A2 Biology 9700: Selective Breeding and Artificial Selection

A dairy farmer keeps a herd of cattle whose average daily milk yield per cow varies continuously, from about 18 to 30 litres per day. The farmer wants to increase the herd's average milk yield over the next several generations and decides to use selective breeding (artificial selection) rather than relying on natural processes alone.

(a) Describe how the farmer could use selective breeding, over several generations, to increase the average milk yield of the herd. [4]

(b) Explain one key difference between artificial selection, as used by the farmer here, and natural selection, in terms of what determines which individuals breed. [2]

(c) Suggest one disadvantage, for the long-term health of the herd, of repeatedly choosing only the highest-yielding cows and bulls as breeding parents over many generations. [2]

Part (a): The process of selective breeding

Selective breeding works by repeatedly choosing which individuals are allowed to reproduce, based on a desired characteristic:

The farmer measures the milk yield of every cow (and, indirectly, of every bull, via the yields of its female relatives or offspring) in the current herd.
Only the cows and bulls with the highest milk yields are chosen as breeding parents; lower-yielding individuals are prevented from breeding.
The offspring produced are reared, and their milk yields are measured once they mature.
Once again, only the highest-yielding offspring are selected as parents for the next generation, and the process is repeated.

Because alleles associated with higher milk yield are consistently passed on by the selected parents while alleles associated with lower yield are excluded from breeding, the frequency of “high-yield” alleles increases in the herd with each generation. Over several generations, this raises the herd’s average milk yield, potentially beyond the range present in the original herd. This is the cumulative effect of selective breeding.

Part (b): Artificial selection versus natural selection

The key difference lies in who or what determines which individuals breed:

In natural selection, the environment determines which individuals survive and reproduce, based on which characteristics happen to improve an individual’s own chances of survival and reproduction in that environment. No conscious choice is involved.
In artificial selection, a human breeder (the farmer) deliberately chooses which individuals breed, based on a characteristic that is useful or desirable to humans (here, high milk yield). A characteristic that may have no particular survival advantage for the cow itself.

Part (c): A disadvantage of intensive selective breeding

Because only the highest-yielding individuals are used for breeding, generation after generation, many alleles that were present in the lower-yielding (excluded) individuals are gradually lost from the herd’s gene pool. This causes:

A reduction in genetic diversity across the herd.
An increased risk of inbreeding depression, since breeding is repeatedly concentrated among closely related, high-yielding lines.
Greater vulnerability: with less genetic variation remaining, the herd has fewer alternative alleles available if a new disease, parasite or environmental change arises. There may be no resistant individuals left for natural selection to favour.

Final answers

(a) Repeatedly select and breed only the highest-yielding cows and bulls each generation, so alleles for high milk yield increase in frequency and the herd’s average yield rises over time.
(b) Natural selection is driven by the environment acting on survival/reproduction; artificial selection is driven by a human breeder deliberately choosing which individuals reproduce, based on a trait useful to humans.
(c) Reduced genetic diversity, increasing the risk of inbreeding depression and making the herd more vulnerable to disease or environmental change.