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Chapter 23
The Evolution of Populations
Overview: The Smallest Unit of Evolution
• One misconception is that organisms evolve during
their lifetimes
• Natural selection acts on individuals, but only
populations evolve
• Consider, for example, a population of medium
ground finches on Daphne Major Island
– During a drought, large-beaked birds were more
likely to crack large seeds and survive
– The finch population evolved by natural selection
Overview: The Smallest Unit of Evolution
• One misconception is that organisms evolve during
their lifetimes
• Natural selection acts on individuals, but only
populations evolve
• Consider, for example, a population of medium
ground finches on Daphne Major Island
– During a drought, large-beaked birds were more
likely to crack large seeds and survive
– The finch population evolved by natural selection
Gene flow is the movement of alleles between
populations
• Gene flow: movement
of alleles from one pop.
to another
• Occurs when individuals
join new populations
and reproduce.
• Gene flow keeps
neighboring populations
similar.
• Low gene flow increases
the chance that two
populations will evolve
into different species.
bald eagle migration
Adaptive radiation
10% of
population
natural disaster kills
five green frogs
20% of
population
Hardy-Weinberg
• The Hardy-Weinberg principle:
– Allele frequencies in a population will remain
constant assuming:
• No Mutations
• No Gene Flow
• Random Mating
• No Genetic Drift
• No Selection
Outline
• Population genetics
– Variations in terms of allele differences.
• Microevolution
– Hardy-Weinberg
– Causes of Microevolution
• Natural Selection
– Types of Selection
• Macroevolution
Population Genetics
• Population
– All members of a single species
– Occupying a particular area at the same time.
HapMap Project
• People inherit patterns of sequence differences, called
haplotypes
– If one haplotype of a person has an A rather than a G at a particular
location in a chromosome, there are probably other particular base
differences near the A
– Genetic data from African, Asian, and European populations will be
analyzed
• A HapMap is a catalog common sequence differences that
occur in a species
– The goal of the project is to link haplotypes to risk for specific illnesses
– May lead to new methods of preventing, diagnosing, and treating
disease
haplotype map
(haploid genotype)
SNP = Single Nucleotide Polymorphism
HapMap Project
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Microevolution
• In 1930s population geneticists described
variations in a population in terms of alleles
• Microevolution pertains to
changes within a population.
evolutionary
– Various alleles at all the gene loci in all individuals
make up the gene pool of the population.
– Gene pool of a population:
• Genotype
• Allele frequencies
Frequency of Gametes Calculation
• From genotype frequencies, the allele and
gamete frequencies can be calculated
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genotypes
frequency of genotypes
in the population
frequency of alleles and
gametes in the population
DD
Dd
dd
0.04
0.32
0.64
0.04 + 0.16
0.16 + 0.64
0.20 D
0.80 d
Hardy-Weinberg
• The Hardy-Weinberg principle:
– Allele frequencies in a population will remain
constant assuming:
• No Mutations
• No Gene Flow
• Random Mating
• No Genetic Drift
• No Selection
Hardy-Weinberg Equilibrium
F1generation
DD
Dd
dd
Genotypes:
Genotype frequencies:
0.04
0.32
0.64
Allele and gamete frequencies:
D = 0.20
d = 0.80
eggs
sperm
F2generation
0.20 D
0.80 d
0.20
D
0.04 DD
0.16 Dd
0.80
d
0.16 Dd
0.64 dd
Offspring
Genotype frequencies:
0.04 DD + 0.32 Dd + 0.64 dd = 1
2
2
p + 2pq + q = 1
p2 = frequency of DD genotype (dark-colored) = (0.20)2
2pq = frequency of Dd genotype (dark-colored) = 2(0.20)(0.80)
q2 = frequency of dd genotype (light-colored) = (0.80)2
=0.04
=0.32
=0.64
1.00
Industrial Melanism and Microevolution
Early observation
36% dark-colored phenotype
Later observation
64% dark-colored phenotype
Peppered moth (Biston betularia)
Hardy-Weinberg
• Required conditions are rarely (if ever) met
– Changes in gene pool frequencies are likely
– When gene pool frequencies change, microevolution has
occurred
• Deviations from a Hardy-Weinberg equilibrium
indicate that evolution has taken place
• The founding of a small population can lead to genetic drift.
– It occurs when a few individuals start a new population.
