Transcript Life: The Science of Biology, 8e

David Sadava H. Craig Heller Gordon H. Orians
William K. Purves David M. Hillis
Biologia.blu
B – Le basi molecolari della vita e
dell’evoluzione
Evolution and Its
Mechanisms
Evolution and Its Mechanisms
• What facts form the base of our understanding of
evolution?
• What are the mechanisms of evolutionary change?
• What evolutionary mechanisms result in adaptation?
• How is genetic variation maintained within populations?
• What are the constraints on evolution?
Evolution and Its Mechanisms - What facts form the base of our understanding of
evolution?
Population genetics has three main
goals:
• explain the origin and maintenance of
genetic variation;
• explain patterns and organization of
genetic variation;
• understand mechanisms that cause
changes in allele frequencies.
Evolution and Its Mechanisms - What facts form the base of our understanding of
evolution?
Different forms of a gene are called alleles.
The gene pool is the sum of all copies of all
alleles at all loci in a population.
Evolution and Its Mechanisms - What facts form the base of our understanding of
evolution?
Populations have genetic variation for
many characters.
Artificial selection for different characters
in a single species of wild mustard
produced many crop plants.
Evolution and Its Mechanisms - What facts form the base of our understanding of
evolution?
Calculating Hardy–Weinberg genotype frequencies
(part 1)
Evolution and Its Mechanisms - What facts form the base of our understanding of
evolution?
Calculating Hardy–Weinberg genotype frequencies
(part 2)
Evolution and Its Mechanisms - What facts form the base of our understanding of
evolution?
For generation 1, probability of two A
alleles coming together is:
p  p  p  ( 0 . 55 )  0 . 3025
2
2
Probability of two a alleles:
q  q  q  ( 0 . 45 )  0 . 2025
2
2
Evolution and Its Mechanisms - What facts form the base of our understanding of
evolution?
There are two ways of producing a
heterozygote:
p  q or q  p , or 2 pq
The Hardy-Weinberg equation:
p  2 pq  q  1
2
2
Evolution and Its Mechanisms - What facts form the base of our understanding of
evolution?
Populations in nature never fit the
conditions for Hardy-Weinberg
equilibrium.
But, it is useful in predicting genotype
frequencies from allele frequencies; and,
because the model describes conditions
that would result in no evolution, patterns
of deviation from the model help identify
specific mechanisms of evolution.
Evolution and Its Mechanisms - What are the mechanisms of evolutionary change?
Hardy-Weinberg equilibrium is a null
hypothesis that assumes evolutionary
forces are absent.
Known evolutionary mechanisms:
• mutation;
• gene flow;
• genetic drift;
• nonrandom mating;
• natural selection.
Evolution and Its Mechanisms - What are the mechanisms of evolutionary change?
Mutation is the origin of genetic
variation.
Mutation is any change in DNA; it
appears to be random with respect to
the adaptive needs of an organism.
Most mutations are harmful or neutral,
but if conditions change, could become
advantageous.
Evolution and Its Mechanisms - What are the mechanisms of evolutionary change?
Mutations can also restore alleles that
other processes remove.
Mutation rates are low—about one per
locus in a million zygotes.
Creates a lot of variation because of the
number of genes that can mutate,
chromosome rearrangements that can
change many genes simultaneously,
and large numbers of individuals.
Evolution and Its Mechanisms - What are the mechanisms of evolutionary change?
Because mutation rate is low, mutations
in themselves result in only minor
deviations from Hardy-Weinberg
equilibrium.
If large deviations are found, it is
appropriate to look for other
mechanisms.
Evolution and Its Mechanisms - What are the mechanisms of evolutionary change?
Gene flow is a result of the migration of
individuals and movements of gametes
between populations.
New alleles can be added to the gene
pool, or allele frequencies changed.
Evolution and Its Mechanisms - What are the mechanisms of evolutionary change?
Genetic drift results from random
changes in allele frequencies.
In large populations, genetic drift can
influence frequencies of alleles that
don’t affect survival and reproduction.
If populations are reduced to a small
number of individuals—a population
bottleneck, genetic drift can reduce the
genetic variation.
