Selective breeding

People were putting the rules of this chapter to work thousands of years before anyone could state them. Every crop in a Sri Lankan field is the result of it.

Selective breeding

Choosing as parents the individuals that show desirable characteristics, so that their offspring are more likely to show them too, and repeating that over many generations.

How it is done

Decide which characteristic is wanted — a higher yield, resistance to a disease, a shorter growing season, better milk.
Select the individuals that show it most strongly and cross them with each other.
Grow or raise the offspring and select again from among them.
Repeat for generation after generation, until the desired characteristic is reliably present.

Figure to be added

Figure 20.27 — improved rice varieties in the field beside an unimproved one, the improved plants shorter and carrying heavier grain heads.

Figure to be added

Figure 20.28 — improved livestock: a high-yielding dairy cow beside an unimproved animal of the same species.

Why it works at all

It works because the characteristics being selected are inherited. Selecting the biggest animal is pointless if its size came from being well fed; it is powerful if its size came from its genes, because only genes are passed to the next generation.

It is slow, and it cannot make something from nothing

Selective breeding can only work with alleles the species already has. If no variety of rice anywhere carries resistance to a particular disease, no amount of selecting will produce it. That limit is the reason the next skill exists.

The first skill of this chapter, used deliberately

The very first page asked why no two individuals are alike. Selective breeding is that variation put to work: without differences between individuals there would be nothing to select from, and every field would still be growing wild grass.