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Influence of geological thinking on Darwin

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Title: Influence of geological thinking on Darwin


1
Influence of geological thinking on Darwin
  • By time of The Beagle voyage idea that Earth
    was young was being challenged.
  • Opposition based on principle of
    Uniformitarianism.
  • Idea that geological processes happening today
    are the same as have operated in the past.

2
Influence of geological thinking on Darwin
  • Uniformitarianism contrasted with Catastrophism
    which proposed that current geological formations
    had resulted from catastrophic events (such as
    biblical flood) which occurred on scale unknown
    today.
  • Uniformitarianism first proposed by James Hutton
    and championed by Charles Lyell.

3
Influence of geological thinking on Darwin
  • Hutton and Lyell inferred Earth must be very old
    based on measurements of rate of ongoing rock
    forming processes (e.g. deposition of mud and
    sand).
  • These developments in geology focused Darwin on
    the potential importance of gradual change in
    shaping structures.

4
Darwinian Natural SelectionArtificial Selection
  • Artificial Selection. Humans have selectively
    bred for desirable traits in domestic animals and
    plants for millenia.
  • Process has produced our crop plants, garden
    plants, pets, and domestic animals.
  • Darwin closely studied pigeon breeding.

5
Artificial Selection
  • Cauliflower, broccoli, kale, brussels sprouts all
    descended from wild cabbage.
  • All these crops can be crossed and produce
    fertile offspring.
  • Cauliflower edible bit is the inflorescence or
    flower stalk.

6
Artificial Selection
  • Cauliflower has large dense infloresence. This
    results from mutant loss of function alleles of
    two genes that affect flower structure and
    infloresence density.

7
Artificial Selection
  • Early farmers choosing among their crops selected
    those with largest infloresences. Process has
    resulted in cauliflowers that are homozygous for
    both loss of function alleles.

8
Evolution by Natural Selection
  • Darwin envisaged process similar to artificial
    selection that had produced organisms we see
    today. He called it Natural Selection.

9
Evolution by Natural Selection
  • Darwin proposed evolution the inevitable outcome
    of 4 postulates
  • 1. There is variation in populations.
    Individuals within populations differ.
  • 2. Variation is heritable.

10
Evolution by Natural Selection
  • 3. In every generation some organisms are more
    successful at surviving and reproducing than
    other. Differential reproductive success.
  • 4. Survival and reproduction are not random, but
    are related to variation among individuals.
    Organisms with best characteristics are
    naturally selected.

11
Evolution by Natural Selection
  • If 4 postulates are true then the population will
    change from one generation to the next.
  • Evolution will occur.

12
Evolution by Natural Selection
  • Darwinian fitness ability of an organism to
    survive and reproduce in its environment.
  • Fitness measured relative to others of its species

13
Evolution by Natural Selection
  • Adaptation is a characteristic or trait of an
    organism that increases its fitness relative to
    individuals that do not possess it.

14
Testing the postulates
  • Evolution of beak shape in Darwins Finches.
  • Peter and Rosemary Grants (and colleagues) work
    on Medium Ground Finches Geospiza fortis
  • On Daphne Major since 1973.

15
Evolution of beak shape in Darwins Finches.
  • Postulate 1. Is the population variable?
  • Finches vary in beak length, beak depth, beak
    width, wing length and tail length.

16
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17
Evolution of beak shape in Darwins Finches.
  • Postulate 2 Is variation among individuals
    heritable?
  • Variation can be a result of environmental
    effects.
  • Heritability proportion of the variation in a
    trait in a population that is due to variation in
    genes.

18
Evolution of beak shape in Darwins Finches.
  • Peter Boag compared average beak depth of parents
    with that of their adult offspring.
  • Strong relationship between offspring and parent
    beak depths.

19
FIG 3.7
20
Evolution of beak shape in Darwins Finches.
  • Postulate 3 Do individuals differ in their
    success at survival and reproduction?
  • 1977 drought 84 of G. fortis individuals died,
    most from starvation. In two other droughts 19
    and 25 of the population died.

21
Evolution of beak shape in Darwins Finches.
  • Seed densities declined rapidly during drought
    and the small soft seeds were consumed first.
  • Average size and hardness of remaining seeds
    increased over the course of the drought.

