A species of land snail has two forms of shell, a banded form and a plain form. The snails are eaten by thrushes, which are birds that hunt the snails by si...

This question tests natural selection using shell-form data over three generations. On bare soil, the form that is harder for thrushes to see survives better, so its proportion rises.

(a) [2] Natural selection is the process by which the organisms best adapted to their environment survive and reproduce [1], passing on their advantageous alleles to the next generation [1].

(b) [3] Over the three generations the banded form decreases in proportion / becomes rarer [1] while the plain form increases in proportion / becomes commoner [1]; for example the banded form falls from 3 out of 5 in generation 1 to 1 out of 5 in generation 3 [1].

(c) [5] There is variation, with banded and plain forms in the population [1]; on bare soil the plain snails are better camouflaged, so fewer are seen and eaten by thrushes [1]; the banded snails stand out and are eaten more [1]; the surviving plain snails reproduce and pass on the alleles for the plain shell [1]; so the proportion of the plain form increases over the generations [1].

(d) [1] New shell-colour alleles arise by mutation [1].

(e) [2] The change is gradual because only a proportion of the less-camouflaged snails is eaten in each generation [1]; so the allele frequency shifts only a little at a time, taking several generations of reproduction to change greatly [1].

(f) [2] Any two differences: natural selection is driven by the environment, whereas selective breeding is driven by humans choosing the parents [1]; natural selection has no chosen/planned outcome, whereas selective breeding aims at a chosen feature [1]; (accept: selective breeding is usually faster) [max 2].