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Evolution and Natural Selection

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Title: Evolution and Natural Selection


1
Evolution and Natural Selection
  • Chapters 1.4-1.6, Bush

2
Introduction to Natural Selection
  • History of Evolutionary Thought
  • Theory of Natural Selection
  • Examples of Natural Selection

3
Introduction to Natural Selection
  • History of Evolutionary Thought
  • Theory of Natural Selection
  • Examples of natural selection

4
Paving the way for Darwin
  • Charles Darwins theory relied upon the findings
    of other scientists
  • Casting doubt on Divine Creation
  • Cuvier, Georges
  • Lyell, Charles
  • Darwin, Erasmus
  • Contributing to the theory itself
  • Lamarck, Jean-Baptiste
  • Malthus, Thomas Robert
  • Wallace, Alfred Russell
  • http//goldberg.history.ohio-state.edu/naturalsele
    ction/

5
Georges Cuvier (1769-1832)
  • leading palaeontologist of his time
  • found that many species have gone extinct

6
Charles Lyell (1797-1875)
  • geologist
  • Earth was way older than the 5000 years or so
    allowed according to Biblical chronology

7
Erasmus Darwin (1731-1802)
  • Charles Darwins grandfather
  • proponent of the theory that species change over
    time

8
Jean-Baptiste Lamarck (1744-1829)
  • Believed scientists like Erasmus Darwin that life
    forms could change over time
  • Lamarckism acquired traits can be inherited
  • e.g. a giraffe with a short neck stretches to get
    at vegetation high up a tree and manages to make
    its neck longer. This giraffe passes its long
    neck to its offspring
  • got Darwin thinking about inheritance

9
Thomas Malthus (1766-1834)
  • found that all species have the potential to
    create far more offspring than there are
    resources to support

10
Alfred Russell Wallace (1823-1913)
  • came up with the theory of natural selection
    independently of Darwin
  • spurred Darwin to publish his own work on the
    subject

11
Charles Darwin (1802-1882)
  • I have called this principle, by which each
    slight variation, if useful, is preserved, by the
    term Natural Selection.
  • (The Origin of Species)

http//www.interaktv.com/Darwin/Darwin.html
12
Summary of the history of evolutionary thought
13
Introduction to Natural Selection
  • History of Evolutionary Thought
  • Theory of Natural Selection
  • Examples of natural selection

14
The Theory of Natural Selection
  • Natural Selection is a Theory in the same way
    that we consider gravity or Einsteins relativity
    to be a theory

15
Theory of Natural Selection
  • Three conditions for Natural Selection
  • 1) Variation in traits
  • 2) Heritability
  • 3) Survivorship/Competition
  • Natural selection ? Survival of the fittest

16
Variation and Heritability
  • Observations from Lamarck and Erasmus Darwin that
    offspring are not exactly like parents (change
    can occur in a single generation)
  • Observed the commonly known facts that
  • all individuals are not alike (i.e., there are
    different phenotypes)
  • Offspring inherit the majority of their traits
    from their parents.

17
Variation within a species
  • Variation can be
  • CONTINUOUS having a multitude of variants (e.g.,
    colour bands in the snail)
  • DISCRETE limited of types (such as blood types)

18
Heritability in Diploids
  • Two copies of each gene (diploid)
  • Humans have 23 chromosomes, 2 copies of each, for
    a total of 46 chromosomes)
  • Each egg or sperm has only one copy of each
    chromosome

19
Passing on genes is like tossing coins
  • Two copies exist for each gene
  • Whether you pass on a certain copy of a gene is
    an independent event for each child
  • If you have two children, sometimes you will pass
    on the same copy to both children (leaving the
    second copy passed on to neither child)

20
Heritability of simple traits
21
Competition
  • From Malthus more offspring are produced than
    there are resources to support
  • Creates a struggle for existence
  • Some offspring will be better at surviving and
    reproducing than others (i.e., have higher
    fitness)

22
Fitness
  • FITNESS
  • the number of offspring an individual produces
    that survive to reproduce themselves
  • Fitness 1.0 means that individuals of this
    phenotype are successfully passing on 100 of
    their genes, on average

23
How is fitness calculated
  • Fitness the number of genes passed on to the
    next generation
  • Because diploid organisms (i.e., most organisms)
    only pass on half of their genes to each child,
    they must have two offspring living to
    reproductive age to have Fitness 1
  • Fitness 1 does not exactly mean that you have
    passed on 100 of your genes to the next
    generation (Remember sometimes you send two
    copies of the same gene and zero copies of the
    other)

24
Outcome
  • Some phenotypes will be better represented in the
    next generation than they are in the present
    generation
  • Could be extended some entire lineages may be
    more successful than others as well resulting in
    some lineages going extinct (as Cuvier had found)

