Download The Economy of Nature 6/e

+ Chapter 6: Evolution and adpatation
Darwin’s finches
+ Heavy rains during El Nino events support lush plant growth in
the archipelago.
+
What this does us?

Finches do not survive or die at random

Because the average hardness of seeds increased as the
drought intensified and the softest seeds were consumed 
birds with larger beaks that could generate the forces
needed to crack hard seeds survived better than those with
smaller beaks

The average beak size of surviving individuals and their
progeny increased significantly

What is necessary for such ‘evolution in action’?
+
Genetics review

The phenotype is the outward expression of an individual's
genotype

Genotype: unique genetic constitution

Phenotype: outward expression of that genotype

A genotype = set of genetic instructions; blueprints

Phenotype = the expression of that genotype in the form of an
organism

(is that enough? Are there external factors?)

Effects of environmental influences are like details in a
blueprint that are left to the discretion of the building
contractor.. What does that mean?
+
More genetics

All phenotypic traits have:

Genetic basis + influence by variations in the environment

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What kind of environmental variations?
Phenotypic plasticity

Capacity of an individual to exhibit different responses to its
environment

How the individual responds to environmetnal variation
+
Genetic variation (review, right?)

Alleles

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Heterozygous
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Different forms of a particular gene
Two different alleles for a particular gene
Homozygous

Both copies of a gene are the same

Dominant… Recessive…
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Gene pool

All the alleles of the genes of every individual in a population
+
Sources of genetic variation

How does genetic variation arise?

Mutation


Any change in the sequence of the nucleotides that make up a
gene or in regions of the DNA that control the expression of a
gene
Consequence?
 Drastic – maybe lethal – changes in the phenotype
 No detectable effect – silent mutations
 New phenotypes produced  better suited to the local
environment  phenotypes increase
 Multiple effects  pleiotropy (effects of a single gene on
multiple traits)
+
Genetic basis of continuously
varying phenotypic traits

Many phenotypic traits with ecological relevance vary
continuously over a range of values (eg: body size)
+ Adaptations result from natural
selection on heritable variation in traits
that effect evolutionary fitness

The most important consequence of genetic variation for the
study of ecology is evolution by natural selection

Evolution

Any change in a population’s gene pool (what is a gene pool?)

Individuals whose traits enable them to have higher rates of
reproduction have more offspring  alleles increase

Adaptations or evolutionary adaptation

Process = adaptation
+
Adaptation (process of evolution
by natural selection)
Variation among individuals
1.

Eg – bird beaks; different individuals have
different-sized beaks
Inheritance of that variation
2.

Size of bird’s beak has an existence of its own in
a population; individual is borrowing that trait
Differences in survival and reproductive
success (or fitness) related to that variation
3.

Fitness: production of descendants over an
individual’s lifetime.
+
Evolutionary change
Change in a California citrus
pest
Cyanide fumigation no longer
effective
+
Stabilizing, directional, and
disruptive selection

Stabilizing selection

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Directional



Individuals with intermediate (average) phenotypes have higher reproductive
success
Population moved towards an optimum point
Maintains a single fittest phenotype
When the environment of a population is relatively unchanging: dominant
mode; little evolutionary change
Fittest individual have a more extreme phenotype;
When new optimum reached – becomes stabilizing selection
Disruptive

Increase genetic and phenotypic variation within a population and in the
extreme case creates a bimodal distribution of phenotypes; relatively
uncommon; eg: individuals specializing on one of a small number of food
resources; strong competition among individuals
+
Example of
Disruptive selection
+ Selection and change in melanistic moths
(peppered moths)
Dark form: more popular in forests
near industrialized regions
Industrial
melanism
+
Criticism of Kettlewell’s research

Moths that were used for the mark – recapture experiments
were reared in the lab

This might have affected their behavior – eg – choice of resting
locations

Experimental moths released at unnaturally high densities –
might have affected the behavior of predators

So?

But: with pollution control  forests became cleaner 
frequencies of melanistic moths decreased (as predicted by
evolutionary theory)
+
Population genetics and the
prediction of evolutionary change

Population genetics


Study of the dynamics of natural selection and genetic change in
populations
Populations are continually engaged in dynamic evolutionary
relationships with their environment that shape their ecological
interactions
(one) Goal of population genetics  to develop methods for
predicting changes in gene frequencies in response to selection
Why?
Ability to predict them can tell us whether the genetic changes we
observe are consistent with our understanding of evolution
(check out the ‘more on the web’ links)
+
Population genetics and ecologists
every population harbors some genetic variation that
influences fitness .. Potential for evolution exists in all
populations
1.

Except?
Changes in the environment will almost always be met by an
evolutionary response that shifts the frequencies of
genotypes within the population. (translate?)
2.

Magnitude of the evolutionary response depends on genetic
variation present in the pop at a given time
Rapid environmental changes brought about by the
appearance of new adaptations in populations of enemies or
by human-caused changes in the environment (eg?) can
exceed the capacity of a population to respond by evolution
3.

So?
+
Individuals can respond to their
environments and increase their fitness

Evolution: less fit individuals replaced by the progeny of
more fit individuals in a population over time

Individual himself/herself does not benefit from evolution.
Explain?

Still: individuals can undergo changes that help them cope
with variation in their environment during their lifetime

 phenotypic plasticity  capacity to respond to environmental
variation
How?
+
Cactus wren: adapted to the desert
environment

Insectivorous bird that lives in
deserts

No source of drinking water  must
not get too much heat from the
environment

In the desert: seeks favorable
microhabitats

Cool mornings: forage;

Afternoon: finds cooler parts;
shade
+
Temperature affects
microhabitat use by cactus
wrens
+
Orientation of cactus wren nest
entrances changes over the
breeding season
Lengths of the bars represent
relative number of nests with
each orientation

Behavioral flexibility of the
cactus wren in choosing where
to forage and how to orient its
nest is a good example of the
more general ability of the
phenotype to respond to
variation in the environment
+
Phenotypic plasticity allows individuals
to adapt to environmental change

Reaction norm  observed relationship between the
phenotype of an individual and the environment
+
+
Phenotypic plasticity allows individuals
to adapt to environmental change

Some reaction norms are a simple consequence of the
influence of the physical environment on life (heat energy 
accelerates most life processes  certain caterpillars grow
faster at higher temperatures … but individuals of the same
butterfly species from MI and AL have different relationships
between growth rate and temperature…)
+

Reaction norms
of populations
adapted to
different
environments
may differ
+

Reaction norms may be modified by evolution

May diverge when two populations of the same species exist
for long periods under different conditions…
+
acclimatization

 a shfit in an individual’s range of physiological tolerances

 generally useful in response to seasonal and other
persistent changes in conditions

 reversible

But – increased tolerance of one extreme often brings
reduced tolerance of another extreme
+

A species’ capacity for
acclimatization may reflect the range
of conditions in its environment
+
Irreversible developmental
responses

Developmental responses  when conditions persist for
long periods – env may influence individual development so
as to modify the size or other attributes of the individual for
long periods

Striking example: the African grasshopper – changes color to
match the color of their environment
+
Most grasshoppers
complete their life
cycle within a
single season
So in habitats where
this color
progression occurs
– the pigment
systems in the
epidermis develop
in such a way that
the nymphs an
adult grasshoppers
match the
background
+
Genotype – environment
interaction

When the reaction norms of two genotypes cross for some
aspect of performance, then individuals with each genotype
perform better in one environment and worse in another
environment (eg: swallowtail butterfly)

This relationship  genetoype – environment interaction
because each genotype responds differently to
environmental variations

How to identify them?  reciprocal transplant experiment