6.1.2(h) - Principles of Artificial Selection

• Many animals and crop plants have been selectively bred over hundreds or thousands of years.

• Each example below comes up with traits that have been selectively bred into the organism.

How Artificial Selection Works

• Effectively it is similar to natural selection except that the selective agent is the human choosing desirable characteristics (rather than those that the environment would naturally select)

• Individuals with desirable characteristics are chosen.

• Only those chosen are allowed to interbreed.

• Offspring that show the desired/improved combination of characteristics are then selected to breed with one another.

• This inbreeding of closely related species may continue for many generations. 

• Over time there will be changes in the allele frequency within the population (i.e. changes to the gene pool of the next generation)

How could a plant breeder carry out a selective breeding programme to develop a new variety of salt tolerant wheat?

• Grow wheat plants in Salty soil;

• select those that grow best; 

• cross-pollinate and collect seeds grown from these, 

• also in salty conditions; 

• select those that grow best and use as parents; 

• keep repeating over many generations.

Consequences of artificial selection

• Organisms with desired characteristic often leading to higher yields and greater profit (for crop plants and animals).

• May lead to speciation

• However:

  • Reduced/ loss of genetic diversity so reduced gene pool.

  • Organisms become much more homozygous.

  • Selection for certain traits will often select others inadvertently. (due to being linked)

  • Recessive genetic disorders become more common within the population (as inbreeding increases the likelihood of inheriting 2 recessive alleles)

  • i.e. Inbreeding depression: Lack of vigour, loss of fertility and reduction in population size, due to organisms being inbred and becoming homozygous at many gene loci.
    (Outbreeding leads to hybrid vigour)

  • Species is less able to cope with changes in the environment (e.g. new diseases…e.g. Cavendish banana / climate change)
    some cultivated species are in danger of being lost.

  • Ethical concerns (see later)

Getting around the problems

• Importance of maintaining wild type breeds /old breeds/ conservation of wild type species.

  • Seed banks: storing seeds from wild and domesticated varieties in seed banks.

  • Gene banks: Store sperm and eggs and tissues in cluture (frozen)

  • Also have living gene banks.
    Rare breed farms, wild populations, botanic gardens and zoos etc.

• Outbreeding to unrelated or distantly related species can increase genetic diversity and also lead to Hybrid vigour. (Organisms are heterozygous at many loci)

Ethical considerations

• Selection of juvenile characteristics (docile, playful, friendly) makes them less able to defend in the wild

• Lack of fat in pigs → susceptible to cold

• Animal welfare: rapid growth rate + large breast size of meat chickens/ turkeys can cause injury as bones too weak/unable to move easily

• Increases susceptibility to diseases/inheritance of genetic disorders in pedigree dogs

• Most selectively bred organisms biologically unfit in wild. e.g. not camouflaged, no horns for defence etc…

compare natural and artificial selection

what can’t artificial selection do?

• variation/allele/gene must be in gene pool to select from.

• can only select for variation that’s there (as result of natural random mutations), selection doesn’t make mutations occur.