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David Sadava, David M. Hillis,
H. Craig Heller, May R. Berenbaum
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Genetica, biologia molecolare ed
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Mechanisms of Evolution
What Is the Relationship between Fact and Theory in Evolution?
Evolution is the change in genetic composition of
populations over time.
Evolutionary change is observed in lab experiments,
natural populations, and the fossil record.
These genetic changes drive the origin and extinction
of species and the diversification of life.
Biologists have also accumulated evidence on how
evolutionary changes occur.
Evolutionary theory is the understanding of the
mechanisms of evolutionary change.
What Is the Relationship between Fact and Theory in Evolution?
Even before Darwin, several biologists had suggested
that species change over time.
But no one had proposed a viable mechanism for
evolution.
The young Charles Darwin was passionately
interested in geology and natural history.
In 1831 he was recommended for a position on the
HMS Beagle, for a five-year survey voyage around
the world.
What Is the Relationship between Fact and Theory in Evolution?
Darwin published his book, The Origin of Species, in
1859.
The book provided exhaustive evidence from many
different fields to support evolution and natural
selection.
But the genetic basis of evolutionary change was not
understood until after 1900.
What Is the Relationship between Fact and Theory in Evolution?
In the twentieth century, Gregor Mendel’s publications
were rediscovered, the roles of chromosomes and
mutations were discovered, and population genetics
was established.
A “modern synthesis” of genetics and evolution took
place over the period 1936–1947.
In 1953, James Watson and Francis Crick published
their paper on the structure of DNA.
What Is the Relationship between Fact and Theory in Evolution?
Evolution is genetic change occurring in a
population—a group of individuals of a single
species that live and interbreed in a particular
geographic area.
Individuals do not evolve, populations do.
The gene pool is the sum of all copies of all alleles at
allcontains
loci in a the
population.
It
genetic variation
that produces the phenotypic
traits on which natural
selection acts.
How Does Natural Selection Result in Evolution?
Much of evolution occurs through gradual changes in
the relative frequencies of alleles in a population from
one generation to the next.
Allele frequencies (p) are estimated by counting
alleles in a sample of individuals:
How Does Natural Selection Result in Evolution?
If a locus has two alleles, A and a, there could be
three genotypes: AA, Aa, and aa. The population is
polymorphic at that locus.
If p is the frequency of allele A, and q is the frequency
of allele a,
p+q=1
q=1–p
If there is only one allele at a locus, its frequency = 1.
The population is monomorphic at that locus; the
allele is said to be fixed.
How Does Natural Selection Result in Evolution?
If certain conditions are met, the genetic structure of a
population does not change over time.
The Hardy–Weinberg equilibrium describes a model
situation in which allele frequencies do not change.
Genotype frequencies can be predicted from allele
frequencies.
How Does Natural Selection Result in Evolution?
Conditions that must be met for Hardy–Weinberg
equilibrium:
• No mutation
• No selection among genotypes
• No gene flow
• Population size is infinite (no genetic drift)
• Mating is random
How Does Natural Selection Result in Evolution?
If these conditions hold:
• Allele frequencies remain constant
• After one generation, genotype frequencies occur in
these proportions:
Genotype
AA
Aa
aa
Frequency p2
2pq
q2
How Does Natural Selection Result in Evolution?
There are two ways of producing a heterozygote:
p  q or q  p, or 2pq
The Hardy–Weinberg equation:
p2 + 2pq + q2 = 1
One Generation of Random Mating Restores Hardy–Weinberg Equilibrium
How Does Natural Selection Result in Evolution?
Populations in nature never fit the conditions for
Hardy–Weinberg equilibrium.
But it is useful for predicting genotype frequencies
from allele frequencies.
Also, patterns of deviation from the model help
identify mechanisms of evolutionary change.
What Are the Mechanisms of Evolutionary Change?
In addition to natural selection there are four other
mechanisms of evolution:
• Mutation
• Gene flow
• Genetic drift
• Nonrandom mating
What Are the Mechanisms of Evolutionary Change?
Mutation is the origin of genetic variation.
Mutation is any change in the nucleotide sequences of
DNA.
Mutations are random with respect to the needs of an
organism; selection acting on the random variation
results in adaptation.
Most mutations are harmful or neutral; a few are
beneficial.
Mutations (even low rates) can also restore genetic
variation that other processes have removed.
What Are the Mechanisms of Evolutionary Change?
Gene flow is a result of the migration of individuals
and movement of gametes between populations.
New individuals can add alleles to the gene pool or
change allele frequencies.
What Are the Mechanisms of Evolutionary Change?
Genetic drift results from random changes in allele
frequencies.
Harmful alleles may increase in frequency, and rare
advantageous alleles may be lost.
In large populations, genetic drift can influence
frequencies of alleles that do not affect survival and
reproduction.
What Are the Mechanisms of Evolutionary Change?
In small populations, genetic drift can be significant.