– The founder effect is genetic drift that occurs after start
of new population.
The current population is thought to have descended from
only seven females and eight males. One of the early
colonists apparently carried a recessive allele for retinitis
pigmentosa, a progressive form of blindness that afflicts
homozygous individuals. The frequency of the allele that
causes this disease is ten times higher on Tristan da Cunha
than in the populations from which the founders came.
Causes of Microevolution
• Genetic Mutations
– The raw material for evolutionary change
– Provides new combinations of alleles
– Some might be more adaptive than others
Causes of Microevolution
• Gene Flow
– Movement of alleles between populations when:
• Gametes or seeds (in plants) are carried into another
population
• Breeding individuals migrate into or out of population
– Continual gene flow reduces genetic divergence
between populations
Gene Flow
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selfpollination
stigma
stamen
Pisum sativum
Causes of Microevolution
• Nonrandom Mating
– When individuals do not choose mates randomly
• Assortative mating:
– Individuals select mates with their phenotype
– Individuals reject mates with differing phenotype
• Sexual selection:
– Males compete for the right to reproduce
– Females choose with males possessing a particular phenotype
– Both of these cause an increase in homozygotes
Causes of Microevolution
• Genetic Drift
– Occurs by disproportionate random sampling from
population
– Can cause the gene pools of two isolated populations to become
dissimilar
– Some alleles are lost and others become fixed (unopposed)
– Likely to occur:
• After a bottleneck
• When severe inbreeding occurs, or
• When founders start a new population
– Stronger effect in small populations
Genetic Drift
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10% of
population
natural disaster kills
five green frogs
20% of
population
Genetic Drift
• Bottleneck Effect
– A random event prevents a majority of individuals
from entering the next generation
– Next generation composed of alleles that just
happened to make it
Genetic Drift
• Founder Effect
– When a new population is started from just a few
individuals
– The alleles carried by population founders are
dictated by chance
– Formerly rare alleles will either:
• Occur at a higher frequency in the new population, or
• Be absent in new population
Figure 23.1
Average beak depth (mm)
Figure 23.2
10
9
8
0
1978
1976
(similar to the (after
prior 3 years) drought)
• Microevolution is a change in allele frequencies in
a population over generations
• Three mechanisms cause allele frequency change:
– Natural selection
– Genetic drift
– Gene flow
• Only natural selection causes adaptive evolution
Concept 23.1: Genetic variation makes
evolution possible
• Variation in heritable traits is a prerequisite for
evolution
• Mendel’s work on pea plants provided evidence of
discrete heritable units (genes)
Genetic Variation
• Genetic variation among individuals is caused by
differences in genes or other DNA segments
• Phenotype is the product of inherited genotype
and environmental influences
• Natural selection can only act on variation with a
genetic component
Humans appeared late in Earth’s history.
Humans share a common ancestor with
other primates.
• Primates are mammals with flexible hands and feet,
forward-looking eyes and enlarged brains.
• Primates evolved into prosimians and anthropoids.
– Prosimians are the oldest living primates.
– They are mostly small and nocturnal.
– Anthropoids are humanlike primates.
– They are subdivided into the New World monkeys, Old
World monkeys, and hominoids.
– Homonoids are
divided into
hominids, great
apes, and lesser
apes.
– Hominids include
living and extinct
humans.
• Bipedal means walking on two legs.
– foraging
– carrying infants and food
– using tools
• Walking upright has
important adaptive
advantages.
There are many fossils of extinct
hominids.
• Most hominids are either the genus Australopithecus or
Homo.
• Australopithecines were a successful genus.
• The Homo genus first evolved 2.4 million years ago.
Modern humans arose about 200,000 years ago.
• Homo sapiens fossils date to 200,000 years ago.
• Human evolution is influenced by a tool-based culture.
• There is a trend toward increased brain size in hominids.
Australopithecus
afarensis
Lucy – 4 mybp
Homo habilis
Homo
neanderthalensis
1.5 mybp
Homo sapiens
196,000 thousand ybp
• Many species evolve from one species during adaptive
radiation.
– ancestral species diversifies into many descendent
species
– descendent species
usually adapted to
wide range of
environments
Evolutionary biology today
Evolution unites all fields of biology
"Nothing in Biology Makes Sense Except in
the Light of Evolution"
-Theodosius Dobzhansky 1973
What is the study of Paleontology?
