Transcript darwin`s theory of evolution

Chapter 13
How Populations Evolve
Clown, Fool, or Simply Well Adapted?
• The blue-footed booby has many specialized
characteristics that are very functional in water
but less useful on land
• Such evolutionary adaptations are inherited
traits that enhance an organism's ability to
survive and reproduce in its particular
environment
• Evolution is the changes in organisms over
time
DARWIN'S THEORY OF EVOLUTION
13.1 A sea voyage helped Darwin frame his
theory of evolution
• Pre-Darwinian ideas about the origin of
species
– Early Greek philosophers: Simpler life forms
preceded more complex ones
– Aristotle: Species are fixed and do not
evolve; had a great impact on Western
thinking
– Judeo-Christian biblical view: All species
were individually designed by a divine
creator
• In the century prior to Darwin, only a few
scientists questioned the belief that species
are fixed
– Buffon: The study of fossils suggested that
Earth is older than 6,000 years, and fossil
forms might be early versions of modern
forms
– Lamarck: Fossils are related to modern
forms because life evolves; acquired
characteristics are inherited
• Charles Darwin made a round-the-world sea
voyage as a naturalist on HMS Beagle in the
1830s
– Darwin observed similarities between living
and fossil organisms and the diversity of life
on the Galápagos Islands
– Darwin's experiences during the voyage
helped him frame his ideas about evolution
– Lyell's Principles of Geology led him to
realize that still-operating natural forces
gradually change Earth
• After his return, Darwin began to document his
observations and his new theory of evolution
– Alfred Wallace conceived a theory almost
identical to Darwin's; both works were
presented to the scientific community
– Darwin's On the Origin of Species by Means
of Natural Selection was published in 1859
• "Descent with modification" summarizes
Darwin's view of life
– All organisms are related through descent
from a remote common ancestor
– Descendants spread into diverse habitats
over millions of years and acquired
adaptations to their environments
– The history of life resembles a tree with
multiple branchings from a common trunk
– Species that are closely related share
characteristics
13.2 Darwin proposed natural selection as the
mechanism of evolution
• The essence of Darwin's theory of natural
selection is differential success in reproduction
– Organisms produce more offspring than the
environment can support
– Organisms vary in many characteristics that
can be inherited
– Excessive numbers of organisms lead to a
struggle for survival
– Individuals whose characteristics are best
adapted to their environment are more likely
to survive and reproduce
– The unequal ability of individuals to survive
and reproduce leads to a gradual change in
the characteristics of a population over
generations
• Natural selection is supported by evidence
from artificial selection
13.3 The study of fossils provides strong
evidence for evolution
• Fossils are the hard parts of organisms that
remain after organic materials decay
– Rarely, an entire organism is fossilized
• The fossil record strongly supports the theory
of evolution
– Changes in sea level and drying and
refilling of lakes over time result in rock
strata that trap organisms
– Fossils appear in an ordered array within
layers of sedimentary rocks
– The fossil record reveals that organisms
have evolved in a historical sequence
• Many fossils link early extinct species with
species living today
Video: Grand Canyon
13.4 A mass of other evidence reinforces the
evolutionary view of life
• Biogeography
– The geographic distribution of species
suggested to Darwin that organisms evolve
from common ancestors
– Isolated organisms resemble each other
more than organisms in similar but distant
places
• Comparative anatomy
– Homologous structures are features that
often have different functions but are
structurally similar because of common
ancestry
– Vestigial structures are remnants of
structures that served important functions in
an organism's ancestors
LE 13-4a
Human
Cat
Whale
Bat
• Comparative embryology
– Common embryonic structures in all
vertebrates are evidence for common
descent
• Molecular biology
– Comparisons of DNA and amino acid
sequences between different organisms
reveal evolutionary relationships
– Molecular biology provides strong evidence
that all life forms are related
CONNECTION
13.5 Scientists can observe natural selection in
action
• Examples of evolutionary adaptation observed
over a short time
– Different camouflage adaptations in
different environments
– Development of pesticide resistance in
insects
Video: Seahorse Camouflage
LE 13-5b
Chromosome with gene
conferring resistance
to pesticide
Additional
applications of the
same pesticide will
be less effective, and
the frequency of
resistant insects in
the population
will grow
Pesticide application
Survivor
• Examples of evolutionary adaptation reveal
three key points about natural selection
– Natural selection is more of an editing
process than a creative mechanism
– Natural selection is contingent on time and
place
– Significant evolutionary change can occur in
a short time
POPULATION GENETICS AND THE MODERN
SYNTHESIS
• 13.6 Populations are the units of evolution
– Population
• A group of individuals of the same species
living in the same place at the same time
• May be isolated from other groups or
concentrated
• The smallest unit that can evolve
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• Population genetics
– Combines Darwin's and Mendel's ideas in
studying how populations change
genetically over time
– The modern synthesis
• Connects population genetics with other
sciences
• Focuses on population as the unit of
evolution and central role of natural
selection
• Studying evolution at the population level
– Evolution: change in the prevalence of
certain heritable characteristics in a
population over a span of generations
– Gene pool: the total collection of genes in a
population at any one time
– Microevolution: a change in the relative
frequencies of alleles in a gene pool
