Natural Selection

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Natural Selection

• Is the process by which organisms that are better
  adapted to their environment survive and breed
  while those less well adapted fail to do so.
• The better adapted organisms are more likely to
  pass their characteristics to succeeding
  generations.

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A wee story to set the scene..

• Mice are not genetically identical. They are
• produced by sexual reproduction which
• ensures that they possess different
• combinations of genes from their parents.
• Important characteristics such as coat colour
• and thickness, speed and reactions, food
• finding ability, resistance to disease and
• aggressiveness will vary from mouse to mouse.

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• Only those mice with the best combinations
• of genes for their habitat will survive. This
• means that their gene combination will be
• passed on to their offspring. Mice with
• less useful combinations of genes die and
• so these genes are not passed on.
• This is the story of natural selection and if
• it operates over millions of years then it is
• thought that it can give rise to new species.

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Natural Selection in the Peppered Moth

• The peppered moth is a nocturnal insect which
  rests during
• the day on tree trunks. Insect-eating birds prey
  on the
• resting moths by picking them from the tree
  trunks.
• In the early 19th century, the moths were well
  adapted to
• survive as they had lightcoloured speckled wings
  which
• camouflaged them against the lichen covered tree
  trunks.
• This meant that the birds could not see them and
  it gave
• the moths some protection.

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• There is another form of the peppered moth
• which is melanic it is nearly all black
• coloured.
• (If these moths were on a lichen covered
• tree such as the one in the picture, do you
• think they would survive the beady eyes of
• an insect-eating bird?)
• Melanic moths show up well against the lichen-
• covered trees and birds can easily hunt them.
• so melanic moths rarely survive and pass on their
  
• genes.

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Environmental change and Natural Selection in the
Peppered Moth

• During the late 19th century, Britain saw the
• Industrial Revolution.
• Large towns and cities were formed where
• factories and industries burned up large
• volumes of coal to power them.
• Burning the coal caused lots of sooty smoke
• and sulphur dioxide.

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• As you know, lichens are very sensitive to
  pollution
• and so will not grow in polluted areas.
• The trees became less lichen-covered and more
• coated with black soot.
• Which moths would be more camouflaged now?
• (The melanic form or the light speckled
• form?)
• Which form would have the greatest survival and
  increase
• in number and pass on their genes to future
  generations?
• Which form would die out and not be able to pass
  on their
• genes?

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Lichen-covered tree trunk in unpolluted area
Soot-covered tree trunk in very polluted area
Today, due to the Clean Air Act, the pale form of
moth is becoming more numerous again in what were
once polluted areas. Perhaps the situation we
had before the Industrial Revolution will return
one day?
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Selective Breeding

• So far we have looked at natural selection and
  how this has led to the great biodiversity found
  today.
• We are now going to look at Selective Breeding
  where humans deliberately choose which
  individuals to breed. This can happen in both
  plants and animals.

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Selective Breeding in Domesticated Animals

• Humans deliberately breed desirable
  characteristics so that the offspring will also
  show these characteristics.
• Over time and several generations later, an
  improvement may result.
• However, it takes quite a long time and the
  results are not always guaranteed.

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Example cows and milk yield
Farming is a huge industry in Britain today. It
would not be cost effective to our farmers if
only some of their cows produced large volumes of
milk. So, they will only breed cattle with
desirable characteristics (large milk yield). The
aim is that the female offspring should be able
to produce large quantities of milk.
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More Examples of Selectively Bred Animals
Selectively Bred Animals Characteristics Selected For
Cattle Milk and Beef production
Sheep Milk and Wool quality
Pigs Growth rate and litter size
Poultry Growth rate and egg production
Horses Strength, size and speed
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Selective breeding in plants

• In 1895 a group of American Biologists began a
  breeding experiment using a variety of maize
  (sweet corn) whose seed grains varied slightly in
  oil and protein content.
• Firstly they selected only those plants that
  produced seed grains with the highest oil content
  strain O.
• Then they selected only those plants that
  produced seed grains with the highest protein
  content strain P.
• They repeated this over 50 generations. Strain O
  showed improved oil content and strain P showed
  improved protein content.

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Limitations of Selective Breeding

• Relies on sexual reproduction
• - fertilisation not always guaranteed and there
  may be difficulties getting individuals to breed.
  
• - can take months or even years to get just one
  generation so can be very slow.

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Limitations of Selective Breeding.

• Inbreeding is a common method used in selective
  breeding.
• - can lead to an increase in recessive genes.
• - since many of these normally hidden recessive
  genes give inferior qualities, inbred lines may
  show disadvantages.

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Limitations of Selective Breeding

• Individuals may not have the genes for the
  desired characteristic.
• - If they are not present, they cannot be
  created e.g. if a gene for disease resistance
  does not exist in a species then the
  characteristic cannot be developed in this way.