Introduction - methodology of
selective breeding and reasons for it
Selective breeding has been practised for
thousands of years producing the forerunners of our modern crops and
farm animals.
Even Darwin had noticed and admired the successful
breeding of livestock in agriculture and 'fancy' pigeon enthusiasts.
BUT note, this initially does not
produce a new species.
In principle selective breeding of plants or
animals by us humans, the basic process is quite simple.
You pick the plants and animals with the best
features you want and interbreed them to get the best possible
offspring - you are cross-breeding varieties to get the best
outcome - the best set of genes.
The basic process of
selective breeding
(i) From your existing plant/animal stock you
pick those with the best characteristics you desire.
(ii) Breed your selection with each other.
(iii) Repeat the process selecting the best offspring
from your initial stock from (ii).
(iv) You continue this process over several
generations so that the desired trait gets stronger and
stronger. The offspring should, in principle,
display the desired characteristic to its full potential.
Unlike natural selection, 'in the wild',
selective breeding is when we artificially pick the plants or
animals to breed to keep the genes for the selected characteristic
in the population.
This selective breeding develops the features
that are e.g. most useful or attractive or resilient to the
environment.
There is nothing new in selective breeding
- for thousands of years, for their own use, people have been
domesticating animals from the wild (e.g. dogs and cows) and
producing useful edible grain like wheat, oats and barley from wild
grasses.
Examples of selective breeding and their
advantages
Crops that give the highest yield e.g. cereals
or vegetables e.g.
Tall crop plants (e.g. wheat) give high
grain yields but are easily damaged by rain and wind. Dwarf crop
plants are better weather resistant, but give lower grain
yields.
If you cross-breed the tall and the
dwarf plant and then cross-breed the offspring, after
several generations you get a new breed of crop variety that
combines the good characteristics of the original varieties
- a compromise of improved yields an weather resistance
Crops that are edible for our digestive
system were bred from wild grasses thousands of years ago.
Crops that are disease resistant, without the
controversy of genetically modified crops.
Farm animals give better yields e.g. milk from cows
or beef from cattle.
You interbreed, through several
generation, the best bulls or cows that give you the greatest
volume of milk per cow or the greatest mass of meat per head of
cattle.
This is not new in the 20th/21st
centuries - this has been done for thousands of years e.g.
domestic cows
have been bred from wild cattle, domestic woolly sheep from wild
sheep.
With these and other animals you may also want to breed
for other good phenotypes e.g. good mothering skills, amiable temperament, successful fertility
rates and good health in general.
Developing a particular breed of dog - size,
colour, quality of fur, facial looks, amiable temperament.
It is believed the first domesticated dogs
were first bred from wolves by people in China 16,000 years ago.
Race horses and greyhounds are bred from thoroughbred stock
known for their speed in racing.
Laboratory bred animals with reared with
particular preferences (e.g. different foods, testing drugs?) and
the results compared e.g. their behavioural activity or brain
activity.
Higher quality of flowers with bigger petals of
a particular or unusual colours produced in a plant nursery.
For more on plant breeding see
How to produce new varieties of plants
Disadvantages of selective breeding
Problems can arise because in selective
breeding for specific characteristics, you are inevitably reducing the gene pool in the population.
This reduction in alleles results e.g. from
the farmer/horticulturalist repeatedly breeding from the best
animals/plants which are closely related genetically - this is known
as inbreeding.
In general, for animals or plants, there
is more chance of organisms inheriting harmful genetic
defects from a more limited gene pool.
This is why it is inadvisable for close
human relatives to interbreed - there is a much greater chance
of inheriting a genetic disorder or disease.
Certain 'modern' dog breeds are quite
susceptible to defects e.g. breathing problems in pugs, invertebral
disc problems of the spine in corgis and dachshunds, Caesarean
operations needed for the safe birth of pups.
This raises ethical issues as to whether
we should breed animals with negative characteristics because of
other desired characteristics.
The same ethical issue crops up if animals
'pure' bred for medical research.
Cows have been bred with larger udders to
give greater milk yields, BUT, many cows have suffered from
all sorts of problems e.g. lameness (due to increase in body
weight), shorter lives, mastitis (the udder tissue becomes inflamed
and painful) and metabolic diseases.
Due to the narrower gene pool of a given
organism population, serious problems can arise if a new disease
crops up, especially with crops!
selective breeding ==> reduction in forms
of genes (alleles) ==> less chance of alleles being present in
the population that can help the organism resist the disease.
Because all the animal/plant stock are
closely related, there is not much genetic variation in the
population.
So if one individual in the population is
susceptible to a disease, all the others may be equally at risk
and can result in the death of most, if not all, of a population
of animals or plants.
For more on plant breeding see
How to produce new varieties of plants
