The arms race
Predator/prey coevolution can
lead to an evolutionary arms race.
Consider a system of plant-eating insects. Any plant that happens
to evolve a chemical that is repellent or harmful to insects will be
favored. But the spread of this gene will
put pressure on the insect population and
any insect that happens to have the ability to overcome this defense
will be favored. This, in turn, puts pressure on the plant population,
and any plant that evolves a stronger chemical defense will be favored.
This, in turn, puts more pressure on the insect population...and so
on. The levels of defense and counter-defense will continue to escalate,
without either side "winning." Hence, it is called an arms
race. This sort of evolutionary arms race is probably relatively
common for many plant/herbivore systems.
Other predator/prey systems have also engaged in arms races. For example, many
molluscs, such as Murex snails, have evolved thick shells and spines to avoid
being eaten by animals such as crabs and fish. These predators have, in turn, evolved
powerful claws and jaws that compensate for the snails' thick shells and spines.