Biology 265 EVOLUTION

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Biology 265EVOLUTION

• Lecture 7

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Overview

• Evolution and adaptation
• Adaptation or design?
• Natural selection is the only scientific
  explanation for adaptation
• Complex organs (e.g. the eye)
• Imperfections and constraints

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The Importance of Variation

• Random, heritable variation drives evolution
• Neutral variation is subject to stochastic forces
  such as drift
• Deleterious mutations are eradicated by natural
  selection
• Advantageous mutations are favored by natural
  selection gt adaptation

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Environment - Genotype

• Some variation is due to environmental effects
• Such variation is not heritable
• For example, human height has increased due to
  improved nutrition not Darwinian evolution
• Natural selection only works on inherited
  variation

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Adaptation

• Feature of an organism enabling it to survive and
  reproduce in its natural environment better than
  if it lacked the feature

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William Paley (1743-1805)

• Natural Theology or, Evidences of the Existence
  and Attributes of the Deity, Collected from the
  Appearances of Nature. (1802)
• the watch must have had a maker

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Design and Adaptation

• Natural theology argued that the perfect design
  of organisms was evidence of a designer (i.e.
  God).
• The adaptive explanation put forward by Darwin
  needed no designer
• For Darwin, the watchmaker was blind...

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In fact, Nature is not very perfect

• Appendix
• Wisdom tooth
• Tough for natural theology as it implied that
  either God was not perfect or deliberately
  created such things

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Natural Theology A Circular Argument

• Need to explain the origin of complex entities
  that are highly adapted/designed to fit their
  environment
• A designer?
• But the designer must also be a complex entity,
  so the question remains

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Lamarkian explanations are not much better

• Escaping predators requires running fast
• Running a lot strengthens muscles
• Maybe these stronger muscles are passed on and
  faster runners evolve
• But why should muscles get stronger with
  exercise?
• This is an adaptation that requires explanation

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The only scientific explanation for adaptive
evolution is Darwinian natural selection
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Evolution of an Eye

• To suppose that the eye could have been formed
  by natural selection, seems, I freely confess,
  absurd in the highest degree. - Charles Darwin
  (1859) The Origin of Species

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But Darwin went on to show that

• The eye could evolve from simple light sensitive
  cells
• Intermediate stages were useful
• Rudimentary eyes are common in nature
• 40-60 independently evolved eyes among
  invertebrates

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D.-E. Nilsson and S. Pelger, (1994)

• A Pessimistic Estimate Of The Time Required For
  An Eye To Evolve, Proceedings of the Royal
  Society London B, 256, pp. 53-58.

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Simulated evolution of an eye

• Allows shape to change at random but not more
  than 1
• Each new generation in the computer model was
  derived from the most optically superior eyes
  from the previous generation
• Lens also evolved from simple transparent cover
• 2,000 steps (maybe 400,000 generations, 1/2
  million years)

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Stage 1

• Simple eye spot
• Light sensitive cells in skin

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Stage 2

• Dimple appears
• Protects light sensitive cell
• Provide more visual acuity

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Stage 3

• Dimpling increases
• Dept of pit approximately equals its width
• Flatworm eye

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Stage 4

• The rim constricts
• Develops an aperture

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Stage 5

• Pit fills with jelly
• Secreted by cells
• Protects and maintains shape of pit (keeps mud
  out)
• Nautilus eye

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Stage 6

• Transparent layer for protection
• Thickens
• Simple lens
• Transparent materials available - protein in
  human cornea also found elsewhere in body

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Stage 7

• Adjustment of lens position
• Adjustment of aperture

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Stage 8

• Lens thickens
• Refractive index higher in center of lens
• Possible as lens consists of mixture of proteins
• Fish eye

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Historic/Current Functions

• In the beginning, an organ may have had the same
  function as it does now
• or it may have had a different function
• in which case the early stage is considered a
  preadaptation for the later stage

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Tetrapod limb

• Two groups of bony fish ray-finned
  (Actinopterygians) and lobe-finned fish
  (Sarcopterygians)
• Only single main rod of skeletal support in
  lobe-finned fish
• Used fins to walk on bottom of lakes

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Preadaptation

• All tetrapods evolved from lobe-finned fish
• None evolved from the ray-finned group
• Lobe-finned skeletal structure was a
  preadaptation for walking on land

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An imperfect world

• Organisms are not perfectly adapted to their
  environment
• If they were, they would live forever, escape all
  predators, lay eggs at an infinite rate etc.
• Often organisms are not even very close to being
  perfect (e.g. wisdom tooth)

Why?
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Evolutionary Constraints

• Time lags
• Genetic
• Developmental
• Historical
• Trade-offs

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Time lags

• Natural selection takes time to catch up with
  environmental changes
• For example, trees in Central America evolved in
  an environment containing large herbivores like
  the giant ground sloth
• fruit now appear to be inappropriately
  designed
• overprotected, excessive production

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Constraints

• Time lags
• Genetic
• Developmental
• Historical
• Trade-offs

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Genetic constraints

• Heterozygous advantage
• Sickle-cell anemia
• Balanced lethal system in European crested newt,
  Triturus cristatus
• Two types of chromosome 1 (1A and 1B)
• Homozygotes die, heterozygotes live
• Each generation half of offspring die

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Constraints

• Time lags
• Genetic
• Developmental
• Historical
• Trade-offs

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Developmental constraints

• A bias on the production of variant phenotypes
  or a limitation on phenotypic variability caused
  by the structure, character, composition, or
  dynamics of the developmental system. (Maynard
  Smith et al. 1985)
• Pleiotropy - a gene may control more than one
  trait

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Why not?

• Why do some forms not occur? E.g. only a subset
  of all possible shell shapes are actually used in
  nature
• Do organisms have their current form because it
  is the only one possible given the developmental
  constraints?
• Or is it because natural selection favored that
  form over the others that are now extinct?

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Testing between selection and constraint

• Adaptive prediction
• a theoretical explanation for why some forms
  occur and others do not
• Measure selection
• have to make experimentally forms that dont
  occur
• Measure heritability
• constraint expects little genetic variation in
  trait

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Testing between selection and constraint

• Cross-species evidence
• allometric relations (brainbody size in
  vertebrates) imply pleiotropy and constraint
• but experimental evidence (selection experiments)
  suggests that allometric relations are malleable
• natural selection may tune allometric relations
  to establish a favorable shape

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Constraints

• Time lags
• Genetic
• Developmental
• Historical
• Trade-offs

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Historical constraint

• Shifting balance theory of Sewell Wright
• Recurrent laryngeal nerve
• evolved from fish where it passed through gill
  arches
• during evolution gill arches have moved in
  mammals
• laryngeal nerve still moves behind its gill
  arch which is now near the heart
• it goes from the brain, round the aorta and back
  up to the larynx

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Contingency

• Chance may lead some groups to solve problems in
  one way, while other groups find different though
  not necessarily worse solutions
• For example, kangaroos and gazelles have
  different but equally (?) successful modes of
  locomotion

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Constraints

• Time lags
• Genetic
• Developmental
• Historical
• Trade-offs

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Trade-offs

• Organs may be adapted for more than one function
• For example, mouths are used for feeding and
  eating
• Amphibians must hold their breath while chewing
• Mammals can chew and breath at the same time due
  to the secondary palate separating nose and mouth

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Higgledy Piggledy

• Natural selection is not progressive
• It does not inevitably lead to an harmonious and
  stable state
• It does not optimize
• It merely suffices
• It is myopic - species can be perfected to
  extinction!