The Mechanisms of Evolution

Newts and other salamanders can move only slowly, so they are easy prey for garter snakes. But some salamanders have evolved defensive toxic chemicals that make them less desirable as prey. The rough-skinned newt, Taricha granulosa, is a salamander that lives on the Pacific Coast of North America. Taricha sequesters in its skin a potent neurotoxin called tetrodotoxin (TTX). TTX paralyzes nerves and muscles by blocking sodium channels (see Chapter 5). Most snakes die if they eat a rough-skinned newt, but some populations of the garter snake Thamnophis sirtalis have evolved TTX-resistant sodium channels in their nerves and muscles. These snakes are able to eat the newts and survive—but the addition to their diet comes at a price. TTX-resistant snakes can crawl only slowly for several hours after eating a newt, and they never crawl as fast as nonresistant snakes. Thus, TTX-resistant snakes are more vulnerable to their own predators.

Pufferfish, octopuses, tunicates, and some species of frogs also use TTX as a defensive chemical. Many other species use a variety of chemicals to defend themselves against predators, and many predators have evolved resistance to those chemicals. But production of and resistance to defensive chemicals like all other adaptations, has costs as well as benefits. Such adaptations may impose a cost in the form of speed of movement, as they do on garter snakes. They may reduce the ability of the organism to function efficiently, or they may be energetically costly to develop and maintain. That is, to improve its performance in one area, the organism must accept reduced performance in some other area—a trade-off.

Biologists try to identify and measure the tradeoffs that different adaptations impose because the nature and strength of these trade-offs influences how adaptations evolve. If there were no cost to TTX resistance, then snakes that live in places where toxic newts are rare would probably also be resistant to TTX—which they are not.

Charles Darwin's main contribution to biology was to propose a plausible and testable hypothesis for a mechanism that could result in the adaptation of organisms to their environments. In effect, Darwin offered a mechanistic explanation for the evolution of life on Earth, the last component of the known universe that lacked such an explanation. The mechanism that Darwin proposed can explain the evolution of all forms of life, including humans. It has been difficult for many people to accept that the same processes that determined the evolution-

An Evolutionary War Rough-skinned newts (below) evolved the ability to secrete a paralyzing neurotoxin in their skin, a trait that deters most of their predators. Some common garter snakes (above) have evolved a resistance that allows them to turn the poisonous newts into a meal.

Galapagos Islands SnakesMarchena Island Galapagos

Galápagos Islands

: Pinta

Marchena

Tasmania New

Zealand

Genovesa

Galápagos Islands

: Pinta

Marchena

Santiago

Santiago

Bartolome?

Santa Cruz

San Cristobal

Fernandina

Isabela 0

Tortuga

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