Natural Selection and
Adaptation
Natural Selection
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Evolution: changes in a population’s gene pool (genetic makeup) over
time.
Natural selection: differential survival and reproduction of individuals
that differ from one another in [heritable] characteristics (phenotypes).
Survival of the fittest, not the strongest.
Fitness: survival + reproductive output (via female and male functions).
Snail coloration
best adapted
to conditions
Average
Natural
selection
Number of individuals
Number of individuals
Directional Natural Selection
New average
Coloration of snails
Previous
average
Average shifts
Coloration of snails
Proportion of light-colored
snails in population increases
Light snails
eliminated
Dark snails
eliminated
Natural
selection
Number of individuals
Number of individuals
Stabilizing Natural Selection
Snails with
extreme
coloration are
eliminated
Coloration of snails
Coloration of snails
Average remains the same,
but the number of individuals with
intermediate coloration increases
Intermediate-colored snails
are selected against
Light
Dark
coloration
coloration
is favored
is favored
Natural
selection
Number of individuals
Number of individuals
Diversifying Natural Selection
Snails with light and dark
colors dominate
Coloration of snails
Coloration of snails
Number of individuals
with light and dark coloration
increases, and the number with
intermediate coloration decreases
Adaptation
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A trait that enhances fitness
compare with at least some
alternative traits.
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Ahistorical definition:
“A phenotypic variant that results in
the highest fitness among a
specified set of variants in a given
environment”
(Reeve & Sherman 93)
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Historical definition:
“A derived trait that evolved in
response to a specific selective
agent”
(Harvey & Pagel 1991)
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Adaptation ≠ phylogenetic history
Adaptation
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Natural selection on populations
results in organisms better fit to
their environment
Mimicry of leaves by insects is an
adaptation for evading predators.
The creosote bush is a desertdwelling plant that produces toxins
that prevent other plants from
growing nearby, thus reducing
competition for nutrients and
water.
Echolocation in bats is an
adaptation for catching insects.
Fish species that live in
completely dark caves have
vestigial, non-functional eyes.
Adaptation
Selection for ≠ selection of
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Selection for a trait may result in the selection of
another trait correlated with it
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Traits targeted by selection are selected for
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Preadaptation – a trait that fortuitously serves a new
function
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Exaptation – a feature that performs a function but that
was not produced by natural selection for its current
use. A co-opted preadaptation: the trait was produced
by natural selection for a function other than the one it
currently performs and was then co-opted for its current
function. For example, feathers might have originally
arisen in the context of selection for insulation, and only
later were they co-opted for flight. In this case, the
general form of feathers is an adaptation for insulation
and an exaptation for flight.
Sexual Selection
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Differential reproductive
success resulting from
competition for mates
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Natural selection with a
twist
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Can result in runaway
selection
Recognizing adaptations
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Byproducts of physics or
chemistry
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Phylogenetic history
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Random genetic drift
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Trait Correlation
– Genetic hitchhiking
– Pleiotropy
Recognizing adaptations
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Complexity: product of natural selection
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Design: function of a trait inferred from
– Engineering design
– Prediction of a model
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Experiments: comparing fitness of
individuals possessing a trait with those in
which the feature is modified or absent
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The comparative method: comparing
sets of species to test adaptation or other
evolutionary phenomena
Recognizing adaptations
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The comparative method: comparing sets of species to test
adaptation or other evolutionary phenomena
Nectarinia pulchella
Cyanerpes caeruleus
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Vestiaria coccinea
Campylopterus hemileucurus
Phylogenetic relationships among taxa can ‘inflate’ observed
correlation of traits (samples not independent)
Y
Y
X
X
Evolutionary changes may be maladaptive
Via e.g. mutation, inbreeding, drift, gene flow, the effect of a
single gene on multiple traits.
House of
Habsburg
Evolution doesn’t have to result in the best design
E.g. Vertebrate eye's blind spot.
Limits on adaptation
– Natural selection can only work on the variation that is available
through genetic recombination and mutation
– Slow process
Fitness Components
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Fitness = reproductive success (R)
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Survival (L)
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Female fecundity (M)
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R = LM
Sexual / asexual reproduction
Iteroparous
Overlapping generations (r)
Age at female maturity
Allele frequencies in gametes
– Zygotic selection
– Gametic selection
Fitness
Populations
Individuals
Net reproductive rate:
Ro=LRS, lx = 0 or 1
Characteristic equation:
r = IRI = indv fitness
24
Phenotype
(body size)
22
20
AA
Aa
aa
Genotype
5
Fitness
(no. offspring)
2
1
AA
Aa
aa
Genotype
Topic 4b.ppt
Multivariate Selection
Frequency dependent
selection
Topic 4b.ppt
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Positive frequency
dependence:
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Eliminates variation
within populations
Frequency
dependent selection
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Negative frequency
dependence:
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Maintains variation within
populations
Topic 4b.ppt
Population Genetics Synthesis
Within-pop
Variation
Mutation
Migration
Assort. mating
Genetic Drift
Selection
Between-pop
Variation
Affects all
loci?
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No
Yes
No
Yes
No
Topic 4b.ppt
Response to Selection
Response to selection depends on the selection
differential and the heritability of the trait in
question:
R = h2 * S
http://www.micro.utexas.edu/courses/levin/bio304/popgen/selection.html
Meyers & Bull 2002, TREE
Meyers & Bull 2002, TREE
Natural Selection
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Although several different environmental forces can change the
genetic structure of organisms and populations, natural selection
is the only one that causes organisms to become well adapted to
their environment.
Natural selection is not the same as evolution.
Natural selection is not the same as evolution by natural
selection.
Natural selection is variation in average reproductive success
among different phenotypes.
Natural selection can have no evolutionary effect unless
phenotypes are at least partially heritable (meaning that some of
the phenotypic variation is due to genetic variation).
Topic 4b.ppt
Levels of Selection
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Genes
Individuals
Populations
Species
Levels of Selection
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Genes: e.g. meiotic drive and retrotransposons
http://www.cartoonstock.com
Levels of Selection
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Individual
http://www.cartoonstock.com
Levels of Selection
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Group: e.g. altruism, kin selection
Levels of Selection
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Species: e.g. differential speciation rates among higher taxa
Orchidaceae
Iridaceae