Aug31_lecture2

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Lecture 2A brief introduction to evolutionary
thinking
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Today

• brief history of evolutionary theory
• natural selection
• evolutionary thinking and some important
  evolutionary themes (following the paper by
  Stephen Stearns)
• Adaptation, Constraints, Trade-offs, Conflict

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A very brief history of evolution

• Evolutionary ideas go back long before Darwin
  (Lamarck, Erasmus Darwin)
• Darwin was the first to present an overwhelming
  case for descent with modification
• Critically, he also articulated a mechanism for
  evolution natural selection

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I have called this principle, by which each
slight variation, if useful, is preserved, by the
term Natural Selection. Charles Darwin
(1809-1882)
Charles Darwin in 1854, five years before
publishing The Origin of Species.
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evolution by natural selection

• Alfred Russel Wallace (while suffering from a
  bout of malaria) hit upon the same insight before
  Darwin had published
• On the Tendency of Varieties to Depart
  Indefinitely From the Original Type

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evolution by natural selection

• Alfred Russel Wallace (while suffering from a
  bout of malaria) hit upon the same insight before
  Darwin had published
• They co-published a paper in 1858 but it wasnt
  until the publication of The Origin of Species
  (1859) that the idea caught on

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evolution by natural selection

• Natural selection was eclipsed for several
  decades because of misunderstandings about
  inheritance.
• Darwin, unaware of Gregor Mendels discoveries
  about genetics, adopted incorrect ideas about
  genetics pangenesis, blending inheritance
• In fact, genetics depends on particulate
  inheritance

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evolution by natural selection

• In 1900, Mendels findings were rediscovered.
• But instead of leading directly to a positive
  re-appraisal of Darwins ideas on natural
  selection, early geneticists were originally
  opposed
• The main problem was their focus on mutations of
  large effect
• JBS Haldane, RA Fisher, Sewall Wright were
  population geneticists who synthesized genetics
  and evolution the modern synthesis

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evolution by natural selection

• Darwin started his argument for natural selection
  with insights from pigeon-breeding (he spent a
  lot of time drinking with animal breeders and
  became a pigeon fancier himself)
• But man can and does select the variations given
  to him by nature, and thus accumulate them in any
  desired manner. He thus adapts animals and plants
  for his own benefit or pleasure. He may do this
  methodically, or he may do it unconsciously by
  preserving the individuals most useful to him at
  the time, without any thought of altering the
  breed.

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evolution by natural selection

• It is certain that he can largely influence the
  character of a breed by selecting, in each
  successive generation, individual differences so
  slight as to be quite inappreciable by an
  uneducated eye. This process of selection has
  been the great agency in the production of the
  most distinct and useful domestic breeds. That
  many of the breeds produced by man have to a
  large extent the character of natural species, is
  shown by the inextricable doubts whether very
  many of them are varieties or aboriginal
  species.

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evolution by natural selection

• A process much like artificial selection, used by
  breeders of domesticated plants and animals to
  select for desirable traits,also happens in
  nature
• Why, if man can by patience select variations
  most useful to himself, should nature fail in
  selecting variations useful, under changing
  conditions of life, to her living products?

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evolution by natural selection

• A process much like artificial selection, used by
  breeders of domesticated plants and animals to
  select for desirable traits,also happens in
  nature
• Individuals within populations are variable
• The variations among individuals are, at least in
  part, passed on from parents to offspring.
• In every generation, some individuals are more
  successful at surviving and reproducing than
  others
• The survival and reproduction of individuals are
  not random those with the most favorable
  variations are naturally selected

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evolution by natural selection

• That many and grave objections may be advanced
  against the theory of descent with modification
  through natural selection, I do not deny. I have
  endeavoured to give to them their full force.
  Nothing at first can appear more difficult to
  believe than that the more complex organs and
  instincts should have been perfected not by means
  superior to, though analogous with, human reason,
  but by the accumulation of innumerable slight
  variations, each good for the individual
  possessor.

