Transcript File

All populations shows some sort of
variation.
Natural selection has led to great biodiversity.
For many centuries humans have also altered the
characteristics of organisms by deliberately
choosing which individuals to breed.
This is known as selective breeding.
First the desirable characteristic is identified.
For example size of cob in maize ( corn )
It is obviously better for the farmer if the cob is
large and gives a high yield.
Since there is always variety in a
population some plants will produce
larger cobs than others.
For breeding purposes the farmer will select the
plants with the desired characteristic. In this case
plants with large cobs.
X
The next generation will have a higher
percentage of plants with large cobs.
The farmer will again select only those plants with
the desired characteristic for breeding.
X
The next generation will have a higher
percentage of plants with large cobs.
The farmer will again select only those plants with
the desired characteristic for breeding.
X
The next generation will have a higher
percentage of plants with large cobs.
The farmer will again select only those plants with
the desired characteristic for breeding.
X
In this way over a number of years the
farmer will have improved his crop.
This is known as Selective Breeding as the
farmer selects the characteristic to improve
and then selects the individuals in the
population which he will allow to breed.
It is not just plants which can be improved in
this way.
Animals are also selectively bred for a
number of characteristics.
For example race horses are bred for speed
or stamina.
= Fast Horse
= Slow Horse
In a normal population of horses there will be
some fast horses but mostly slower ones.
X
The horse breeder will only allow his fastest
horses to breed
X
X
X
X
Agriculture has produced many
examples of both animals and plants
which have been improved by the use
of selective breeding.
Selective Breeding In Animals
Animal
Characteristic
Improved
Cattle
Beef and Milk Production
Pig
Growth Rate and Litter Size
Sheep
Wool Quality and Quantity
Chicken
Growth Rate and Egg
Production
Behaviour, Appearance and
Abilities
Dog
Selective Breeding In Plants
Plant
Characteristic
Improved
Maize
Oil Content and Size of Cob
Tomato
Size, Colour and Flavour
Wheat
Apple
Height ( being the same height
makes it easier to harvest )
Size, Colour and Flavour
Beans
Colour of Bean
The Limitations of Selective Breeding
Difficulty in getting selected individuals to breed.
It may take months or years to obtain just one
generation.
Hybrids may turn out to be inferior due to the new
combination of genes.
The genes for a desired characteristic may not be
present.
Inbreeding can lead to the increase in the frequency of
recessive genes. Often recessive genes give inferior qualities