Population bottleneck: environmental conditions
result in survival of only a few individuals. Genetic drift
can reduce genetic variation in the population.
What Are the Mechanisms of Evolutionary Change?
Genetic drift also affects small populations that
colonize a new region.
The colonizing population is unlikely to have all the
alleles present in the whole population.
This is called a founder effect (equivalent to a
population bottleneck).
What Are the Mechanisms of Evolutionary Change?
Non-random mating occurs when individuals choose
mates with particular phenotypes.
Example: self-fertilization is common, especially in
plants.
If individuals choose the same genotype as
themselves, homozygote frequencies will increase.
How Does Natural Selection Result in Evolution?
Natural selection acts on the phenotype rather than
directly on the genotype.
The reproductive contribution of a phenotype to
subsequent generations relative to other phenotypes
is called its fitness.
Only changes in the relative success of different
phenotypes lead to change in allele frequencies.
How Does Natural Selection Result in Evolution?
Natural selection can act on traits with quantitative
variation in three ways:
• Stabilizing selection
preserves average
phenotype.
• Directional selection
favors individuals that vary
in one direction.
• Disruptive selection
favors individuals that vary
in both directions from the
mean.
What Are the Mechanisms of Evolutionary Change?
Sexual selection is a form of non-random mating that
favors traits that increase the chances of reproduction.
Sexual selection may favor traits that enhance an
individual’s chances of reproduction but reduce its
chances of survival.
How Is Genetic Variation Maintained within Populations?
Many mutations do not affect the function of the
resulting proteins.
An allele that does not affect fitness is a neutral allele.
They tend to accumulate in a population.
Sexual reproduction results in new combinations of
genes through the combination of gametes, crossing
over, and independent assortment.
Sexual recombination produces genetic variety that
increases evolutionary potential.
How Is Genetic Variation Maintained within Populations?
Frequency-dependent selection: a polymorphism
can be maintained when fitness depends on its
frequency in the population.
Example: A scale-eating fish in Lake Tanganyika.
“Left-mouthed” and “right-mouthed” individuals are
both favored; the host fish can be attacked from either
side.
How Is Genetic Variation Maintained within Populations?
Much genetic variation is maintained in
subpopulations of a species in different geographic
regions.
The subpopulations may be subjected to different
environmental conditions and selective pressures.
Example: Populations of white clover
that produce cyanide are more likely
to be killed by frost.
In Europe, there is gradual change
in phenotype—a clinal variation.
How Is Genetic Variation Maintained within Populations?
Environmental variation also helps to preserve
genetic variation.
Example: Colias butterflies live in an environment
with temperature extremes. The population is
polymorphic for an enzyme (PGI) that influences flight
at different temperatures.
Heterozygotes are favored because they can fly over
a larger temperature range.
What Are the Constraints on Evolution?
Adaptation: trait that increases the chance that a
given individual will survive and reproduce, increasing
the frequency of the trait in the next generation.
All evolutionary innovations are modifications of
previously existing structures.
Example: Two lines of bottom-dwelling fishes.
• Skates and rays
• Sole and flounder
What Are Species?
Biological species concept: groups of actually or
potentially interbreeding populations that are
reproductively isolated from other such groups.
Species are the result of speciation: the divergence
of biological lineages and emergence of reproductive
isolation.
Reproductive isolation, when groups can no longer
exchange genes, is a key factor in the divergence of
sexually reproducing organisms.
What Is the Genetic Basis of Speciation?
Not all evolutionary change results in new species.
Speciation usually requires the evolution of
reproductive isolation.
How can one lineage ever split into two reproductively
isolated species?
What Barriers to Gene Flow Result in Speciation?
Allopatric speciation occurs
when populations are
separated by a physical
barrier.
Thought to be the dominant
mode of speciation in most
groups.
The environments in which
the isolated populations live
are different, and so the
populations evolve differently.
What Barriers to Gene Flow Result in Speciation?
Sympatric speciation is speciation without physical
isolation.
This can occur with disruptive selection if certain
genotypes have a preference for distinct microhabitats
where mating takes place.
Sympatric speciation most commonly occurs by
polyploidy—duplication of whole sets of
chromosomes.
Chromosome duplication in a single species is
autopolyploidy; combining of chromosomes from two
species is allopolyploidy.
What Happens when Newly Formed Species Come into Contact?
Prezygotic isolating mechanisms prevent
hybridization from occurring.
• Mechanical
• Temporal
• Behavioral
• Habitat
• Gametic
What Happens when Newly Formed Species Come into Contact?
Postzygotic isolating mechanisms reduce the
fitness of hybrid offspring.
• Low hybrid zygote viability—zygotes fail to mature or
have severe abnormalities.
• Low hybrid adult viability—offspring have lower
survival rates.
• Hybrid infertility—offspring are infertile (e.g., mules).
Adapted from
Life: The Science of Biology, Tenth Edition, Sinauer Associates, Sunderland, MA, 2014
Inc. All rights reserved