Download Chapter 2 - Green Resistance

CHAPTER 2
Ecology’s Evolutionary backdrop
Ecology’s evolutionary backdrop
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“Nothing in biology makes sense except in the light
of evolution” – Russian-American biologist
Dobzhansky
Very little in evolution makes sense except in the
light of ecology
Why are there so many types of organisms? Why
are their distributions so restricted?
Why: all species are so specialized that they are
almost always absent from almost everywhere
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Evolution by natural selection
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Darwin, 1859, ‘On the origin of species’
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Malthus, 1798, ‘An essay on the principle of population’
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Gradually developed the view that the natural diversity of
nature was the result of a process of evolution in which natural
selection favored some variants within species through a ‘struggle
for existence.’
The human population – capable of doubling every 25 years and
so…
Darwin and Wallace – field ecologists – realized this argument
applied equally to plant and animal kingdoms
The great fecundity of some species + every species ‘must
suffer destruction during some period of its life and during
some season or occasional year – otherwise…’
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Fundamental truths of evolutionary
theory
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1.
2.
3.
4.
Individuals that form a population of a species are not
identical
Some of the variation between individuals is heritable
All populations could grow at a rate that would
overwhelm the environment – but most individuals die
before reproductions and most (usually all) reproduce
at less than their maximal rate
Different ancestors leave different numbers of
descendants (not just offspring): they do not all
contribute equally to subsequent generations.  those
that contribute most have the greatest influence on the
heritable characteristics of subsequent generations
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In other words …
1.
2.
3.
Variation among individuals
Inheritance of that variation
Differences in survival and reproductive success (or
fitness) related to that variation
Human selection vs natural selection
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Human selection – example?
Natural selection – example?
Difference?
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Natural selection
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No aim for the future
Past environments may have selected certain
characteristics of individuals that we see today –
but those traits are ‘suited’ to present day
environments ONLY because environments tend to
remain the same – or change quite slowly
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Evolution within species
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To understand the evolution of species – we need to
understand evolution WITHIN species
Evolution by natural selection does far more than
create new species
Natural selection and evolution occur within species
– and we can study them in action within our own
lifetime
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Geographic variation within species
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The characteristics of a species may vary over its
geographic range
Note:
Evolution forces characteristics of populations to
diverge from each other IF:
 There
is sufficient heritable VARIATION on which
selection can act; and
 Forces of selection favoring divergence are strong
enough to counteract the mixing and hybridization of
individuals from different sites
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Reciprocal transplant experiments – what do they
tell us?
Beware of assuming that all species exhibit
geographically distinct variants with a genetic basis
Difficult to detect local specialization of animals by
transplanting them into each other’s habitat. Why?
 Can
do with invertebrates (corals, sea anemones, …)
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Geographic variants of species – i.e. influence of
physical environment
Also
Influence of other selective forces
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Evolutionary
experiments
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The guppy, small
freshwater fish from S
America
Present in all water
bodies – subdivided by
waterfalls – thus
isolating fish populations
Populations differ from
each other; 47 traits
tend to vary with
intensity of risk from
predators
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w/o predators: male
guppies are brightly
decorated. Females
prefer but…
Controlled field
experiment
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Variation within a species with
human-made selection pressures
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Sites in Britain and
Ireland where
frequencies of the pale
(forma typica) and
melanic forms of Biston
betularia were recorded
More than 20,000
specimens
Pollutants spread
toward the east;
unpolluted west
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Selection and change in melanistic moths (peppered
moths)
Dark form: more
popular in forests
near industrialized
regions
Industrial
melanism
Field experiment
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Large numbers of
melanic and pale
moths reared and
released in rural and
unpolluted areas
190 moths captured
by birds; 164 melanic
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Experiment repeated
near a city
Moths of the typical
form disadvantaged in
the polluted industrial
environment
Criticism of Kettlewell’s research
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Moths that were used for the mark – recapture
experiments were reared in the lab
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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)
Evolution and coevolution
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Physical conditions are not heritable; not subject to
natural selection
What about when two species interact?
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Coevolution
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When populations of two or more species
interact, each may evolve in response to
characteristics of the other that affect its own
evolutionary fitness. This process is referred to as
coevolution:
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plants and animals employ structures and behaviors to
obtain food and to avoid being eaten or parasitized:
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much of this diversity is the result of coevolution: natural
selection on the means of food procurement and escape
Coevolution is mediated by biological
agents.
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The evolutionary effects of biological agents are
unlike those of physical factors in two important ways:
biological factors stimulate mutual evolutionary responses;
adaptations of organisms in response to changes in the
physical environment have no effect on that environment
 biological agents foster diversity of adaptations rather than
promoting similarity
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Convergence
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In response to biological factors, organisms tend to
diversify:
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organisms specialize, approaching feeding, avoidance of
predators and mutually beneficial arrangements in unique
ways
In contrast, organisms responding to similar physical
stresses in the environment tend to evolve similar
adaptations:
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this familiar process is known as convergence
1.
Coevolution involves mutual responses
in the two species
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predator species evolves to be faster, allowing it
to catch more
results in greater selection on speed in the prey
for “regular” evolution, there isn’t any response by
the environment
2.
Coevolution promotes diversity of
adaptations
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evolution often converges on the same solution for
the same problems
coevolution involves specific, unique responses to
specific challenges
The confusing definition of coevolution
Species A evolves an
adaptation in response
to species B
Species B evolves in
response to the
adaptation of species A
This isn’t coevolution
Species A has some trait
unrelated to species B
Species B evolves in
response to that trait in
species A
What is a species?
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(Mayr and Dobzhansky) – two populations were
part of the same species if they could – potentially
– breed together to produce fertile offspring
Not enough time to apply this test before
recognizing every species
Test recognizes a crucial element in the evolutionary
process:
2
parts of a population can evolve into distinct species
ONLY IF a barrier prevents gene flow between them
 Biospecies (species) do not exchange genes
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Orthodox speciation
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Two species of gull
diverged from a
common ancestry
(from the lesser
black-backed
gull in Siberia) as
they colonized
and encircled N
Hemisphere
Where they occur
together in N
Europe they are
unable to
interbreed and
are two distinct
species
They are linked
along their
ranges by a
series of freely
interbreeding
races or
subspecies
Islands and speciation: Darwin’s finches
-Genetic distance between species
is shown by length of horizontal
lines
-This process happened in less than
3 million years
-But is continuing…
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Heavy rains during El Nino events support lush plant growth in the archipelago.
Evolution in action – Geospiza fortis
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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 (drought in mid-1970s)
The average beak size of surviving individuals and their progeny
increased significantly
In the exceedingly wet year of 1983, small seeds produced; birds
with smaller beaks handled smaller seeds more efficiently … 
average beak size returned to a lower value
Evolutionary responses were small – still they illustrate the capacity
of a population to respond to changes in the environment – changes
in climate
What is necessary for such ‘evolution in action’?
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Test 1 (Chapters 1 and 2)
Next Friday – October15
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