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Chapter 13
How Populations Evolve
PowerPoint® Lectures for
Campbell Essential Biology, Fifth Edition, and
Campbell Essential Biology with Physiology,
Fourth Edition
– Eric J. Simon, Jean L. Dickey, and Jane B. Reece
Lectures by Edward J. Zalisko
© 2013 Pearson Education, Inc.
CHARLES DARWIN AND THE ORIGIN OF
SPECIES
• Darwin made three observations from these facts.
1. Life shows rich diversity.
2. There are similarities in life that allow the
classification of organisms into groups nested
within broader groups.
3. Organisms display many striking ways in which
they are suited for their environments.
© 2013 Pearson Education, Inc.
CHARLES DARWIN AND THE ORIGIN OF
SPECIES
• In The Origin of Species, Darwin
– proposed a hypothesis, a scientific explanation for
his observations,
– used hundreds of pages in his book to describe the
evidence supporting his hypothesis,
– made testable predictions, and
– reported the results of numerous experiments he
had performed.
© 2013 Pearson Education, Inc.
CHARLES DARWIN AND THE ORIGIN OF
SPECIES
• Darwin hypothesized that
– present-day species are the descendents of
ancient ancestors that they still resemble in some
ways and
– change occurs as a result of “descent with
modification,” with natural selection as the
mechanism.
© 2013 Pearson Education, Inc.
CHARLES DARWIN AND THE ORIGIN OF
SPECIES
• Natural selection is a process in which organisms
with certain inherited characteristics are more likely
to survive and reproduce than are individuals with
other characteristics.
• As a result of natural selection, a population, a
group of individuals of the same species living in
the same place at the same time, changes over
generations.
© 2013 Pearson Education, Inc.
CHARLES DARWIN AND THE ORIGIN OF
SPECIES
• Natural selection leads to evolutionary adaptation,
a population’s increase in the frequency of traits
suited to the environment.
• Natural selection thus leads to evolution, seen
either as
– a change in the genetic composition of a population
over time or
– on a grander scale, the entire biological history, from
the earliest microbes to the enormous diversity of
organisms that live on Earth today.
© 2013 Pearson Education, Inc.
Descent with Modification
• Darwin made two main points in The Origin of
Species.
1. Organisms inhabiting Earth today descended from
ancestral species.
2. Natural selection is the mechanism for descent
with modification.
© 2013 Pearson Education, Inc.
EVIDENCE OF EVOLUTION
• Evolution leaves observable signs.
• We will examine five of the many lines of evidence
in support of evolution:
1. the fossil record,
2. biogeography,
3. comparative anatomy,
4. comparative embryology, and
5. molecular biology.
© 2013 Pearson Education, Inc.
Figure 13.7-3
Figure 13.8
Australia
Common
ringtail
possum
Koala
Common wombat
Red kangaroo
Figure 13.9
Human
Cat
Whale
Bat
Comparative Anatomy
• Vestigial structures
– are remnants of features that served important
functions in an organism’s ancestors and
– now have only marginal, if any, importance.
© 2013 Pearson Education, Inc.
Molecular Biology
• The hereditary background of an organism is
documented in
– its DNA and
– the proteins encoded by the DNA.
• Evolutionary relationships among species can be
determined by comparing
– genes and
– proteins of different organisms.
© 2013 Pearson Education, Inc.
Figure 13.11
Primate
Percent of selected DNA
sequences that match a
chimpanzee’s DNA
92%
Chimpanzee
Human
Gorilla
Orangutan
Gibbon
Old World
monkey
96%
100%
NATURAL SELECTION
• Darwin noted the close relationship between
adaptation to the environment and the origin of
new species.
• The evolution of finches on the Galápagos Islands
is an excellent example.
© 2013 Pearson Education, Inc.
Figure 13.12
(a) The large ground finch
(b) The warbler finch
(c) The woodpecker finch
Darwin’s Theory of Natural Selection
• Darwin based his theory of natural selection on two
key observations.
1. All species tend to produce excessive numbers of
offspring.
2. Organisms vary, and much of this variation is
heritable. Natural expression of genotype
© 2013 Pearson Education, Inc.
Figure 13.14
Darwin’s Theory of Natural Selection
• Inference: Unequal reproductive success
(natural selection)
– Those individuals with traits best suited to the local
environment generally leave a larger share of
surviving, fertile offspring.
© 2013 Pearson Education, Inc.
Natural Selection in Action
• Examples of natural selection include
– pesticide-resistant insects,
– antibiotic-resistant bacteria, and
– drug-resistant strains of HIV.
© 2013 Pearson Education, Inc.
Figure 13.15-3
Insecticide application
Chromosome with gene
conferring resistance
to pesticide
Survivors
Reproduction
Evolutionary Trees
• Darwin saw the history of life as analogous to a
tree.
– The first forms of life on Earth form the common
trunk.
– At each fork is the last common ancestor to all the
branches extending from that fork.
– The tips of millions of twigs represent the species
living today.
© 2013 Pearson Education, Inc.
Figure 13.17
Common ancestor of
lineages to the right
Lungfishes
Tetrapods
Amniotes
Amphibians
1
Mammals
2
Tetrapod
limbs
Lizards
and snakes
3
Amnion
Crocodiles
4
Homologous trait
shared by all groups
to the right
Ostriches
6
Feathers
Hawks and
other birds
Birds
5
THE MODERN SYNTHESIS: DARWINISM
MEETS GENETICS
• The modern synthesis is the fusion of
– genetics with
– evolutionary biology.
© 2013 Pearson Education, Inc.
Populations as the Units of Evolution
• A population is
– a group of individuals of the same species, living in
the same place at the same time and
– the smallest biological unit that can evolve.
