natural Selection
•
Differential reproductive success results in certain alleles being passed to
the next generation in greater proportions
–
Ex) The fruit fly has an allele that confers resistance to several
insecticides, including DDT
• This allele has a frequency of 0% in laboratory strains established
from flies collected in the wild in the early 1930s (prior to DDT
usage)
• Strains established from flies collected after 1960 (after 20+ years
of DDT usage) had an allele frequency of 37%
–
We can infer that this allele either arose by mutation between 1930 and
1960 or that this allele was present in the population in 1930 but very
rare
• In either case, the observed increase in the frequency of this allele
most likely occurred because DDT is a powerful poison that is a
strong selective force in exposed fly populations
– This type of evolution is known as adaptive evolution, or
evolution that results in a better match between organisms and
their environment
Genetic Drift
•
The smaller a sample, the greater the chance of deviation from a predicted result
–
•
Ex) An outcome of 7 heads to 3 tails out of 10 coin tosses is not as surprising
as 700 heads to 300 tails out of 1000 tosses
Chance events can also cause allele frequencies to fluctuate unpredictably from one
generation to the next, especially in small population, a process called genetic drift
–
Genetic drift tends to reduce genetic variation through losses of alleles
–
Ex) The CW allele is lost in a small wildflower population, perhaps because a
large animal happened to step on three CWCW individuals (completely by
chance)
Fig. 23-8-3
•
This increases the chance
that only the CR allele will
be passed to the next
generation
CR CR
CR C R
C W CW
CR CW
C R CW
C R CR
CW CW
C R CW
CR C R
Animation: Causes of Evolutionary Change
CR C W
C R CW
Generation 1
p (frequency of CR) = 0.7
q (frequency of CW ) = 0.3
CW C W
C R CW
C R CR
C R CR
C W CW
C R CR
CR C W
CR C R
CR C R
C R CR
CR C R
C R CR
C R CR
C R CR
CR C R
CR C R
CR C W
Generation 2
p = 0.5
q = 0.5
C R CR
CR C R
Generation 3
p = 1.0
q = 0.0
The Founder Effect
• One type of genetic drift is called the founder effect
–
The founder effect occurs when a few individuals become
isolated from a larger population
• Ex) A few members of a population are randomly blown by a
storm to a new island
–
This smaller group may then establish a new population whose
gene pool differs from the original population
• The founder effect probably accounts for the relatively high frequency
of certain inherited disorders among isolated human populations
–
Ex) The frequency of a recessive allele causing retinitis
pigmentosa (a progressive form of blindness) is 10x higher on a
small island called Tristan da Cunha than in the populations from
which the founders came
• This island was settled in 1814 by 15 British colonists, at
least one of which carried this allele causing blindness
The Bottleneck Effect
•
Another type of genetic drift is called the bottleneck effect
–
The bottleneck effect is a sudden reduction in population size due to a
change in the environment
•
–
Ex) A natural disaster, like a fire or flood
The resulting gene pool may no longer be reflective of the original population’s
gene pool
•
By chance alone, certain alleles may be over-represented among the
survivors, others may be under-represented, and some may be absent
altogether
•
If the population remains small, it may be further affected by genetic drift
due to chance effects (recall coin
flip example)
Fig. 23-9
–
Even if a population that has
passed through a bottleneck
ultimately recovers in size, it
may have low levels of genetic
variation for a long period
of time
Original
population
Bottlenecking
event
Surviving
population
Case Study: Impact of Genetic Drift on the Greater
Prairie Chicken
•
Understanding the bottleneck effect can increase understanding of how
human activity affects other species
–
Loss of prairie habitat caused aFig.severe
reduction in the population of
23-10
greater prairie chickens in Illinois
Pre-bottleneck Post-bottleneck
(Illinois, 1820)
–
The surviving birds had low levels of genetic
variation, and only 50% of their eggs