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evolution by natural selection

• Nevertheless, this difficultycannot be
  considered real if we admit the following
  propositions, namely, -- that gradations in the
  perfection of any organ or instinct, which we may
  consider, either do now exist or could have
  existed, each good of its kind, -- that all
  organs and instincts are, in ever so slight a
  degree, variable, -- and, lastly, that there is a
  struggle for existence leading to the
  preservation of each profitable deviation of
  structure or instinct. The truth of these
  propositions cannot, I think, be disputed.

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evolution by natural selection

• Natural selection is a blind watchmaker
• Its what?

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evolution by natural selection

• William Paleys argument for a designer
• In crossing a heath, suppose I pitched my foot
  against a stone and were asked how the stone came
  to be there, I might possibly answer that for
  anything I knew to the contrary it had lain there
  forever nor would it, perhaps, be very easy to
  show the absurdity of this answer.

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evolution by natural selection

• William Paley
• But suppose I had found a watch upon the ground,
  and it should be inquired how the watch happened
  to be in that place, I should hardly think of the
  answer which I had before given, that for
  anything I knew the watch might have always been
  there.

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evolution by natural selection

• William Paley
• Yet why should not this answer serve for the
  watch as well as for the stone? Why is it not as
  admissible in the second case as in the first?
  For this reason, and for no other, namely, that
  when we come to inspect the watch, we
  perceivewhat we could not discover in the
  stonethat its several parts are framed and put
  together for a purpose.

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evolution by natural selection

• Nothing at first can appear more difficult to
  believe than that the more complex organs and
  instincts should have been perfected not by means
  superior to, though analogous with, human reason,
  but by the accumulation of innumerable slight
  variations, each good for the individual
  possessor.
• Let us hope that what Mr. Darwin says is not
  true but, if it is true, let us hope that it
  will not become generally known."

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Evolutionary thinking

• The Stearns paper is the introduction to a book
  about evolutionary medicine, hence the medical
  focus
• Evolutionary biology is a rich collection of
  well-developed alternative approaches to the
  interpretation of biological diversity and
  organismal design.

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Evolutionary thinking

• Evolutionary topics
• Adaptations
• Relationships and history
• Neutral versus selectively advantagous variation
• The study of conflicts and cooperation
• Maladaptation Why am I not perfect?

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Evolutionary thinking

• Wikipedia Evolution
• The basic mechanisms that produce evolutionary
  change are natural selection (which includes
  ecological, sexual, and kin selection) and
  genetic drift
• these two mechanisms act on the genetic variation
  created by mutation, genetic recombination and
  gene flow.
• Natural selection is the process by which
  individual organisms with favorable traits are
  more likely to survive and reproduce. If those
  traits are heritable, they are passed to
  succeeding generations, with the result that
  beneficial heritable traits become more common in
  the next generation.
• Given enough time, this passive process can
  result in varied adaptations to changing
  environmental conditions.6

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Evolutionary thinking

• Different sorts of evolutionary biologists
• Population geneticists (genes, alleles, change or
  maintenance in variation)
• Evolutionary ecologists (design of phenotypes for
  survival and reproduction, life history, sexual
  selection, behaviour)

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Evolutionary thinking

• Different sorts of evolutionary biologists
• Molecular evolutionists (history stamped into
  genomes, patterns in DNA and the processes
  underlying them)
• Systematists (phylogenies, relationships among
  taxa
• Paleontologists (fossils, deep time, major
  trends)
• No clear boundaries, and many evolutionists where
  several hats. All try to observe patterns and
  infer process that underly evolution

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Evolutionary thinking

• Different evolutionary approaches
• Changes in gene frequencies (population and
  quantitative genetics)
• Optimization approach
• Game theory approach
• Phylogenetic approach

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natural selection and adaptation

• In short
• Heritable variation
• plus
• Differential survival and reproductive success
• Leads to
• Non-random survival and reproduction such that
  favorable variation is naturally selected

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natural selection and adaptation

• The ultimate source of variation is mutation (in
  DNA, it turns out)
• Mutation is random
• Selection is NOT!
• Natural selection filters and preserves random
  mutation-derived variation and is the antithesis
  of randomness (it is, however, still blind)

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natural selection and adaptation

• Selection extracts order from randomness
• THERE IS GRANDEUR IN THIS VIEW OF LIFE
• 31 genes, each with 26 alleles
• This is just one of the 2631 possible
  haplotypes
• http//home.pacbell.net/s-max/scott/weasel.html