22
FIG 3.8b
23
FIG 3.8A
24
Fig 3.8c
25
Evolution of beak shape in Darwins Finches.
  • Postulate 4 Are survival and reproduction
    nonrandom?
  • Do those who survive and reproduce have different
    characteristics than those that dont?

26
Evolution of beak shape in Darwins Finches.
  • As drought progressed small soft seeds
    disappeared and large, hard Tribulus seeds became
    a key food item.
  • Only birds with deep, narrow beaks could open
    them.

27
Evolution of beak shape in Darwins Finches.
  • At end of the 1977 drought the average survivor
    had a deeper beak than the average non-survivor
    and also a larger body size.

28
FIG 3.9
29
Did the population evolve?
  • Chicks hatched in 1978 had deeper beaks on
    average than those hatched in 1976.
  • Population evolved.

30
Fig 3.10
31
Evolution of beak shape in Darwins Finches.
  • Variation in weather from year to year on Daphne
    Major over 30 years has led to variation in the
    traits that are favored by selection.
  • Population has evolved over time.

32
Fig 3.11 A
Over the course of 30 years (1970 to 2000) beak
size evolved. Rose sharply during drought (red
line) then declined to pre-drought dimensions.
33
Fig 3.11 B
Over same 30-year period birds evolved more
pointed beaks and (next slide) significantly
smaller body size.
34
Fig 3.11 C
35
The nature of Natural Selection
  • Many misconceptions about how selection operates
    and evolution occurs.
  • Points to remember about natural selection

36
Natural selection acts on individuals, but its
effects accumulate in populations
  • Individual finches live or die during a drought
    (the selection event).
  • But change occurs in the characteristics of the
    population, not in individuals.

37
Natural selection acts on individuals, but its
effects accumulate in populations
  • During drought individual finchs beaks did not
    change, but average beak dimensions changed
    because more small-beaked birds died than
    large-beaked birds.

38
Evolution causes changes in allele frequencies
  • Evolution only occurs when traits have a genetic
    basis.
  • If beak dimensions were environmentally induced,
    no evolution could take place. After drought,
    frequencies of phenotypes in next generation
    might have been the same as before.

39
Natural selection does not plan ahead.
  • Each generation is result of selection by
    environmental conditions of the previous
    generation.
  • Evolution always one generation behind
    environmental changes.

40
New traits evolve even though selection acts on
existing traits.
  • This occurs because
  • 1. mutation produces new alleles.
  • 2. In sexually reproducing organisms meiosis and
    fertilization recombine existing alleles to
    produce new genotypes.

41
New traits evolve even though selection acts on
existing traits.
  • Artificial selection for oil content in corn.
  • After 60 generations oil levels were well above
    starting values.

42
Fig 3.12
43
New traits evolve even though selection acts on
existing traits.
  • Natural selection can also modify existing
    features over time for a new purpose e.g. Pandas
    thumb.
  • Trait used in novel way and eventually developed
    into a new structure referred to as a
    preadaptation. This does NOT mean there is
    pre-planning by natural selection.

44
Natural selection does not produce perfect
solutions
  • Pandas thumb not a perfect solution.

45
Natural selection does not produce perfect
solutions
  • On Daphne Major during drought finches with
    narrow beaks survived better than those with
    wider bills.
  • At end of drought, however, selection for deeper
    bills and bigger body size resulted in wider
    beaks even through deeper narrower beaks would
    have been a better solution.
  • Presumably same genes control all three traits.
    And solution is not perfect.

46
Natural selection does not produce perfect
solutions
  • Similarly, many characteristics of organisms are
    the result of compromises between different
    selection pressures.
  • The wings of various species of auk (seabirds
    that dive and swim) are a compromise between the
    need to fly (i.e. swim) underwater and in the
    air, two very different media.

47
Little Auk
polar.alaskapacific.edu/aharding/images/Littl...
Razorbill
http//media-2.web.britannica.com/eb-media/16/2601
6-004-13D8FA4C.jpg
48
Natural selection is nonrandom, but not
progressive
  • There is no goal of natural selection.
  • Evolution makes organisms better adapted to their
    environments, but there is no trend towards being
    more advanced.
  • E.g. Tapeworms have no digestive tract. They are
    simpler than their ancestors.

49
Selection does not act for the good of the
species
  • Apparently altruistic acts (e.g. giving an alarm
    call) are favored because they enhance relatives
    survival.
  • Infanticide in lions benefits individual male
    lions not the species as a whole.
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