Lesson 5: Life's History: The Great Adventure 5.7 The Rules of Evolution Most of the commonly cited rules of evolution center around the Darwinian concept of adaptation through natural selection, within the animal kingdom. The selection works because there are always more offspring than can reach maturity. Otherwise, the world fills up with the organism, in a geometric expansion; a population explosion. Thus, it is a logical requirement that resources diminish and hazards increase when a population expands. From this environmental feedback, a population is kept in check. Selection prevents some members of the population from reproducing. The better adapted use more of the resources and avoid hazards more readily. They have a better chance to reproduce. In a way, much of what has been written on the subject is a commentary on Darwin's book, except for two new developments. One is the concept of quantum inheritance through genes. It produced the idea of "gene fitness", which proposes that genes will act to maximize their abundance in a population. This moves the concept of "success" from the level of organism to that of the gene. The other new development is the demonstration that physical catastrophe is important in making organisms extinct even though they are perfectly well adapted to their surroundings. (Well-adapted except to being hit by a bomb from space). The "gene fitness" idea is behind the concept of the "selfish" gene. The discovery of extinction-by-bombing led to the phrase "survival of the lucky" by David Raup and Jack Sepkoski, in contrast to Darwin's "survival of the fittest". A topic that still needs much discussion is the question of precisely where the variation arises on which natural selection operates. How do we get useful mutations within the gene pool of a population? This is not known. We must assume that more or less random chance can produce useful changes as well as deleterious ones. Useful changes will spread in a population. Deleterious ones will eventually be eliminated because the carrier will be at a disadvantage. Some rules of evolution are intuitive and simple. Why does Thompson's gazelle have such big ears and fleet feet? To escape from the Cheetah. And why does the Cheetah run so fast? To catch fast food on the hoof. So, since the early mammals look kind of klutzy and these two look sleek, there is an inference that they evolved together in a kind of arms race. That predator-prey relationships stimulate evolution is seen everywhere: snails on the seashore making thicker and more ornamented shells to thwart the bigger and better claws of crabs, plants making themselves spiny, bitter or poisonous to fend off ever tougher and more tolerant herbivores, fish growing bigger pectoral fins for flying to escape ever faster predators, moths developing sensitivity to ultrasound to cope with bats. A fundamentally different kind of co-evolution arises between collaborating organisms. Practically every eucaryote organism is collaborating with someone else. This is called "symbiosis". A prime example for symbiosis are zooxanthellae. They are minute, single-celled photosynthesizing protists (dinoflagellates). They live within corals, trading products from photosynthesis for nutrients gathered by the coral animal. Without its symbionts, such a coral cannot survive for long. The mutual interdependence is such that the symbiosis requires extensive fine-tuning of the physiology of each organisms to provide and receive maximum benefit. This implies a long evolutionary history. From these few examples, we see that organisms evolve because other organisms around them evolve, so that their environment (their resources and their hazards from others) changes all the time. Figure 5.7.1 The peppered moth. With the air pollution brought about by the Industrial Revolution in Manchester, England, the number of black moths increased by 90% while the white variety nearly disappeared. Scientists discovered that on the soot-coated trees, the white moth was highly visible to the predator birds. The black moths gradually began to dominate the population. This is a very famous example of natural selection. The light-colored moth was selected against while the dark moth had an advantage in the changed environment. The other big reason to evolve is to take advantage of new opportunities in the physical environment. These arise when new potential habitats open up, because the landscape changes or the climate, or both. In some cases, the evolution of some organisms can change the environment for others on a large scale. When animals first learned to burrow into the sea floor, an opportunity arose for organisms to invade and share burrows, or to hunt within the sediment, or for bacteria to burn the materials within the sediment much more efficiently than before. Looking at the distribution of overall diversity, we find that it is high in the tropics and low in the high latitudes, for most organisms for which an inventory has been made. The possible reasons are: a higher rate of evolution in the tropics, a higher rate of extinction in the high latitudes, insufficient time for developing new species in the cold realm of high latitudes (very likely), or inclement physical conditions at high latitudes (also likely). In summary, the rules are simple: make more offspring. But the circumstances are complex, and the tasks to be solved are many: get resources in a competitive setting, improve your abilities to collaborate with other organisms, improve your abilities to avoid and fight predators, parasites and disease. And even if successful in all of this there is no guarantee for survival. Get lucky and avoid the bad storm, the bad winter, the drought, and (ultimately) the big bomb from space.