72 UNDERWATER GUIDE TO MARINE LIFE 



phenomena observed by ecologists and paleoecologists studying evolution are so 

 instructive that they must be mentioned. 



First, there is the phenomenon of convergence. Unrelated animals living in 

 similar environments or having similar habits are under similar environmental 

 stress and frequendy come to look alike. Porpoises, for instance, are adapted 

 for a swift-moving, fish-eating existence on the open sea. Before mammals 

 became dominant, when the reptiles reigned, a group of reptiles called 

 "ichthyosaurs" with porpoiselike habits were remarkably like porpoises in body 

 form. The whales and whale sharks are remarkably alike and have similar 

 habits. There are many eel-shaped fishes all adapted for secretive predacious 

 lives, yet derived from widely different groups of fishes. These are all examples 

 of convergence of body form in response to similar habits and habitat. 



The second phenomenon is faralleUsni, which differs from convergence but 

 little. Again it concerns evolution of animals in response to similar conditions, 

 but, unlike convergence, the groups were not widely divergent to start with 

 and have merely followed parallel courses in evolution. The demoiselles and 

 the butterfly fishes are very similar in habits, size, and form and were derived 

 from fairly closely related suborders of fishes. There are several predacious 

 species of bottom-dwelling fishes such as toadfish, scorpion fish, stargazers, etc., 

 which have developed from closely related stock. 



The third is adaptive radiation. All groups of animals will tend to evolve 

 in all ways that are open to them within the limits of their own anatomies. 

 Thus the reptiles and mammals have both had swimming, running, flying 

 species. The fishes have radiated into almost every conceivable niche from the 

 surface to the deep sea, from plankton and plant-eaters to voracious carnivores, 

 and from life on rocks and reefs to sandy beaches. Even smaller groups of fishes 

 show their own radiation patterns. One has only to examine the varied mackerel- 

 like fishes— swift predators, plankton feeders, eellike forms, etc.— to see radiation 

 at work. 



Evolution is a dynamic, continuing process. It is going on today even in the 

 human species. Rarely does it come to a halt in any group. When such a halt 

 occurs, the result is usually extinction— especially when environmental change 

 occurs— but occasionally a seemingly changeless "living fossil" is left behind: the 

 oyster, the horseshoe crab, the sturgeon, Lingula. Evolution is a peculiar combi- 

 nation of the randomness of mutation and the orientation of natural selection. 

 It is natural selection imposed by the environment that keeps species within 

 bounds. Without it, deleterious genes could accumulate, and the result would 

 almost certainly be genetic pandemonium, which could only result in environ- 

 mental maladjustment and extinction. 



Evolution is highly opportunistic. New situations are quickly met by organisms 

 that seize new opportunities, but such meeting is largely due to the chance 

 that the animals are present to meet these opportunities and the chance that 

 mutations of plus selective value occur. Islands or isolated coral reefs, for in- 

 stance, have notably depauperate faunas simply because many groups of animals 

 have not found their way there. 



Evolution seems slow to humans who measure time in minutes, hours, and 

 years. Geologic time is not so reckoned. To the human eye, the world is a 

 static place, but this eye sees only one frame of a moving picture. We should 



