Artificial Selection

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Artificial Selection

• Selection pressures applied by humans
• For example cattle bred for thousands of years,
  selected for particular features such as high
  milk yield and docile behaviour
• The presence of these features varies in the
  population, individuals with high degree of them
  are bred and offspring are likely to possess
  alleles which code for them (if they are
  inherited).
• Over many generations, allele frequency for the
  desired characteristic will have increased

2
Evolution

• Theory credited to Darwin, but developed also by
  Wallace
• Organisms produce more offspring than needed to
  replace parents
• Populations remain roughly stable in size
• Therefore there is a struggle for survival
• There is variation among the individuals of a
  given species
• The best adapted individuals will survive and
  breed natural selection and survival of the
  fittest
• Darwin made journey on the Beagle in 1931-6 to
  Galapagos Islands
• Made notes on Evolution of Species by Natural
  Selection in 1937 (Wallace was only 14) but did
  not publish his book until 1859.
• Wallace and Darwin did not explain why new
  variations are continually arising
• Mendels work on pea plants (1856-63) could have
  helped but was not generally known until 20th
  Century, long after Mendels and Darwins death.
• Also failed to explain how new species arise.

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Darwin aged 31
Wallace Mendel
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Species and Speciation

• A species is a group of organisms with similar
• Morphological
• Physiological
• Biochemical
• Behavioural features
• Which can interbreed to produce fertile offspring
  and are reproductively isolated from other
  species.
• Example horses and donkeys are different
  species. Each can breed with members of the
  sames species to produce fertile offspring.
• Horses and donkeys can interbreed to produce
  mules but mules are infertile.
• Tests of breeding are not always possible so
  other features are usually considered, including
  DNA testing.

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Allopatric Speciation

• Geographical isolation very important in the
  evolution of new species
• Many islands have own unique species.
• A geographical barrier allows two distinct
  evolutionary scenarios each with different
  selection pressures. The same species evolves
  into two different species.

Sympatric Speciation
Speciation occurring in the same place Polyploidy
more than two sets of chromosomes Mistake in
meiosis results in diploid gametes, two fertilise
to form tetraploid Often sterile as problems in
pairing of chromosomes in Meiosis I Can reproduce
by mitosis as no chromosome pairing is needed If
diploid gamete is formed, and joins with haploid
gamete the result is triploid Triploid is always
sterile as chromosomes cannot be separated
evenly. The triploid is a new species in one
generation
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Polyploids

• Autopolyploid all four sets of chromosomes from
  the same species
• Allopolyploid two sets from one and two sets
  from another species
• Easier for chromosomes to pair up as more likely
  to be different, allopolyploids may be fertile.
  But allopolyploids cannot breed with parental
  generation so new species is formed.
• Example Spartina cord grass growing in salt
  marshes

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Spartina (cord grass)
Spartina maritima diploid
Spartina alterniflora diploid
Spartina townsendii diploid
Hybrid sterile New species
Spartina anglica Tetraploid
Allotetraploidy fertile new species formed
(more vigorous)