© 2013 Pearson Education, Inc.
Figure 13.18
(a) Two dense populations of
trees separated by a lake
(b) A nighttime satellite view of
North America
Populations as the Units of Evolution
• The total collection of alleles in a population at any
one time is the gene pool.
• When the relative frequency of alleles changes
over a number of generations, evolution is
occurring on its smallest scale.
© 2013 Pearson Education, Inc.
Sources of Genetic Variation
• Genetic variation results from processes that both
involve randomness:
1. mutations, changes in the nucleotide sequence of
DNA, and
2. sexual recombination, the shuffling of alleles
during meiosis.
© 2013 Pearson Education, Inc.
Sources of Genetic Variation
• For any given gene locus, mutation alone has little
effect on a large population in a single generation.
• Organisms with very short generation spans, such
as bacteria, can evolve rapidly with mutation as the
only source of genetic variation.
© 2013 Pearson Education, Inc.
Analyzing Gene Pools
• A gene pool
– consists of all the alleles in a population at any one
time and
– is a reservoir from which the next generation draws
its alleles.
• Alleles in a gene pool occur in certain frequencies.
© 2013 Pearson Education, Inc.
Figure 13.20
Figure 13.21
p  0.8
(R)
Allele frequencies
q  0.2
(r)
Eggs
R
r
p  0.8
q  0.2
RR
p2  0.64
Rr
pq  0.16
rR
pq  0.16
rr
q2  0.04
p2  0.64
(RR)
2pq  0.32
(Rr)
R
p  0.8
Sperm
r
q  0.2
Genotype frequencies
q2  0.04
(rr)
Population Genetics and Health Science
• The Hardy-Weinberg formula can be used to
calculate the percentage of a human population
that carries the allele for a particular inherited
disease.
© 2013 Pearson Education, Inc.
Microevolution as Change in a Gene Pool
• How can we tell if a population is evolving?
• A non-evolving population is in genetic equilibrium,
also known as Hardy-Weinberg equilibrium,
meaning the population’s gene pool is constant
over time.
• From a genetic perspective, evolution can be
defined as a generation-to-generation change in a
population’s frequencies of alleles, sometimes
called microevolution.
© 2013 Pearson Education, Inc.
MECHANISMS OF EVOLUTION
• The main causes of evolutionary change are
– genetic drift,
– gene flow, and
– natural selection.
• Natural selection is the most important, because it
is the only process that promotes adaptation.
© 2013 Pearson Education, Inc.
Figure 13.23-3
Only 5 of 10
plants leave
offspring
RR
RR
Only 2 of 10
plants leave
offspring
Rr
Rr
RR
rr
rr
Rr
RR
rr
Rr
Rr
Generation 1
p  0.7
q  0.3
Rr
RR
RR
RR
Rr
RR
RR
RR
rr
RR
RR
RR
RR
RR
RR
Rr
Generation 2
p  0.5
q  0.5
RR
RR
Generation 3
p  1.0
q  0.0
The Bottleneck Effect
• The bottleneck effect
– is an example of genetic drift and
– results from a drastic reduction in population size.
• Passing through a “bottleneck,” a severe reduction
in population size,
– decreases the overall genetic variability in a
population because at least some alleles are lost
from the gene pool, and
– results in a loss of individual variation and hence
adaptability.
© 2013 Pearson Education, Inc.
The Bottleneck Effect
• Cheetahs appear to have experienced at least two
genetic bottlenecks:
1. during the last ice age, about 10,000 years ago,
and
2. during the 1800s, when farmers hunted the
animals to near extinction.
• With so little variability, cheetahs today have a
reduced capacity to adapt to environmental
challenges.
© 2013 Pearson Education, Inc.
Figure 13.25
The Founder Effect
• The founder effect is likely when a few individuals
colonize an isolated habitat.
• This represents genetic drift in a new colony.
• The founder effect explains the relatively high
frequency of certain inherited disorders in some
small human populations.
© 2013 Pearson Education, Inc.
Figure 13.26
Africa
South
America
Tristan da Cunha
Gene Flow
• Gene flow
– is another source of evolutionary change,
– is separate from genetic drift,
– is genetic exchange with another population,
– may result in the gain or loss of alleles, and
– tends to reduce genetic differences between
populations.
© 2013 Pearson Education, Inc.
Natural Selection: A Closer Look
• Of all causes of microevolution, only natural
selection promotes adaptation.
• Evolutionary adaptation results from
– chance, in the random generation of genetic
variability, and
– sorting, in the unequal reproductive success
among the varying individuals.
© 2013 Pearson Education, Inc.
Evolutionary Fitness
• Relative fitness is
– the contribution an individual makes to the gene
pool of the next generation
– relative to the contributions of other individuals.
© 2013 Pearson Education, Inc.
Figure 13.28
Three General Outcomes of Natural Selection
1. Directional selection shifts the overall makeup
of a population by selecting in favor of one
extreme phenotype.
2. Disruptive selection can lead to a balance
between two or more contrasting phenotypic
forms in a population.
3. Stabilizing selection favors intermediate
phenotypes, occurs in relatively stable
environments, and is the most common.
© 2013 Pearson Education, Inc.
Frequency of
individuals
Figure 13.29
Evolved
Original
population population
(a) Directional selection
Original
population
Phenotypes (fur color)
(b) Disruptive selection
(c) Stabilizing selection
Sexual Selection
• Sexual selection is a form of natural selection in
which individuals with certain traits are more likely
than other individuals to obtain mates.
• Sexual dimorphism is a distinction in appearance
between males and females not directly associated
with reproduction or survival.
© 2013 Pearson Education, Inc.
Figure 13.30
(a) Sexual dimorphism in a finch species (b) Competing for mates