Lecture 26
Natural Selection
How Does Biological Evolution Happen?
How do heritable traits (genes) pass to the next generation?
 Theory that individual’s evolve
 Inheritance of acquired
characteristics
 Somehow passed on to offspring
 Theory that populations evolve
 Selection of genes already in the
population
 Changes the gene frequency of a
population
Evidence for Selection Theory of Change
 Human Directed Selection: (demonstrates the reality of selection)
 Domestication of plants and
animals
 Breeding for extreme variation
Evidence for Selection Theory of Change
 Natural Selection (the same process – without human direction)
 Drug & pesticide resistance
 Analogous anatomy
 Ecological equivalents
 Homologous anatomy
 Common ancestor
Charles Darwin and Natural Selection
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Darwin was an ordained Anglican minister who was fully convinced that
species were immutable
In 1831, Charles Darwin took on the role of naturalist of the ship HMS Beagle
The Beagle set sail on a five-year navigational trip around the world
Darwin studied a wide variety of plants and animals across the globe
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His observations eventually convinced him that evolution took place
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In 1859, he published his book On the Origin of Species
 Particularly on the Galapagos Islands
 Fossils of extinct species resembled living species in the same area
 Galapagos finches differed slightly in appearance but resembled those on the S.
American mainland
 In it he proposed that evolution occurs through natural selection
The Theory of Natural Selection
 Darwin observed 14 different finch species that differed mainly in beaks
and feeding habitats
 He concluded that this resulted from “descent with modification” from a
common ancestor, or Evolution
 Darwin was familiar with artificial selection used by breeders to produce
animals/plants with particular traits
 Darwin proposed that such trait selection could also occur in nature
which he termed natural selection
 Darwin was influenced by Thomas Malthus’
Essay on the Principle of Population (1798)
 Populations increase geometrically, while food
supply increases only arithmetically
 Thus, food supply will limit population growth
Four Fundamentals of Natural Selection
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Proposed independently by
Charles Darwin and Alfred
Wallace
1. Heritability of traits
2. Limiting factors exist in all
environments
3. Overproduction of offspring
4. Reproduction by those with
“best fit”
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Adaptation of a population is
the result of natural selection
On the Origin of Species
 Darwin drafted a preliminary transcript in
1842
 However, he shelved it for 16 years, probably
because of its controversial nature
 Alfred Russel Wallace (1823-1913)
independently developed a similar theory
 Correspondence between the two spurred
Darwin to publish his theory in 1859
 Darwin’s Origin of Species was disturbing
to many
 It suggested that humans and apes have a
common ancestor
 Darwin presented this argument directly in a
later book, The Descent of Man
How Natural Selection Produces Diversity
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Darwin believed that the
Galapagos finches all
evolved from a single
common ancestor
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The ancestor came from the
South American mainland
New arrivals occupied
different niches and were
subject to different
environmental pressures
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This resulted in a cluster of species
 A phenomenon termed adaptive radiation
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The 14 finch species that Darwin studied now occupy four types of niches
 Ground finches
 Tree finches
 Warbler finches
 Vegetarian finch
The Beaks of Darwin’s Finches
 The Grants’ research supported Darwin’s hypothesis
Population Genetics: The Hardy-Weinberg Rule
 Population genetics is the study of the properties of genes in a population
 Genetic variation in populations puzzled scientists
 Dominant alleles were believed to drive recessive alleles out of populations
 In 1908, G. Hardy and W. Weinberg pointed out that in large populations
with random mating, allele frequencies remain constant
 Dominant alleles do not, in fact, replace recessive ones
 Hardy and Weinberg came to their conclusion by analyzing allele
frequencies in successive generations
 If a population of 100 cats has 84 black and 16 white
 Then the frequencies of black and white phenotypes are 0.84 and 0.16,
respectively
 A population that is in Hardy-Weinberg equilibrium is not evolving
The Hardy-Weinberg equilibrium equation
(p + q)2 = p2 + 2pq + q2
Individuals homozygous
for allele b
Individuals homozygous
for allele B
Individuals heterozygous
for alleles B and b
By convention
The more common allele (B) is designated p
The less common allele (b) is designated q
p+q=1
B allele  Black color
b allele  White color
Calculating Allele Frequencies
Frequency of white (bb) cats = 16/100 = 0.16
=> q2 = 0.16
=> q = √ 0.16 = 0.4
p + q =1 => p = 1 – q = 1 – 0.4 = 0.6
What about genotype frequencies?
