Origin of Species Revisited: Natural selection and laws of variation

In which Darwin explores the forces influencing the variation upon which natural selection works

11 November 2009

Sexual signals involve a trade-off, as they can attract predators too
(Image: Dr. Paul Zahl/SPL)

Chapter Four

In which Darwin explains how inherited differences in the ability to survive and reproduce have shaped nature

Chapter Five

In which he explores the forces influencing the variation upon which natural selection works

Ten thousand years ago, much of North America was covered with ice. As the glaciers retreated they scooped out an intricate landscape of low hills, small lakes and rushing streams. Slowly, sticklebacks moved in from the sea. They found two distinct kinds of place to live in and adopted two ways of life. In lakes, where most of the food was available in open water, they became slim and active swimmers. In streams, where the shallow bottom was a better place to forage, they evolved to be robust and tough so as to grub around for food. Although they are not yet classified as distinct species, each form now prefers to stay in its own home and to mate with its own kind. Natural selection has worked to adapt the fish to the challenges they face, with only those individuals best suited to passing the test of survival getting the chance to hand on their genes.

As Darwin recognised, the entrance examination to the next generation has two papers. The first involves staying alive, but there is a second paper that turns on successful reproduction. It leads to what he called 鈥渟exual selection鈥�. Males of many species are forced to compete for the attention of females, and it pays a female to choose the best mate to father her offspring, the vessel of her own precious genes. This struggle for sex can lead to the evolution of bizarre structures such as the peacock鈥檚 tail. Far from helping their bearers to stay alive, these sexually attractive traits are often a handicap. That may be why they work, for they indicate that those who bear them have what it takes to survive despite their costly sexual signal.

That trade-off between sex and death is hard at work in a certain Trinidad guppy. In some streams the males have bright orange spots that entrance the females. In others there lurks a nasty predator that spots the spots and eats those who carry them. Here, the males have evolved to be dull. But move them to a stream without enemies and in just a few generations the bright spots reappear, testimony to the power of sex.

麻豆传媒