hatched
• This data suggests that genetic drift
during the bottleneck may have led
to a loss of genetic variation and an
increase in the frequency of harmful
alleles
(Illinois, 1993)
Range
of greater
prairie
chicken
(a)
Location
Population
size
Percentage
Number
of alleles of eggs
per locus hatched
Illinois
1930–1960s
1,000–25,000
5.2
93
<50
3.7
<50
Kansas, 1998
(no bottleneck)
750,000
5.8
99
Nebraska, 1998
(no bottleneck)
75,000–
200,000
5.8
96
Minnesota, 1998
(no bottleneck)
4,000
5.3
85
1993
• To investigate this hypothesis, researchers extracted DNA from
15 museum specimens of Illinois greater prairie chickens
– Of these 15 birds, 10 had been collected in the 1930s
during which time there were still 25,000 greater prairie
chickens in Illinois
• The other 5 birds were collected in the 1960s, when
there were 1,000 greater prairie chickens in Illinois
– Researchers were then able to obtain a minimum, baseline
estimate of how much genetic variation was present in the
Illinois population before the population shrank to extremely
low numbers
• The researchers surveyed 6 loci and found that the 1993
population had lost 9 alleles present in museum species
– The 1993 population also had fewer alleles per locus than
the pre-bottleneck Illinois or the current Kansas, Nebraska,
and Minnesota populations
• Thus, as predicted, genetic drift had reduced the
genetic variation of the small 1993 population, likely
leading to an increase in the frequency of harmful
alleles and a decrease in the egg-hatching rate
• Researchers introduced greater prairie chickens from
population in other states and were successful in introducing
new alleles and increasing the egg hatch rate to 90%
Effects of Genetic Drift: A Summary
There are 4 key points regarding genetic drift:
1.
Genetic drift is significant in small populations
–
2.
Genetic drift causes allele frequencies to change at random
–
3.
Chance events can cause alleles to be over- or under-represented in
new generation, especially in very small populations
Unlike natural selection, which consistently favors some alleles over
others in a given environment, genetic drift causes allele frequencies
to change at random over time
Genetic drift can lead to a loss of genetic variation within populations
–
Genetic drift can eliminate alleles from a population by causing allele
frequencies to fluctuate randomly over time
•
4.
Such losses can influence how effectively a population can adapt
to a change in the environment
Genetic drift can cause harmful alleles to become fixed
Gene Flow
•
Allele frequencies can also change due to gene flow, 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
since alleles are exchanged among these populations
• If gene flow is extensive enough, it can even result in neighboring
populations combining into a single population with a common
gene pool
Fig. 23-11
– Ex) Migration of humans throughout the
world has increased gene flow between
populations that were once isolated from
one another
•
Because it can occur at a higher rate, gene flow is
more likely than mutation to alter allele frequencies
directly
When neighboring populations live in different environments, alleles
transferred by gene flow may prevent a population from fully adapting to its
environment, decreasing the fitness of that population
–
Ex) In bent grass, alleles for copper tolerance are beneficial in
populations near copper mines, but harmful to populations in other soils
• Copper-tolerant plants on uncontaminated soil reproduce poorly
compared to non-tolerant plants
–
Windblown pollen, however, moves these alleles between populations
• The movement of these unfavorable alleles into a population may
thus result in a decrease in fit between organism and environment
Fig. 23-12
70
Index of copper tolerance
•
60
MINE
SOIL
NONMINE
SOIL
NONMINE
SOIL
50
Prevailing wind direction
40
30
20
10
0
20
0
20
0
20
40
60
80
Distance from mine edge (meters)
100
120
140
160
• Gene flow can also increase the fitness of a population
– Ex) Gene flow has resulted in the worlwide spread of
several insecticide-resistance alleles in mosquitoes
that carry West Nile virus and malaria