• Heritability is the ability of a trait to be passed down.
• There is a struggle for survival due to overpopulation
and limited resources. Populations would grow
geometrically if resources were unlimited.
• Darwin proposed that adaptations arose over many
generations.
• Natural selection is a mechanism by which individuals that
have inherited beneficial adaptations produce more
offspring on average than do other individuals.
Natural selection explains how evolution can occur.
• There are four main principles to the theory of natural
selection.
– Variation: the heritable differences in each population
are the basis of natural selection.
– Overproduction: having many offspring increases the
chances of survival but creates competition
– Adaptation: a certain variation that allows an organism to
survive better than organisms it competes against.
– Descent with modification: heritability of adaptations.
• Fitness is the measure of survival ability and ability to
produce more offspring.
A population shares a common gene pool.
Variation Between Populations
• Most species exhibit geographic variation,
differences between gene pools of separate
populations
• For example, Madeira is home to several isolated
populations of mice
– Chromosomal variation among populations is due
to drift, not natural selection
Genetic variation comes from several sources.
• Mutation is a random change in the DNA of a gene.
– can form new allele
– can be passed on to
offspring if in
reproductive cells
• Recombination forms new combinations of alleles.
– usually occurs during meiosis
– parents’ alleles
arranged in new
ways in gametes
Figure 23.5
Ldh-Bb allele frequency
1.0
0.8
0.6
0.4
0.2
0
46
44
42
Maine
Cold (6°C)
40
38
36
Latitude (ºN)
34
32
Georgia
Warm (21ºC)
30
Sources of Genetic Variation
• New genes and alleles can arise by mutation or gene
duplication
Formation of New Alleles
• A mutation is a change in nucleotide sequence of
DNA
• Only mutations in cells that produce gametes can
be passed to offspring
• A point mutation is a change in one base in a gene
• The effects of point mutations can vary:
– Mutations in noncoding regions of DNA are often
harmless
– Mutations in a genes can be neutral because of
redundancy in the genetic code
Rapid Reproduction
• Mutation rates are low in animals and plants
• The average is about one mutation in every 100,000
genes per generation
• Mutations rates are often lower in prokaryotes and
higher in viruses
Natural selection acts on distributions of traits.
• A normal distribution graphs as a bell-shaped curve.
– highest frequency near
mean value
– frequencies decrease
toward each extreme
value
• Traits not undergoing
natural selection have a
normal distribution.
Natural selection can change the distribution
of a trait in one of three ways.
• Microevolution is evolution within a
population.
– observable change in the allele frequencies
– can result from natural selection
Hardy-Weinberg
• Required conditions are rarely (if ever) met
– Changes in gene pool frequencies are likely
– When gene pool frequencies change, microevolution has
occurred
• Deviations from a Hardy-Weinberg equilibrium
indicate that evolution has taken place
Genetic variation in a population increases the
chance that some individuals will survive.
Where does that variation occur?
Hardy-Weinberg Equilibrium
F1generation
DD
Dd
dd
Genotypes:
Genotype frequencies:
0.04
0.32
0.64
Allele and gamete frequencies:
D = 0.20
d = 0.80
eggs
sperm
F2generation
0.20 D
0.80 d
0.20
D
0.04 DD
0.16 Dd
0.80
d
0.16 Dd
0.64 dd
Offspring
Genotype frequencies:
0.04 DD + 0.32 Dd + 0.64 dd = 1
2
2
p + 2pq + q = 1
p2 = frequency of DD genotype (dark-colored) = (0.20)2
2pq = frequency of Dd genotype (dark-colored) = 2(0.20)(0.80)
q2 = frequency of dd genotype (light-colored) = (0.80)2
=0.04
=0.32
=0.64
1.00
Industrial Melanism and Microevolution
Early observation
36% dark-colored phenotype
Later observation
64% dark-colored phenotype
Peppered moth (Biston betularia)
• Genetic variation leads to phenotypic variation.
• Phenotypic variation is necessary for natural selection.
• Genetic variation is stored in a population’s gene pool.
– made up of all alleles in a population
– allele combinations form when organisms have offspring
• Allele frequencies measure genetic variation.
