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Gradualism and Punctuated Equilibrium Throughout most of the 20th century, researchers developing the synthetic theory of evolution primarily focused on microevolution , which is slight genetic change over a few generations in a population. Until the 1970's, it was generally thought that these changes from generation to generation indicated that past species evolved gradually into other species over millions of years. This model of long term gradual change is usually referred to as gradualism. It is essentially the 19th century Darwinian idea that species evolve slowly at a more or less steady rate. A natural consequence of this sort of macroevolution would be the slow progressive change of one species into the next in a line, as shown by the graph on the right. Gradualism Beginning in the early 1970's, this model was challenged by Stephen J. Gould, Niles Eldredge, and a few other leading paleontologists. They asserted that there is sufficient fossil evidence to show that some species remained essentially the same for millions of years and then underwent short periods of very rapid, major change. Gould suggested that a more accurate model in such species lines would be punctuated equilibrium. (illustrated by the graph on the left). Punctuated equilibrium: Long periods of stability and short episodes of change The punctuated, or rapid change periods, were presumably the result of major environmental changes in such things as predation pressure, food supply and climate. During these times, natural selection can favor varieties that were previously at a comparative disadvantage. The result can be an accelerated rate of change in gene pool frequencies in the direction of the varieties that become the most favored by the new environmental conditions. It would be expected that long severe droughts, major volcanic eruptions, and the beginning and ending of ice ages would be likely triggers for rapid evolution. In such stressful situations, populations would be expected to initially diminish and become isolated. Genetic drift would then potentially speed up the rate of evolution. If by chance nature favored successful adaptations, the population would again increase in numbers as a radically changed species. Conversely, if it favored maladaptive variations, the population would decrease in numbers further and possibly even become extinct. Random mutations provide variations that help a species survive. Mutations in regulator genes in particular can quickly result in radically new variations in the organization of the body and its important structures. As a consequence, changes in these genes can result in a greater likelihood that at least some individuals will have variations that will allow them to survive during times of extinction level events. In this situation, subsequent generations would be significantly changed from the generations before the period of severe natural selection. In other words, regulator genes probably play an important part in the rapid change phases of punctuated evolution. Short-lived species with quick generation replacement times usually evolve at a faster rate than do large, long-lived species. This is because new genetic variations normally appear each generation as a consequence of mutation in sex cells. Those variations may be selected for or against depending on the environment at the time. As a consequence, quicker reproductive cycles generally result in speeded up species divergence. It is not surprising that there are far more species of insects and microscopic organisms than species of large trees and big animals such as elephants, horses, and humans. Tropical species also generally evolve at a faster rate than do those from colder temperate climates. Subsequently, tropical forests are more diverse ecosystems than forests in colder regions. This is probably because warm environments promote shorter generation times and higher mutation rates. A relatively new but extremely important factor in affecting rates of evolution has been people. There are now nearly 7 billion of us, and our numbers are growing rapidly. We have already severely changed most environments on our planet to suit our needs. In addition, we are the super predator around the globe, bringing many species to the brink of extinction and beyond. As a consequence, humans have dramatically altered natural selection. The surviving animal and plant species have responded to this pressure in a variety of ways. For instance, fish species that are heavily exploited by people now usually have smaller bodies as adults and begin to reproduce at an earlier age. It is also likely that because humans increasingly live in urban environments and rely on ever more technology, the evolution of our species has accelerated and changed in ways that are yet to be discovered. It is apparent that the evolutionary history of life on this planet is extremely complicated. Different species have evolved at different rates and those rates have changed through time in response to complex patterns of interaction with other species and other environmental factors. In addition, it is clear that most species lines have already become extinct as a result of their inability to adapt to changed conditions. Vocab: Synthetic theory of evolution - the 20th century conception of evolution caused by a number of complex and often interacting processes. This is essentially a combination of Darwin's natural selection, Mendel's basic genetics, and theories of population genetics and molecular biology. Regulator gene - also called homeotic gene is a gene that can initiate or block the functions of other genes. Shortly after conception, regulator genes work as master switches orchestrating the timely development of our body parts. Microevolution - evolutionary change within a species or small group of organisms, especially over a short period. Macroevolution- major evolutionary transition from one type of organism to another occurring at the level of species and higher taxa.