Ecology

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CHAPTER 2

• Ecologys Evolutionary backdrop

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Ecologys evolutionary backdrop

• 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

• Darwin, 1859, On the origin of species
• 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.
• Malthus, 1798, An essay on the principle of
  population
• 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

1. Individuals that form a population of a species
   are not identical
2. Some of the variation between individuals is
   heritable
3. 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
4. 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. Variation among individuals
2. Inheritance of that variation
3. Differences in survival and reproductive success
   (or fitness) related to that variation

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Human selection vs natural selection

• Human selection example?
• Natural selection example?
• Difference?

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Natural selection

• 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

• 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

• 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 others
  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

• 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

• 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

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Field experiment

• Large numbers of melanic and pale moths reared
  and released in rural and unpolluted areas
• 190 moths captured by birds 164 melanic

• Experiment repeated near a city
• Moths of the typical form disadvantaged in the
  polluted industrial environment

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Criticism of Kettlewells 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)

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Evolution and coevolution

• Physical conditions are not heritable not
  subject to natural selection
• What about when two species interact?

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Coevolution

• 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
• plants and animals employ structures and
  behaviors to obtain food and to avoid being eaten
  or parasitized
• much of this diversity is the result of
  coevolution natural selection on the means of
  food procurement and escape

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Coevolution is mediated by biological agents.

• 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

• In response to biological factors, organisms tend
  to diversify
• 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
• this familiar process is known as convergence

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• Coevolution involves mutual responses in the two
  species
• predator species evolves to be faster, allowing
  it to catch more
• results in greater selection on speed in the prey
• for regular evolution, there isnt any response
  by the environment

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• Coevolution promotes diversity of adaptations
• evolution often converges on the same solution
  for the same problems
• coevolution involves specific, unique responses
  to specific challenges

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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
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This isnt coevolution
Species A has some trait unrelated to species B
Species B evolves in response to that trait in
species A
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What is a species?

• (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

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Islands and speciation Darwins finches
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• 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.
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Evolution in action Geospiza fortis

• 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