– measures how common allele is in population
– can be calculated for each allele in gene pool
Conditions for Hardy-Weinberg Equilibrium
• The Hardy-Weinberg theorem describes a hypothetical
population that is not evolving
• In real populations, allele and genotype frequencies do
change over time
• The five conditions for non-evolving
populations are rarely met in nature:
1.
2.
3.
4.
5.
No mutations
Random mating
No natural selection
Extremely large population size
No gene flow
Genetic variation comes from several sources.
• Hybridization is the crossing of two different species.
– occurs when individuals can’t find mate of own species
– topic of current scientific research
A zonkey
Populations, not individuals, evolve.
In which of the following scenarios will
natural selection (N.S.) most likely occur?
a) Very little genetic variation is present within a species
b) Harsh environmental conditions result in competition for survival
c) No reproductive isolation barriers exist within a species living in
an area.
d) A geographical area has plenty of food to support individuals
within the species living in that area.
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 success in reproduction results in
certain alleles being passed to the next generation
in greater proportions
• For example, an allele that confers resistance to
DDT increased in frequency after DDT was used
widely in agriculture
Genetic Drift
• The smaller a sample, the greater the chance of
deviation from a predicted result
• Genetic drift describes how allele frequencies fluctuate
unpredictably from one generation to the next
• Genetic drift tends to reduce genetic variation through
losses of alleles
Figure 23.9-1
CRCR
CRCR
CRCW
CWCW
CRCR
CRCW
CRCR
CRCR
CRCW
CRCW
Generation 1
p (frequency of CR) = 0.7
q (frequency of CW) = 0.3
Figure 23.9-2
CRCR
CRCR
CRCW
CWCW
5
plants
leave
offspring
CRCR
CWCW
CRCW
CRCR
CWCW
CRCR
CRCW
CRCW
CRCR
CRCR
CRCW
CRCW
Generation 1
p (frequency of CR) = 0.7
q (frequency of CW) = 0.3
CWCW
CRCW
CRCR
CRCW
Generation 2
p = 0.5
q = 0.5
Figure 23.9-3
CRCR
CRCR
CRCW
CWCW
5
plants
leave
offspring
CRCR
CWCW
CRCW
CRCR
CWCW
CRCR
CRCW
CRCW
CRCR
CRCR
CRCW
CRCW
Generation 1
p (frequency of CR) = 0.7
q (frequency of CW) = 0.3
CWCW
CRCW
2
plants
leave
offspring
CRCR
CRCR
CRCR
CRCR
CRCR
CRCR
CRCR
CRCW
Generation 2
p = 0.5
q = 0.5
CRCR
CRCR
CRCR
CRCR
Generation 3
p = 1.0
q = 0.0
Effects of Genetic Drift: A Summary
1. Genetic drift is significant in small populations
2. Genetic drift causes allele frequencies to change at
random
3. Genetic drift can lead to a loss of genetic variation
within populations
4. Genetic drift can cause harmful alleles to become
fixed
Gene Flow
• 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 variation among
populations over time
• Gene flow can decrease the fitness of a population
• Consider, for example, the great tit (Parus major) on
the Dutch island of Vlieland
– Mating causes gene flow between the central and
eastern populations
– Immigration from the mainland introduces alleles that
decrease fitness
– Natural selection selects for alleles that increase
fitness
– Birds in the central region with high immigration have
a lower fitness; birds in the east with low immigration
have a higher fitness
Figure 23.12
60
Survival rate (%)
50
Population in which the
surviving females
eventually bred
Central
Eastern
Central
population
NORTH SEA
Eastern
population
Vlieland,
the Netherlands
40
2 km
30
20
10
0
Females born
in central
population
Females born
in eastern
population
Parus major
Figure 23.12a
Parus major
• Gene flow can increase the fitness of a population
• Consider, for example, the spread of alleles for
resistance to insecticides
– Insecticides have been used to target mosquitoes that
carry West Nile virus and malaria
– Alleles have evolved in some populations that confer
insecticide resistance to these mosquitoes
– The flow of insecticide resistance alleles into a
population can cause an increase in fitness
• Gene flow is an important agent of evolutionary
change in human populations
Concept 23.4: Natural selection is the only
mechanism that consistently causes
adaptive evolution
• Evolution by natural selection involves both change
and “sorting”
– New genetic variations arise by chance
– Beneficial alleles are “sorted” and favored by natural
selection
• Only natural selection consistently results in adaptive
evolution
Types of Selection
• Most traits are polygenic - variations in the trait
result in a bell-shaped curve
• Three types of selection occur:
– (1) Directional Selection
• The curve shifts in one direction
– Bacteria become resistant to antibiotics
– Guppies become more colorful in the absence of predation
• 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
Three Type of Natural Selection
Number of Individuals
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Phenotype Range
Phenotype Range
Phenotype Range
stabilizing selection
directional selection
disruptive selection
Number of Individuals
Peak narrows.
a.