Frequency of the homozygous dominant
genotype is
p2 = (0.6)2 = 0.36
36 out of 100 cats are black (BB)
Frequency of the heterozygous
genotype is
2pq = 2(0.6)(0.4) = 0.48
48 out of 100 cats are black (Bb)
Why Allele Frequencies Change
 The Hardy-Weinberg equation is true only if the following five
assumptions are met
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Large population size
Random mating
No mutation
No migration
No natural selection
 Five evolutionary forces can significantly alter the allele frequencies of a
population
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Genetic drift
Nonrandom mating
Mutation
Migration
Selection
Why Allele Frequencies Change
1.
Genetic Drift
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2.
Nonrandom Mating
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3.
4.
Random loss of alleles (more likely to occur in smaller
population)
Founder effect: Small group of individuals establishes a
population in a new location
Bottleneck effect: A sudden decrease in population size to
natural forces
Mating that occurs more or less frequently than expected by
chance
Inbreeding Mating with relatives increases homozygosity
Outbreeding Mating with non-relatives increases heterozygosity
Mutation
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Errors in DNA replication
The ultimate source of new variation
Mutation rates are too low to significantly alter allele
frequencies on their own
Migration
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Movement of individuals from one population to another (A very
potent agent of change)
Immigration: movement into a population
Emigration: movement out of a population
5. Selection
 Some individuals leave behind more
offspring than others
 Artificial selection: Breeder selects for
desired characteristics
 Natural selection: Environment selects for
adapted characteristics
 Selection is a statistical concept
 One cannot predict the fate of any single
individual
 But it is possible to predict which kind of
individual will tend to become more
common in a population
Three kinds of natural selection
 Stabilizing Selection
 In humans, infants with intermediate weight at birth
have the highest survival rate
 In chicken, eggs of intermediate weight have the
highest hatching success
 Disruptive Selection
 In the African seed-cracker finch, large- and smallbeaked birds predominate
 Intermediate-beaked birds are at a
disadvantage: unable to open large seeds, too
clumsy to open small seeds
 Directional Selection
 Drosophila flies that flew toward light were eliminated
from the population
 The remaining flies were mated and the experiment
repeated for 20 generations
Sickle-Cell Anemia
 First detected on December 31st, 1904, sickle-cell anemia is a
hereditary disease affecting hemoglobin molecules in the blood
 The sickle-cell mutation changes the 6th amino acid in the b-hemoglobin
chain from glutamic acid to valine resulting in sickled red blood cells
 This causes hemoglobin molecules to clump
 In normal RBCs, the hemoglobin chains do not clump
 Sickle-cell homozygosity leads to a reduced life span
 Heterozygosity produces enough hemoglobin to keep RBCs healthy
Why is the defective allele still around?
