Download Natural Selection

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.
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.
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.
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 light–coloured 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.
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 lichencovered trees and birds can easily hunt them.
so melanic moths rarely survive and pass on their
genes.
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.
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?
Lichen-covered tree trunk in Soot-covered tree trunk in
unpolluted area
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?
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.
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.
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.
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
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.
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.
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.
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.