IGCSE Biology 0610: Selective Breeding of Tilapia for Faster Growth

A fish farmer keeps a large pond of tilapia, a freshwater fish, for food production. When the farmer weighs six-month-old tilapia from the pond, their body mass forms a continuous range from very small to very large fish, with no natural gaps between individual values. The farmer wants to breed a strain of tilapia that reaches a larger body mass more quickly.

(a) State the three stages the farmer would follow, over one generation, to begin selectively breeding tilapia for faster growth to a larger body mass. [3]

(b) Explain why the farmer needs to repeat this process over many generations, rather than reaching the desired growth rate after selecting and breeding from the fastest-growing fish just once. [2]

(c) State why it was essential that the tilapia in the original pond already showed variation in growth rate before the selective breeding programme could begin. [2]

Part (a): Stages of selective breeding

Selective breeding follows the same three stages whatever the desired feature is:

The farmer selects individuals that already show the desirable feature to the greatest extent, here, the fish with the fastest growth and largest body mass at six months.
The farmer crosses (breeds) these selected fish with one another to produce the next generation of offspring.
The farmer selects the offspring that themselves show the desirable feature, so that only these are used to breed the following generation in turn.

Part (b): Why the process must be repeated over many generations

Breeding from the fastest-growing fish only once does not turn every offspring into a fast grower: each offspring inherits a mixture of alleles from its two selected parents, so the offspring generation still shows a range of growth rates, just shifted a little faster on average than the original population. To produce a strain that reliably reaches a much larger body mass more quickly, the farmer must repeat the cycle of selecting the fastest growers and breeding from them, generation after generation. Each repetition shifts the average growth rate a little further, and these small shifts accumulate over many generations into a substantial, lasting improvement.

Part (c): Why pre-existing variation was essential

Selective breeding is a process of choosing between individuals, so it can only work if individuals actually differ from one another in the feature of interest. Because the original pond already showed a continuous range of growth rates, some tilapia already grew faster and larger than others, giving the farmer a real basis for comparison. If every fish in the pond had grown at exactly the same rate, there would have been no faster-growing individuals to identify, and the farmer would have had nothing to select for, so the breeding programme could not have begun.

Final answers

(a) Select fish with the fastest growth; cross them to breed the next generation; select the fastest-growing offspring to breed from in turn.
(b) A single round of breeding only shifts the average growth rate slightly; repeating selection over many generations lets these shifts accumulate into a much greater improvement.
(c) Without pre-existing variation in growth rate, there would be no faster-growing individuals to choose between, so selective breeding would have no variation to act on.