• These alleles originally evolved in some populations as a
result of random mutation
– In their population of origin, these alleles increased
because they provided insecticide resistance
• These alleles were then transferred to new
populations, where again, their frequencies
increased as a result of natural selection
Concept 23.4: Natural selection is the only
mechanism that consistently causes adaptive
evolution
• Only natural selection consistently results in adaptive evolution
– Natural selection brings about adaptive evolution by acting
on an organism’s phenotype
• The phrases “struggle for existence” and “survival of the fittest,”
which are commonly used to describe natural selection are
misleading
– Though there are animal species in which individuals
(usually males) do combat, reproductive success is
generally more subtle and depends on many factors
• Ex) Moths whose coloration helps conceal them in their
environment improve their chances of surviving long
enough to produce more offspring
• Adaptive evolution may therefore lead to greater relative
fitness
– This 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
– The relative fitness conferred by a particular allele depends
on the genetic and environmental context in which it is
expressed
• Ex) An allele that is slightly disadvantageous may
increase in frequency by “hitchhiking,” locating itself
close to an allele at another locus that is strongly
favored by natural selection
Directional, Disruptive, and Stabilizing Selection
• Natural selection can alter the frequency distribution of heritable traits
in 3 ways, depending on which phenotypes in a population are
favored:
–
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
• Regardless of the mode of selection, however, selection always
favors individuals whose heritable phenotypic traits provide higher
reproductive success than do the traits of other individuals
Directional selection occurs when conditions favor individuals exhibiting
one extreme of a phenotypic range
–
This causes a shift in the frequency curve for the phenotypic character
in one direction or the other
–
This type of selection is common when a population’s environment
changes or when members of a population migrate to a new (and
different) habitat
Fig. 23-13a
• Ex)The average size of black bears
in Europe increased during each
glacial period, only to decrease again
during warmer interglacial periods
– Larger bears have smaller
surface-to-volume ratios and are
thus better at conserving body
heat and surviving periods of
extreme cold
Frequency of individuals
•
Original population
Phenotypes (fur color)
Original population
Evolved population
(a) Directional selection
• Disruptive selection occurs when conditions favor
individuals at both extremes of a phenotypic range over
individuals with intermediate phenotypes
– Ex) In a population of
black-bellied seedcracker
finches, birds with
intermediate-sized bills
are relatively inefficient
at cracking both soft and
hard seeds and thus have
lower relative fitness
Frequency of individuals
Fig. 23-13b
Original population
Phenotypes (fur color)
Evolved population
(b) Disruptive selection
• Stabilizing selection acts against both extreme phenotypes
and favors intermediate variants
– This mode of selection thus reduces variation and tends to
maintain an average phenotype within the population
• Ex) The birth weights of
most human babies is in the
range of 6.6-8.8 pounds
– Babies that are much
larger or smaller suffer
high rates of mortality
Frequency of individuals
Fig. 23-13c
Original population
Phenotypes (fur color)
Evolved population
(c) Stabilizing selection
The Key Role of Natural Selection in Adaptive Evolution
•
Adaptations of organisms can arise gradually over time as natural selection
increases the frequencies of alleles
that enhance survival and reproduction
Fig. 23-14
–
As the proportion of individuals that have
favorable traits increases, the match
between a species and its environment
improves and adaptive evolution occurs
• Ex) Cuttlefish have the ability to
rapidly change color, enabling them
to blend into different environments