– Species: a group of populations capable of
interbreeding and producing fertile offspring
13.7 The gene pool of a nonevolving population
remains constant over the generations
• In a nonevolving population, the shuffling of
alleles that accompanies sexual reproduction
does not alter the genetic makeup of the
population
• In Hardy-Weinberg equilibrium, the frequency
of each allele in the gene pool will remain
constant unless acted upon by other agents
• For a population to be in Hardy-Weinberg
equilibrium, it must satisfy five main conditions
– The population is very large
– The population is isolated
– Mutations do not alter the gene pool
– Mating is random
– All individuals are equal in reproductive
success
• The Hardy-Weinberg conditions are rarely met
in nature
– We can follow alleles in a population to
observe if Hardy-Weinberg equilibrium
exists
– Hardy-Weinberg equilibrium provides a
basis for understanding how populations
evolve
LE 13-7a
Webbing
No webbing
LE 13-7b
Phenotypes
Genotypes
WW
Ww
ww
Number of animals
(total = 500)
320
160
20
Genotype frequencies
320
500
160
500
= 0.64
Number of alleles
in gene pool
(total = 1,000)
640 W
Allele frequencies
800
1,000
= 0.32
160 W + 160 w
= 0.8 W
200
1,000
20
500
40 w
= 0.2 w
= 0.04
LE 13-7c
Recombination
of alleles from
parent generation
Sperm
W sperm
p = 0.8
w sperm
q = 0.2
WW
= 0.64
Ww
pq = 0.16
p2
W egg
p = 0.8
Eggs
w egg
q = 0.2
wW
qp = 0.16
q2
ww
= 0.04
Next generation:
Genotype frequencies
Allele frequencies
0.64 WW
0.8 W
0.32 Ww
0.04 ww
0.2 w
CONNECTION
13.8 The Hardy-Weinberg equation is useful in
public health science
• Public health scientists use the HardyWeinberg equation to estimate frequencies of
disease-causing alleles in the human
population
– Example: phenylketonuria (PKU)
13.9 In addition to natural selection, genetic drift
and gene flow can contribute to evolution
• Genetic drift: change in the gene pool of a
population due to chance
– Can alter allele frequencies in a population
– The smaller the population, the greater the
impact
• Bottleneck effect: an event that drastically
reduces population size
• Founder effect: colonization of a new
location by a small number of individuals
• Gene flow: the movement of individuals or
gametes between populations
– Can alter allele frequencies in a population
– Tends to reduce differences between
populations
• Natural selection
– Best-adapted individuals have the most
reproductive success
– Results in accumulation of traits that adapt
a population to its environment
MECHANISMS OF CHANGE
MUTATIONS
GENE FLOW (MIGRATION)
GENETIC DRIFT
NATURAL SELECTION
CONNECTION
13.10 Endangered species often have reduced
variation
• Loss of genetic variability due to bottlenecking
may reduce a population's ability to adapt to
environmental change
– Particularly threatening to endangered
species such as the cheetah
VARIATION AND NATURAL SELECTION
13.11 Variation is extensive in most populations
• Individual variation exists in all sexually
reproducing populations
• Heritable variation results from a combination
of genotype and environmental influences
– Polymorphism: two or more forms of
phenotypic characteristics
– Geographic variation: variation of an
inherited characteristic from place to place
• May occur along a geographic continuum (a
cline)
13.12 Mutation and sexual recombination
generate variation
• Mutations-changes in the nucleotide sequence
of DNA-can create new alleles
– Only mutations in cells that produce
gametes can affect a population's gene pool
– A mutation may rarely improve adaptation
to the environment and thus contribute to
evolution
• Sexual recombination generates variation by
shuffling alleles during meiosis
LE 13-12a
A1
Parents
A2
A1
A3

Meiosis
A1
Gametes
A2
A3
LE 13-12b
A1
A2
A3
Gametes
Fertilization
Offspring,
with new
combinations
of alleles
A1
A1
A2
and
A3
CONNECTION
13.13 The evolution of antibiotic resistance in
bacteria is a serious public health concern
• Natural selection has led to the evolution of
antibiotic-resistant bacteria
• Overuse and misuse of antibiotics has
contributed to the proliferation of antibioticresistant strains
– Example: tuberculosis
13.14 Diploidy and balancing selection preserve
variation
• Diploidy (two sets of chromosomes) helps to
prevent populations from becoming genetically
uniform
– Recessive alleles are "hidden" from natural
selection and remain in the population
• Balancing selection allows two or more
phenotypic forms in a population
– Balanced polymorphism may result from
• Heterozygote advantage; example: sickle-cell
disease
• Frequency-dependent selection
• Neutral variation provides no apparent
advantage or disadvantage
– Example: fingerprints
13.15 The perpetuation of genes defines
evolutionary fitness
• Evolutionary fitness is the relative contribution
an individual makes to the gene pool of the
next generation
• Survival of genes depends on production of
fertile offspring
• Selection indirectly adapts a population to its
environment by acting on phenotype
13.16 Natural selection can alter variation in a
population in three ways
• Stabilizing selection: favors intermediate
phenotypes
• Directional selection: acts against individuals
at one of the phenotypic extremes
• Disruptive selection: favors individuals at both
extremes of the phenotypic range
Frequency of individuals
LE 13-16
Original
population
Phenotypes (fur color)
Original
population
Evolved
population
Stabilizing selection
Directional selection
Disruptive selection
13.17 Sexual selection may produce sexual
dimorphism
• Sexual dimorphism
– The distinction in appearance between
males and females of a species
• Sexual selection
– The determining of "who mates with whom"
– Leads to the evolution of secondary sexual
characteristics that may give individuals an
advantage in mating
13.18 Natural selection cannot fashion perfect
organisms
• There are at least four reasons why natural
selection cannot produce perfection
– Organisms are limited by historical
constraints
– Adaptations are often compromises
– Chance and natural selection interact
– Selection can only edit existing variations