Evolution and Natural Selection

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Evolution and Natural Selection

• Chapters 1.4-1.6, Bush

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Introduction to Natural Selection

• History of Evolutionary Thought
• Theory of Natural Selection
• Examples of Natural Selection

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Introduction to Natural Selection

• History of Evolutionary Thought
• Theory of Natural Selection
• Examples of natural selection

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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/

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Georges Cuvier (1769-1832)

• leading palaeontologist of his time
• found that many species have gone extinct

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Charles Lyell (1797-1875)

• geologist
• Earth was way older than the 5000 years or so
  allowed according to Biblical chronology

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Erasmus Darwin (1731-1802)

• Charles Darwins grandfather
• proponent of the theory that species change over
  time

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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

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Thomas Malthus (1766-1834)

• found that all species have the potential to
  create far more offspring than there are
  resources to support

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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

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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
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Summary of the history of evolutionary thought
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Introduction to Natural Selection

• History of Evolutionary Thought
• Theory of Natural Selection
• Examples of natural selection

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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

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Theory of Natural Selection

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

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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.

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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)

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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

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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)

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Heritability of simple traits
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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)

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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

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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)

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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)

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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

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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

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Aside Darwins nemesis was genetics!
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Gregor Mendel father of genetics

• conducted experiments on pea plants
• discovered that most organisms have two copies of
  their genes, one from each parent.

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Darwin never read Mendels Paper
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Introduction to Natural Selection

• History of Evolutionary Thought
• Theory of Natural Selection
• Examples of natural selection

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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

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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
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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

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Natural selection can occur rapidly
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Rock plants
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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

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Natural Selection reviewed
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Natural Selection continued

• Characteristics of natural selection
• Types of natural selection
• Natural selection ? Evolution

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Natural Selection continued

• Characteristics of natural selection
• Types of natural selection
• Natural selection ? Evolution

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Characteristics of Natural Selection

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

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Sources of variation

• Gene flow immigration
• recombination
• ultimately, from mutation

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Immigration leads to new variation

• Immigration provides new genetic material for
  selection to act upon

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Recombination creates variation in offspring
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Mutation at the Phenotype Level

• Mutations can be
• beneficial
• detrimental
• neutral

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Mutation at the DNA Level

• A mutation is caused when the chromosomal
  machinery makes a mistake

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Mutagens

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

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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

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Natural Selection continued

• Characteristics of natural selection
• Types of natural selection
• Natural selection ? Evolution

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Types of Natural Selection

• Three kinds of natural selection
• Directional selection
• Stabilizing selection
• Disruptive selection

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Directional Selection
Larger individuals may have higher fitness
(i.e., produce more offspring) than
smaller individuals.
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Directional Selection

• Fishing industry produces selection that
  favours smaller cod and can produce a decrease in
  average body size.

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

• Characteristics of natural selection
• Types of natural selection
• Natural selection ? Evolution

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Selection pressures may conflict
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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

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Chance events influence evolution
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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

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