25
Natural selection will not take place if
  • there is no variation
  • E.g., No humans have gills, so we cannot select
    for them, regardless of how beneficial they might
    be
  • If the gene is not heritable
  • E.g., Working out and getting a strong heart
    might make you live longer and have more children
    but selection can not act upon it if is not a
    genetic trait
  • If there is no difference in survivorship or
    reproductive ability between variants
  • E.g., Having attached or free earlobes doesnt
    really matter

26
Survival of the fittest
  • This saying is a bit misleading and doesnt quite
    capture the essence of what is natural selection
  • You can be as fit an individual as can be but
    it is the ability to reproduce that is the key
    feature for an increase in representation in the
    next generation

27
Aside Darwins nemesis was genetics!
28
Gregor Mendel father of genetics
  • conducted experiments on pea plants
  • discovered that most organisms have two copies of
    their genes, one from each parent.

29
Darwin never read Mendels Paper
30
Introduction to Natural Selection
  • History of Evolutionary Thought
  • Theory of Natural Selection
  • Examples of natural selection

31
Ground Finch (Geospiza fortis)
  • beak size has a lot to do with how well a finch
    feeds on certain seeds
  • seeds of Tribulus have the toughest seed coat
    that requires a large beak to break

32
Natural selection in finches
Drought causes collapse of food supply, survival
plummets
High mortality in smaller individuals, strong
selection for large birds that can crack large,
tough seeds
33
Human-induced selection
  1. Natural popn with variation for insecticide
    resistance
  2. Insecticide appln kills all but those with
    resistance
  3. Surviving insects breed new generation of
    insecticide resistance population

34
Natural selection can occur rapidly
35
Rock plants
36
Summary
  • Darwin put together a number of ideas from
    different disciplines to come up with the Theory
    of Natural Selection
  • Natural selection states that heritable
    phenotypes that are well-suited to their
    environment will have more offspring and so will
    be better represented in the next generation.
  • Natural selection can operate so quickly that we
    can observe it in a single generation

37
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38
Natural Selection reviewed
39
Natural Selection continued
  • Characteristics of natural selection
  • Types of natural selection
  • Natural selection ? Evolution

40
Natural Selection continued
  • Characteristics of natural selection
  • Types of natural selection
  • Natural selection ? Evolution

41
Characteristics of Natural Selection
  • Natural Selection
  • dependent on the variation present in the
    population
  • Short-sighted acts only present selection
    pressures

42
Sources of variation
  • Gene flow immigration
  • recombination
  • ultimately, from mutation

43
Immigration leads to new variation
  • Immigration provides new genetic material for
    selection to act upon

44
Recombination creates variation in offspring
45
Mutation at the Phenotype Level
  • Mutations can be
  • beneficial
  • detrimental
  • neutral

46
Mutation at the DNA Level
  • A mutation is caused when the chromosomal
    machinery makes a mistake

47
Mutagens
  • Many things may increase the mutation rate
  • radiation
  • certain chemicals (e.g. carcinogens)

48
Variation is random
  • When a new recombinant or mutant genotype arises,
    there is no tendency for it to arise in the
    direction of improved adaptation
  • Natural selection imposes direction on evolution,
    using undirected variation

49
Natural Selection continued
  • Characteristics of natural selection
  • Types of natural selection
  • Natural selection ? Evolution

50
Types of Natural Selection
  • Three kinds of natural selection
  • Directional selection
  • Stabilizing selection
  • Disruptive selection

51
Directional Selection
Larger individuals may have higher fitness
(i.e., produce more offspring) than
smaller individuals.
52
Directional Selection
  • Fishing industry produces selection that
    favours smaller cod and can produce a decrease in
    average body size.

53
Stabilizing selection
The average members of the population may have
higher fitness than the extremes.
54
Stabilizing Selection
Babies of intermediate birth weight have
higher survivorship than very small and very
large babies
55
Disruptive selection
Natural selection could favour both extremes
over the intermediate types
56
Disruptive Selection
In the finch, Pyrenestes ostrinus both very large
and very small bills are beneficial for eating
large and small seeds, respectively
57
Natural Selection continued
  • Characteristics of natural selection
  • Types of natural selection
  • Natural selection ? Evolution

58
Selection pressures may conflict
59
Other factors in evolution
  • If there is no relation between fitness and the
    character in question, then natural selection is
    not acting on it
  • Chance events can still make these traits show
    change over time RANDOM DRIFT

60
Chance events influence evolution
61
Summary
  • Natural Selection acts on whatever variation is
    present at the time. This variation is generated
    randomly with respect to selection pressures
  • Selection can be directional, stabilizing or
    disruptive
  • Random factors can also play a part in evolution

62
"nothing in biology makes sense except in the
light of evolution" -Theodosius Dobzhansky
(1900-1975)
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