Peak shifts.
b.
Two peaks result.
c.
• Natural selection can take one of three
paths.
– Directional selection favors phenotypes at one
extreme.
• Natural selection can take one of three
paths.
– Stabilizing selection favors the
intermediate phenotype.
Directional Selection
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No
predation
All guppies
are drab
and small
Amount of Color
Low
predation
above waterfall
High
predation
0
below waterfall
Experimental site
Result
© Helen Rodd
4
Months
8
12
Types of Selection
• Three types of selection occur (cont):
– (2) Stabilizing Selection
• The peak of the curve increases and tails decrease
• Ex - when human babies with low or high birth weight
are less likely to survive
– (3) Disruptive
• The curve has two peaks
• Ex – When Cepaea snails vary because a wide
geographic range causes selection to vary
Stabilizing Selection
Due to stabilizing selection, the average
human birth weight stays steady.
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100
20
70
50
15
30
20
10
10
7
5
5
3
2
.9
1.4 1.8 2.3 2.7 3.2 3.6 4.1 4.5
Birth Weight (in kilograms)
Percent Infant Mortality
Percent of Births in Population

Disruptive Selection
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Forested
areas
Low-lying
vegetatio
© Bob Evans/Peter Arnold, Inc.
Natural selection is not the only mechanism through
which populations evolve.
Gene flow is the movement of alleles between
populations
• Gene flow: movement
of alleles from one pop.
to another
• Occurs when individuals
join new populations
and reproduce.
• Gene flow keeps
neighboring populations
similar.
• Low gene flow increases
the chance that two
populations will evolve
into different species.
bald eagle migration
Genetic drift is a change in allele frequencies
due to chance
• Genetic drift: change in allele frequency due to chance
• Common in small populations, some alleles may increase
in frequency, while others decrease or disappear
• A population bottleneck can lead to genetic drift.
– It occurs when an event
drastically reduces
population size.
– The bottleneck effect is
genetic drift that occurs
after an event drastically reduces the population.
• The founding of a small population can lead to genetic drift.
– It occurs when a few individuals start a new population.
– The founder effect is genetic drift that occurs after start
of new population.
The current population is thought to have descended from only
seven females and eight males. One of the early colonists
apparently carried a recessive allele for retinitis pigmentosa, a
progressive form of blindness that afflicts homozygous individuals.
The frequency of the allele that causes this disease is ten times
higher on Tristan da Cunha than in the populations from which the
founders came.
• Genetic drift has negative effects on a population.
(bottle neck and founder effect)
– less likely to have some individuals that can adapt
– harmful alleles can become more common due to
chance
Sexual Selection
• Sexual selection is natural selection for mating
success
• It can result in sexual dimorphism, marked
differences between the sexes in secondary sexual
characteristics
Figure 23.15
• Intrasexual selection is competition among
individuals of one sex (often males) for mates of
the opposite sex
• Intersexual selection, often called mate choice,
occurs when individuals of one sex (usually
females) are choosy in selecting their mates
• Male showiness due to mate choice can increase a
male’s chances of attracting a female, while
decreasing his chances of survival
• How 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 increase in
frequency
© 2011 Pearson Education, Inc.
Sexual selection occurs when certain traits
increase mating success.
• Sexual selection:
processes in which certain
traits increase mating
success and therefore
become more common in
the population.
– males produce many
sperm continuously
– females are more limited
in potential offspring each
cycle
• Females preferentially mate with males that display
certain traits, so those traits get passed on to offspring
and become more exaggerated each generation.
https://www.youtube.com/watch?v=W7QZnwKqopo
Bird of Paradise mating dance
• There are two types of sexual selection.
– intrasexual selection: competition among males
– intersexual selection: males display certain traits to
females
Sexual Selection
• Female Choice
– Choice of a mate is serious consideration
• Good genes hypothesis: Females choose mates on the basis of
traits that improve the chance of survival.
