Theory of Evolution

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[edit] Definition

The theory of evolution explains the facts of biological evolution in terms of the law of natural selection (which in modern parlance is usually taken to include sexual selection) and the facts and laws of genetics, which include such things as mutation, recombination, lateral gene transfer, and genetic drift.

Scientists use this theory to explain how the current diversity of living things came into existence and the relationships between modern organisms and their more "primitive" ancestors.

[edit] Definition of Evolution

Having said that the theory of evolution explains the facts of biological evolution, we should say what we mean by the phrase "biological evolution". A strict definition offered by the biologist Douglas Futuyma in Evolutionary Biology, (Sinauer Associates 1986) is as follows:

"In the broadest sense, evolution is merely change, and so is all-pervasive; galaxies, languages, and political systems all evolve. Biological evolution ... is change in the properties of populations of organisms that transcend the lifetime of a single individual. The ontogeny of an individual is not considered evolution; individual organisms do not evolve. The changes in populations that are considered evolutionary are those that are inheritable via the genetic material from one generation to the next. Biological evolution may be slight or substantial; it embraces everything from slight changes in the proportion of different alleles within a population (such as those determining blood types) to the successive alterations that led from the earliest protoorganism to snails, bees, giraffes, and dandelions."

More tersely, "evolution" in this strict sense refers to the heritable changes in a population of organisms over time (and even more strictly, to the changes in allele frequencies in the appropriate gene pool).

[edit] History

The fact of evolution has been known since ancient times. By breeding selected animals or sowing selected seeds, people have changed plants and animals over time. In some cases, the changes have been dramatic. The domesticated forms of maize, known as sweet corn and field corn, have been modified by people so much that they can no longer reproduce in the wild. Sheep have been modified to grow fluffy coats, suitable for shearing, from an original wild animal that had only a light, stubbly coat. Wheat and barley grow many edible seeds that are huge in comparison to their ancestors. Wild dogs have been modified to everything from the miniature chihuaha to the great dane. This process is known as "evolution by artificial selection."

During the 18th and 19th centuries, it became apparent that the Earth was much older than people had previously believed. It also became apparent that many creatures, beyond those few domesticated creatures already known to have evolved under the influence of humanity, had also evolved before humans had come upon the scene. While the fact of this evolution was widely accepted, a successful theory to describe evolution was still lacking.

One of the easiest ideas was that this was also due to artificial selection under the hand of God rather than the hand of humanity. God had husbanded creatures, much as humans have done, before creating humans, at which time God handed the task over to humans under the "dominion" clause of Genesis. This theory had a number of problems. One is that it failed to explain the extinctions of perfectly good creatures. It is unlikely that human beings would deliberately wipe out a species as useful and as modified as the sheep except out of sheer ignorance, which God is presumed not to have. The fossil record seemed more consistent with the idea of a brilliant but naive tinkerer than of an omniscient God. Furthermore, evidence was mounting for evolution during the lifetime of humans but isolated beyond their control. It was also not very useful as a scientific theory; if God did things this way according to a plan, then the plan seemed haphazard and useless for prediction. Nevertheless, a form of this idea has survived and forms a portion of the ideology known as Intelligent Design.

Another idea that came about was the inheritance of acquired characteristics (see the main article on Lamarckism for more information). It was well known that working some body parts could make them bigger. Blacksmiths developed bulging muscles and enlarged hands. Children raised on mountain tops developed bigger chests and, though it was not known at the time, thicker blood. These are now known as "developmental adaptations." Jean-Baptise Lamarck proposed that if such acquired characteristics were inherited, this could account for evolution. While the understanding of inheritance at the time could not rule this out, Lamarck's ideas never achieved much status in the scientific community. The idea of the inheritance of acquired characteristics was carried well into the 20th century and well beyond where the mechanism of inheritance was well enough understood to discredit it, by Trotim Lysenko. These became a central part of Soviet agricultural science, because they were interpreted as flattering the concept of dialectical materialism.

By 1838, Charles Darwin had developed another idea. Since artificial selection was known to have produced dramatic evolution over a short period of time, might natural conditions, such as climate change, competition for limited resources, habitat, reproductive partners, etc. also produce similar effects? The process might not be as fast as that directed by a human or God, so it would produce a wide variety of creatures evolved over a long period of time, which is exactly what we see. He called this process Natural Selection by analogy with Artificial Selection. He also noted that the tendency of animals to select mates might also be a factor and called this Sexual Selection.

For 20 years, Darwin looked earnestly for flaws with his theories and was reluctant to confide them in anyone other than close friends. In 1858, he learned that others such as Alfred Russel Wallace had been thinking along similar lines, so he published jointly with Wallace. The next year, Darwin published On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life. [1] In this book, he also proposed the Theory of Common Descent. Since species branched from original species, it seemed that in the past, the number of species would be fewer, and all existing species would be descended from those. Darwin proposed that the original forms may perhaps have numbered as few as ten. Investigations over the past century and a half suggest that the number may be as small as one.

These theories of evolution by natural and sexual selection, combined with the theory of common descent, form a central part of modern evolutionary theory. This is sometimes known as Darwinism, though the term is at best misleading, partly because evolutionary theory has come a long way since the time of Darwin, and partly because the term "Darwinism" is used by some scientists to describe what the late Steven Jay Gould also calls "gradualism" and Richard Dawkins called "constant-speedism" (for more information, see the article on Punctuated Equilibrium or the article on the word Darwinism).

The theory of evolution has proven to be one of the most important aspects of modern science and is a key to our modern understanding of biology. Despite this tremendous explanatory and predictive power, the theory of evolution is under attack by people, especially US-based fundamentalist Christians, who believe that the theory of evolution, as expressed by biologists, contradicts the literal meaning of the story of the origin of life as expressed by the book of Genesis.

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