From Julyan Cartwright, University of Granada
The case of monkeys becoming “bleached” over generations
(5 February, p 32)
is perfectly consistent with natural selection. There is no need to invoke
orthogenesis—the theory that organisms are propelled along a predetermined
evolutionary pathway—or other esoteric ideas as explanations.
If it costs energy to make the pigment for coat colour, then in the absence
of any direct selection pressure, the less pigmented animal is better off.
Mutations that tend to colour loss will then be favoured over those enhancing
pigmentation, so an initially dark population will become lighter over time,
unless and until there is selection pressure in the opposite direction—for
protection against predators, sexual display, and so on.
That the world isn’t full of albinos shows that usually there is selection
based on coat colour. The gradual bleaching is nature’s default process in the
relatively few cases where this isn’t so.
Spain