• Runaway hypothesis: Females choose mates on the basis of traits
that improve male appearance.
• Male Competition
– Can father many offspring because they continuously
produce sperm in great quantity.
– Compete to inseminate as many females as possible.
Sexual Selection
• Sexual selection adaptive changes in males
and females to increase ability to secure a
mate.
– Males - ability to compete
– Females choose to select a male with the best
fitness (ability to produce surviving offspring).
Sexual Selection
• The drab
females tend to
choose
flamboyant
males as mates.
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Sexual Selection: Competition
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a.
b.
a: © Y. Arthus-Bertrand/Peter Arnold, Inc.; b: © Neil McIntre/Getty Images
New species can arise when populations are isolated.
The isolation of populations can lead to
speciation.
• Populations become isolated when there is no gene flow.
– Isolated populations adapt to their own environments.
– Genetic differences can add up over generations.
• Reproductive isolation can occur between isolated
populations.
– members of different
populations cannot
mate successfully
– final step to
becoming separate
species
• Speciation is the rise of two or more species from one
existing species.
Populations can become isolated in several
ways.
• Behavioral barriers can cause isolation.
– called behavioral isolation
– includes differences in courtship or mating behaviors
• Geographic barriers can cause isolation.
– called geographic isolation
– physical barriers divide population
• Temporal barriers can cause isolation.
– called temporal isolation
– timing of reproductive periods prevents mating
Evolution occurs in patterns.
Evolution through natural selection
is not random.
• Natural selection can have direction.
• The effects of natural selection add up over time.
Heterozygote Advantage
• 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
Figure 23.17
Key
Frequencies of the
sickle-cell allele
0–2.5%
2.5–5.0%
Distribution of
malaria caused by
Plasmodium falciparum
(a parasitic unicellular eukaryote)
5.0–7.5%
7.5–10.0%
10.0–12.5%
>12.5%
Frequency-Dependent Selection
• In frequency-dependent selection, the fitness of a
phenotype declines if it becomes too common in
the population
• Selection can favor whichever phenotype is less
common in a population
• For example, frequency-dependent selection selects
for approximately equal numbers of “rightmouthed” and “left-mouthed” scale-eating fish
Why Natural Selection Cannot Fashion
Perfect Organisms
1.
2.
3.
4.
Selection can act only on existing variations
Evolution is limited by historical constraints
Adaptations are often compromises
Chance, natural selection, and the environment
interact
• Convergent evolution describes
evolution toward similar traits in
unrelated species.
• Convergent evolution describes the evolution of traits
toward similar features.
• Divergent evolution describes evolution toward different
traits in closely related species.
red fox
kit fox
ancestor
How do convergent and divergent
evolution illustrate the directional
nature of natural selection?
Species can shape each other over time.
• Two or more species can evolve together through
coevolution.
– evolutionary paths become connected
– species evolve in response to changes in each other
• Coevolution can occur in beneficial relationships.
• Coevolution can occur in competitive relationships,
sometimes called evolutionary.
Species can become extinct.
• Extinction is the elimination of a species from Earth.
• Background extinctions occur continuously at a very low
rate.
– occur at roughly the same
rate as speciation
– usually affects a few species
in a small area
– caused by local changes in
environment
• Background extinctions occur continuously at a very low
rate.
– occur at roughly the same rate as speciation
– usually affects a few species in a small area
– caused by local changes in environment
• Mass extinctions are rare but much more intense.
– destroy many species at global level
– thought to be caused by catastrophic events
– at least five mass extinctions in last 600 million years
A small portion of a population becomes geographically
isolated from the rest of the population. As such, it
runs the risk of decreased…
a)
b)
c)
d)
Genetic drift
Mutation rate
Natural selection
Genetic variation
Can you define these words:
Endosymbiosis –
Primate –
Prosimian –
Anthropoid –
Hominid –
Bipedal –
Evolution through natural selection
is not random.
Explain and justify this statement.
REMEMBER!
Speciation often occurs in patterns.
• A pattern of punctuated equilibrium exists in the
fossil record.
–
–
–
–
theory proposed by Eldredge and Gould in 1972.
episodes of speciation occur suddenly in geologic time.
followed by long periods of little evolutionary change.
revised Darwin’s idea that species arose through
gradual transformations.