(a) Color-changing ability in cuttlefish
Movable bones
• Ex) Most snakes have movable
bones in their upper jaws, allowing
them to swallow food much larger
than their head
(b) Movable jaw
bones in
snakes
The Key Role of Natural Selection in Adaptive Evolution
• Adaptive evolution, however, is a continuous dynamic process
– What constitutes a “good match” between an organism and
its environment can change
• Though genetic drift and gene flow can also increase the
frequencies of alleles that increase the match between
organisms and their environment, neither does so consistently
– Both of these mechanisms are just as likely to cause the
frequency of such alleles to decrease
• Natural selection is the only evolutionary mechanism
that consistently leads to adaptive evolution
Sexual Selection
•
Sexual selection is natural selection for mating success
–
In this form of natural selection, individuals with certain inherited
characteristics are more likely than other individuals to obtain mates
–
Sexual selection can result in sexual dimorphism, marked differences
between the sexes in secondary sexual characteristics
• These characteristics are not directly associated with reproduction
or survival
Fig. 23-15
– Sexual dimorphism can
be seen in differences
in size, color,
ornamentation, and
behavior
• Ex) Peacocks and
peahens
•
There are different types of sexual selection:
–
Intrasexual selection is competition among individuals of one sex
(often males) for mates of the opposite sex
• Ex) A single male may patrol a group of females and prevent other
males from mating with them
–
Intersexual selection, often called mate choice, occurs when
individuals of one sex (usually females) are choosy in selecting their
mates
• In many cases, the female’s choice depends on the showiness of
the male’s appearance or behavior
• Though male showiness due to mate choice can increase a male’s
chances of attracting a female, it can actually decrease his chances
of survival
– Ex) Bright plumage may make male birds more visible to
predators
• If the benefit of gaining a mate outweighs the risk of predation,
however, then both the bright plumage and the female preference
for it will be reinforced because they enhance overall reproductive
success
•
One hypothesis for the evolution of female preferences for certain male
characteristics is that females prefer male traits that are correlated with “good genes”
–
•
If the trait preferred by females is indicative of the male’s overall genetic quality,
both the male trait and the female preference for it should increase in frequency
One experiment testing this hypothesis was performed with gray tree frogs
–
Female gray tree frogs prefer to mate with males that give long mating calls
–
Researchers tested whether the genetic makeup of long-calling (LC) males is
superior to that of short-calling (SC) males
•
•
Half of the eggs from each
Fig. 23-16
female were fertilized with
sperm from the LC male and
half were fertilized with sperm
from the SC male
The resulting half-sibling
offspring were raised in a
common environment and
tracked for 2 years
EXPERIMENT
Female gray
tree frog
SC male gray
tree frog
LC male gray
tree frog
SC sperm  Eggs  LC sperm
Offspring of Offspring of
SC father
LC father
Fitness of these half-sibling offspring compared
• Conclusion:
– Offspring fathered by the LC male had higher fitness than
their half-siblings fathered by the SC male
Fig. 23-16
EXPERIMENT
• Thus, the researchers
concluded that the
duration of the male’s
mating call is indicative
of the male’s overall
genetic quality
• These results support the
hypothesis that female
mate choice can be
based on a trait that
indicates whether the
male has “good genes”
Female gray
tree frog
SC male gray
tree frog
LC male gray
tree frog
SC sperm  Eggs 
LC sperm
Offspring of Offspring of
SC father
LC father
Fitness of these half-sibling offspring compared
RESULTS
Fitness Measure
1995
1996
Larval growth
NSD
LC better
Larval survival
LC better
NSD
Time to metamorphosis
LC better
(shorter)
LC better
(shorter)
NSD = no significant difference; LC better = offspring of LC males
superior to offspring of SC males.