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natural selection and adaptation

• Adaptation is both a process and a state
• The process of adaptation what happens over
  successive generations of selection of heritable
  variation in reproductive success
• The state of adaptation a particular trait that
  does a job very well, just as though it were
  designed by an engineer
• E.g. opposable thumb, acute hearing, vision, sex
  drive, camouflage, venom, crystallins in eyes

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Constraints on adaptation

• Natural selection does not produce perfection
• Rather it is a tinkerer that produces
  good-enough solutions to context dependant
  problems.
• Often this occurs through duplication and
  divergence

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Constraints on adaptation

• As natural selection acts by competition, it
  adapts the inhabitants of each country only in
  relation to the degree of perfection of their
  associates so that we need feel no surprise at
  the inhabitants of any one country, although on
  the ordinary view supposed to have been specially
  created and adapted for that country, being
  beaten and supplanted by the naturalised
  productions from another land.

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Constraints on adaptation

• Nor ought we to marvel if all the contrivances
  in nature be not, as far as we can judge,
  absolutely perfect and if some of them be
  abhorrent to our ideas of fitness.

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Constraints on adaptation

• Time is a major constraint on adaptation. It
  takes time to generate variation and select for
  it
• Absorbtion of milk sugar (lactose) by human
  adults
• Normally lactase is effective until weaning age
  (about 4). If you cant digest lactose you
  suffer flatulence, intestinal cramps, diarrhea,
  nausea, vomitting
• But in cultures where dairy has been used for
  thousands of years, gt90 of adults can digest
  lactose, like giant, lumbering babies
• Thousands of years, but probably not decades, are
  sufficient for this selection to shape human
  evolution

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Constraints on adaptation

• Trade-offs are major constraints too
• E.g. sex is dangerous for fruit flies. There is
  a trade-off between survival and reproduction
• Similarly, there is a trade-off between having a
  robust immune system and suffering from asthma,
  or lupus, or diabetes type I
• Other trade-offs?

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Trade-offs
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Constraints on adaptation

• Historical constraints are also important
• How strange it is that a bird, under the form of
  woodpecker, should have been created to prey on
  insects on the ground that upland geese, which
  never or rarely swim, should have been created
  with webbed feet

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Constraints on adaptation

• Historical constraints are also important
• The vertebrate eye has a basic flaw the nerves
  and blood vessels that feed it enter right in the
  middle of the region of photosensitive cells.
• You would fire any engineer who designed an
  optical device in this way, but because of
  developmental constraints deriving from much
  simpler, ancestral eyes, thats what were stuck
  with
• If squid could talk, they would taunt us about
  our poorly designed eyes.

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Release from constraint adaptive radiation

• Adaptive radiations can occur when new ecological
  niches open up, or a new adaptation opens up new
  possibilities (each beetles/flowering plants)
• Natural selection can operate very rapidly in
  such cases to generate new adaptations
• Islands, such as the Galapagos, are classic areas
  for observing evidence of adaptive radiation (eg
  Darwins Finches)
• But other favorable conditions also exist.

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• The Great Lakes of Africa are home to the most
  species rich vertebrate radiation
• Hundreds of species in each lake, with relatively
  recent common ancestors
• A few million years for the 500 species in Lake
  Malawi
• 12,000 years for Lake Victoria

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Evolutionary conflicts

• Lots of conflicts arise in nature
• Predators and prey
• Parents and offspring
• Insects and plants
• Fungi and crop plants they destroy
• Chromosomes competing for transmission through
  gametes

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Evolutionary conflicts

• Conflicts are where much of the action is in
  evolutionary biology
• Conflicts occur when genes have different
  patterns of transmission but interact, directly
  or indirectly, in the organisms that carry them
• Perhaps the most obvious example is genes of
  pathogens and genes of hosts
• Endless cycles of damage and damage control
  adaptations can lead to evolutionary arms
  races

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Evolutionary conflicts

• In one sense, these arms races lead to lots of
  adaptation
• In another, they present a serious constraint on
  adaptation and amount to running just to stay
  still.