Evolution and Its Mechanisms - What are the mechanisms of evolutionary change?
A population bottleneck
Evolution and Its Mechanisms - What are the mechanisms of evolutionary change?
Genetic drift also effects small
populations that colonize a new region.
Colonizing population is unlikely to have
all the alleles present in the whole
population.
Founder effect—equivalent to a
bottleneck.
Evolution and Its Mechanisms - What are the mechanisms of evolutionary change?
Example of founder effect:
Populations of European fruit fly D.
subobscura began in Chile, and then in
Washington state.
Both populations grew and expanded their
ranges.
These populations have very similar genetic
structure, and much less variation than the
European populations.
Evolution and Its Mechanisms - What are the mechanisms of evolutionary change?
A founder effect
Evolution and Its Mechanisms - What are the mechanisms of evolutionary change?
Nonrandom mating occurs when
individuals choose mates with particular
phenotypes. If individuals choose the
same genotype as themselves,
homozygote frequencies will increase.
Nonrandom mating in primroses
(Primula): two flower types—pin and
thrum. Pollen from one type can fertilize
only flowers of the other type.
Evolution and Its Mechanisms - What are the mechanisms of evolutionary change?
Flower structure fosters non-random mating
Evolution and Its Mechanisms - What evolutionary mechanisms result in adaptation?
Natural selection acts on phenotype.
Fitness is the reproductive contribution of a
phenotype to subsequent generations and it
is determined by the average rates of survival
and reproduction of individuals with that
phenotype.
Changes in the relative success of different
phenotypes in a population leads to change
in allele frequencies.
Evolution and Its Mechanisms - What evolutionary mechanisms result in adaptation?
Natural selection can act on characters
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 opposite directions from the
average.
Evolution and Its Mechanisms - What evolutionary mechanisms result in adaptation?
Natural selection can operate on quantitative variation
in several ways (part 1)
Evolution and Its Mechanisms - What evolutionary mechanisms result in adaptation?
Natural selection can operate on quantitative variation
in several ways (part 2)
Evolution and Its Mechanisms - What evolutionary mechanisms result in adaptation?
Natural selection can operate on quantitative variation
in several ways (part 3)
Evolution and Its Mechanisms - What evolutionary mechanisms result in adaptation?
Sexual selection (part 1)
Evolution and Its Mechanisms - What evolutionary mechanisms result in adaptation?
Sexual selection (part 2)
Evolution and Its Mechanisms - 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 “rightmouthed” individuals are both favored;
the host fish can be attacked from either
side.
Evolution and Its Mechanisms - How is genetic variation maintained within populations?
A stable polymorphism
Evolution and Its Mechanisms - 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 that influences flight at different
temperatures.
Heterozygotes are favored because they
can fly over a larger temperature range.
Evolution and Its Mechanisms - How is genetic variation maintained within populations?
A heterozygote mating advantage (part 1)
Evolution and Its Mechanisms - How is genetic variation maintained within populations?
A heterozygote mating advantage (part 2)
Evolution and Its Mechanisms - What are the constraints on evolution?
Lack of genetic variation can prevent
evolution of potentially favorable traits.
If the allele for a given trait does not exist
in a population, that trait cannot evolve,
even if it would be favored by natural
selection.
Evolution and Its Mechanisms - What are the constraints on evolution?
Evolution must work within the boundaries of
universal constraints such as:
• cell size, constrained by surface area-tovolume ratios;
• protein folding, constrained by types of
bonding that can occur;
• laws of thermodynamics that constrain energy
transfers.
Developmental processes also constrain
evolution.
Evolution and Its Mechanisms - What are the constraints on evolution?
All evolutionary innovations are modifications of
previously existing structures.
Example: two lineages of bottom-dwelling
fishes.
A)Skates and rays evolved from a common
ancestor with sharks. They started with a
flattened body plan, and can swim along the
ocean floor.
B) Sole and flounder evolved from laterally
flattened bony fishes. They can’t swim well, but
lie still on the bottom. The eyes gradually
shifted to one side.
Evolution and Its Mechanisms - What are the constraints on evolution?
Two solutions to a single problem