The Preservation of Genetic Variation
•
The tendency of natural selection to reduce genetic variation by removing all
unfavorable genotypes is countered by mechanisms that preserve or restore
it
–
Diploidy maintains genetic variation in the form of hidden recessive
alleles
• Recessive alleles that are less favorable than their dominant
counterparts, or even harmful in the current environment, can
persist by propagation in heterozygous individuals
• This latent variation is only exposed to natural selection when two
parents that carry the recessive allele mate to produce a zygote
with two copies of that allele
– This happens only very rarely if the frequency of the recessive
allele is very low
• Thus, heterozygote protection maintains a huge pool of alleles that
may not be favored under present conditions, but of which some
could bring new benefits when the environment changes
Balancing Selection
• Selection itself may preserve variation at some loci
– Balancing selection occurs when natural
selection maintains stable frequencies of two or
more phenotypic forms in a population
• This type of selection includes:
– Heterozygote advantage
– Frequency-dependent selection
•
Heterozygote advantage occurs when heterozygotes have a higher fitness
than do both homozygotes
•
In such cases, natural selection will tend to maintain two or more alleles
at that locus
• Ex) The recessive sickle-cell allele causes mutations in hemoglobin
but also confers malaria resistance
• Though homozygous recessive individuals are resistant to
malaria, the red blood cells of these individuals become
distorted in shape, often leading to kidney, heart, and brain
damage
Fig. 23-17
• In addition, although
homozygous dominant
individuals are healthy, they are
more susceptible to malaria than
heterozygotes
• Thus, the frequency of the
sickle-cell allele in Africa is
generally highest in areas
where the malaria parasite is
the most common
Frequencies of the
sickle-cell allele
0–2.5%
Distribution of
malaria caused by
Plasmodium falciparum
(a parasitic unicellular eukaryote)
2.5–5.0%
5.0–7.5%
7.5–10.0%
10.0–12.5%
>12.5%
• Heterozygote advantage can represent either stabilizing or
directional selection, depending on the relationship
between genotype and phenotype
•
If the phenotype of the heterozygote is intermediate to
the phenotypes of both homozygotes, heterozygote
advantage will be in the form of stabilizing selection
•
If the heterozygote shows the dominant phenotype,
heterozygote advantage will be in the form of
directional selection
In frequency-dependent selection, the fitness of a phenotype declines if it
becomes too common in the population
•
Ex)The scale-eating fish of Africa attacks other fish from behind, darting
in to remove a few scales from the flank of their prey
• Some of these fish display the dominant phenotype of being “leftmouthed,” always attacking their prey’s right flank
• Others display the recessive phenotype of being “right-mouthed,”
always attacking from the left
• Prey species guard against attack
from whatever phenotype of scaleeating fish is most common
Fig. 23-18
• Thus, from year to year, selection
favors whichever mouth phenotype is
least common
• This causes the frequency of leftand right-mouthed fish to oscillate
over time, and balancing selection
keeps the frequency of each
phenotype close to 50%
“Right-mouthed”
1.0
Frequency of
“left-mouthed” individuals
•
“Left-mouthed”
0.5
0
1981 ’82 ’83 ’84 ’85 ’86 ’87 ’88 ’89 ’90
Sample year
Neutral Variation
• Much of the DNA variation in populations probably has
little or no impact on reproductive success
– This type of variation is known as neutral variation
• Ex) Variation in noncoding regions of DNA
• Ex) Variation in proteins that have little effect on
protein function or reproductive fitness
– Thus, natural selection does not affect this DNA
• Over time, however, the frequencies of these
alleles may increase or decrease as a result of
genetic drift
Why Natural Selection Cannot Fashion Perfect Organisms
•
Though natural selection leads to adaptation, there are several reasons why nature
abounds with examples of organisms that are less than ideally “engineered” for
their lifestyles
1.
Selection can act only on existing variations
•
2.
New advantageous alleles do not arise on demand
Evolution is limited by historical constraints
•
Rather than scrapping ancestral anatomy to build new complex structures
from scratch, evolution can only modify existing structures and adapt them to
new situations
Fig. 23-19
3.
Adaptations are often compromises
•
4.
Ex) The load call that allows the
Tungara frog to attract mates
also attracts predators
Chance, natural selection, and the
environment interact
•
Chance events and changes in
environment may not necessarily
result in adaptive evolution
You should now be able to:
1. Explain why the majority of point mutations
are harmless
2. Explain how sexual recombination generates
genetic variability
3. Define the terms population, species, gene
pool, relative fitness, and neutral variation
4. List the five conditions of Hardy-Weinberg
equilibrium
5. Apply the Hardy-Weinberg equation to a
population genetics problem
6. Explain why natural selection is the only
mechanism that consistently produces
adaptive change
7. Explain the role of population size in genetic
drift
8. Distinguish among the following sets of terms:
directional, disruptive, and stabilizing
selection; intrasexual and intersexual
selection
9. List four reasons why natural selection cannot
produce perfect organisms