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Biological Evolution
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Darwin's finches are a symbol of evolution in the Galapagos. These finches helped Darwin recognize the reality of the evolutionary process Photos courtesy Galapagos |
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While the chemosynthetic? theory explains the origin and development of life itself, biological evolution? is a theory that explains the origin of all the diverse life forms on Earth (in other words, the origin of species). There are millions of different organisms on Earth, ranging from microscopic bacteria to elephants. The theory of evolution states that all these different kinds of living things evolved from common ancestors over a time span of millions of years since the first appearance of life on Earth. It may be hard to see the relationship between two species as diverse as elephants and bacteria, but evolution is a widely-accepted theory which gives a plausible (and well-supported) explanation for the diversity of life on Earth.
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"Nothing in biology makes sense except in the light of evolution.
-Theodosius Dobzhansky
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While it is difficult to pinpoint a single, succinct definition for evolution, the theory revolves around the central idea of change. There is an important distinction between change and evolution: although most evolution is change, not all change is a form of evolution. Changes in individuals is not evolution; only populations evolve. Generally, evolution is the progressive change of genes in a population. Random genetic mutations? and genetic recombination? (through mating) are the main factors which create change within populations.
Not all genetic change is good, and the mechanism that weeds out the positive changes from negative changes is natural selection?. Natural selection was a concept developed by Charles Darwin, who is probably the most famous evolutionist to date. Organisms usually produce more offspring than their habitats can sustain, and inevitably, some of the offspring will die. Natural selection says that the organisms best equipped to cope with their environment are most likely to survive and produce the most successful progeny. In turn, these offspring will pass on their genes to successive generations, and eventually the hereditary characteristics that are conducive to survival will become dominant throughout a population.
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The English naturalist Charles Darwin, famed for his theory of evolution based upon natural selection. Photo courtesy Corbis Images |
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As previously mentioned, evolution occurs over quite a long time span. There are two main and conflicting theories that propose different ideas for the pace of evolution over this timespan. Punctuated equilibria? states that evolution consists of occasional periods of rapid growth, while gradualism? states that evolution is slow, gradual and nearly constant. There are concrete examples supporting both these theories. After the extinction of the dinosaurs, mammals evolved very rapidly. Over a period of 45 million years, large horned beasts called Titanotheres evolved from a small, mouse-like animal. This evidence supports punctuted equilibria. However, fossil evidence of plants and animals that shows that these species have evolved little (if at all) over a long period of time supports the idea of gradualism. Perhaps evolution is actually a combination of the two theories in that it occurs in both spurts and over long, gradual periods.
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Natural selection says that the organisms best equipped to cope with their environments are those most likely to survive and produce the most successful progeny.
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It is extremely important to realize that evolution is change without direction. Evolution is opportunistic and it is not caused by the “urge of individual organisms toward something better, nor is it the series of the trends from the simple to the complex” (G. Ledyard Stebbings). For this reason, the term “evolutionary ladder” is false as it symbolizes evolution as moving somewhere, as being a progression towards the summit of perfection. The most accurate analogy to evolution is that of a branching bush, where each twig grows randomly and in different directions. Contrary to popular belief, humans are not on top of the evolutionary ladder. We are simply another branch on the bush of life. |
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This series of images show fossils of Archaeopteryx specimens. These are thought be transitional fossils of between dinosaurs and birds. Copyright © 1999 by Chris Nedin |
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Because evolution is essentially a non-directional phenomenon, the evolution of extra-terrestrial life will most likely be drastically different from the evolution of life on Earth. The different conditions present on different planets will lead extraterrestrial life down different evolutionary pathways, so to speak. But while life elsewhere in the universe is probably drastically and even unrecognizably different from life on Earth, the same concepts and mechanisms that direct evolution, such as natural selection, will apply. On Earth and beyond, it is a matter of the survival of the fittest.
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