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The disease originated in Central Africa
 It affects 1 in 500 African Americans, but it is almost unknown in other racial
groups
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People who are heterozygous for the sickle-cell allele have less susceptibility to
malaria
 This is an example of heterozygote advantage
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Stabilizing selection is thus acting on the sickle-cell allele
 It occurs because malarial resistance counterbalances lethal anemia
The Biological Species Concept
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Speciation is the species-forming process
 It involves progressive change
1. Local populations become increasingly specialized
2. Natural selection acts to keep them different enough
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Ernst Mayr coined the biological species concept
 “Species are groups of actually or potentially interbreeding natural
populations, which are reproductively isolated from other such
groups”
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Reproductively isolated populations
 Populations whose members do not mate with each other or who
cannot produce fertile offspring
Isolating Mechanisms
 Reproductive isolating
mechanisms are the barriers
that prevent genetic
exchange between species
 Prezygotic isolating
mechanisms
 Prevent the formation
of zygotes
 Postzygotic isolating
mechanisms
 Prevent the proper
functioning of zygotes
after they have
formed
Working with the Biological Species Concept
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Speciation is a two-part process
1. Identical populations must
diverge
2. Reproductive isolation must
evolve to maintain these
differences
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Speciation occurs much more
readily in the absence of gene
flow
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This much more likely in
geographically isolated
populations
Populations can become
geographically isolated for
several reasons
Types of Speciation
 Allopatric speciation
 The differentiation of geographically isolated populations into distinct
species
 Sympatric speciation
 The differentiation of populations within a common geographical
area into distinct species
 Instantaneous sympatric speciation may occur through polyploidy
 More than two sets of chromosomes
 Autopolyploidy  All chromosomes from one species
 Allopolyploidy  Chromosomes derived from two species, via
hybridization
 Much more common in plants than animals
Problems with the Biological Species Concept
 The biological species concept has been criticized for several reasons
 The extent to which all species are truly are reproductively isolated
 It is becoming increasingly evident that hybridization is not that
uncommon in plants and animals
 It can be difficult to apply the concept to populations that do not occur
together in nature
 It is not possible to observe whether they would interbreed naturally
 The concept is more limited than its name would imply
 Many organisms are asexual and reproduce without mating
 For these reasons, other concepts have been proposed to define a
species; however, none has universal applicability
 Because of the diversity of organisms, it may be that there is no single
definition of a species
Natural Selection and Behavior
 Adaptive traits confer evolutionary advantages in different ways
 Some behaviors reduce predation
 Egg-shell removal by gulls reduce predation by crows
 Other behaviors enhance energy intake
 This allows more offspring to be supported
 Other behaviors increase resistance to disease
 Still others enhance the ability to acquire a mate
 Every behavior that offers a survival advantage for an individual comes
with an associated cost
 Thus, for a behavior to be favored by natural selection, the benefits have
to outweigh the costs
Reproductive Behaviors
 Reproductive behaviors encompass a variety of animal behaviors,
including courtship
 Competition for mating opportunities has been termed sexual selection
 Intrasexual selection
 Competition between members of one sex (usually males)
 Intersexual selection
 Essentially, mate choice
 The benefits of mate choice for the female
 The male that provides the best offspring care
 The male that provides the best territory
 The male that provides the best genes
 The typical number of mates an animal has during its breeding season is
called the mating system
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Monogamy – One male and one female
Polygyny – One male and many females
Polyandry – One female and many males
Polygyny is more common than polyandry
Altruism and Group Living
 Altruism is the performance of an action that benefits
another individual at a cost to the actor
 Helpers at the nest in some bird species
 Sentinels that give predator-alarm calls in some mammalian
species
 The existence of altruism among animals is rather
perplexing
 Natural selection should operate against it
 Altruistic behavior may not be truly altruistic after all
 The actor may benefit
 Nest helpers may get parenting experience or inherit
territory
 Sentinels may be able to escape predators in the
confusion following the alarm call
 Individuals may benefit directly if there is a mutual
exchange of altruistic acts
 In reciprocal altruism, “cheaters” (nonreciprocators) are
discriminated against
 These individuals are cut off from receiving future aid
Altruism and Group Living
 An altruist compensates for the reduction in its own reproductive success
by increasing that of relatives
 Selection that favors altruism directed toward relatives is called kin
selection
 The more closely related two individuals are, the greater the
potential genetic payoff
 White-fronted bee-eaters
 Helpers tend to be close
relatives
 Helpers’ assistance
increases with genetic
relatedness