Evolution: Difference between revisions

Revision as of 12:57, 25 February 2004

Revision as of 13:12, 25 February 2004

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:''This article is about evolution in the [[biology|biological]] sense. For other possible meanings, see [[Evolution (disambiguation)]].''

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'''Evolution''' is any [[process]] of growth, change or development. The word stems from the [[Latin]] ''evolutio'' meaning "unfolding" and before the late [[19th century]] was confined to referring to goal-directed, pre-programmed processes such as embryological development. A pre-programmed task, as in a military maneuver, using this definition, may be termed an "evolution." One can also speak of [[stellar evolution]], [[chemical evolution]], [[cultural evolution]] or the [[meme|evolution of an idea]]. Other kinds of evolution include [[evolutionary algorithm]]s (which include [[genetic algorithm]]s) which attempt to mimic processes similar to biological evolution in a [[computer program]], most frequently as an [[optimization]] technique and as an experimental framework for the computational [[model|modelling]] of evolution.

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Most biologists believe in [[common descent]]: that all life on Earth is descended from one common ancestor. This conclusion is based upon the fact that many traits of living organisms, such as the [[genetic code]], seem arbitrary yet are shared by all organisms. Some have suggested that life may have had more than one origin, but the high degree of commonality argues strongly against multiple origins.

The study of the ancestry of [[species]] is [[phylogeny]]. Phylogeny has revealed that organs with radically different internal structures can bear a superficial resemblance and perform similar functions. These examples of ''analogous structures'' show that there are multiple ways to solve most problems and make it difficult to believe that the universal traits of life are all necessary. Likewise other organs with similar internal structures will perform radically different functions. [[Vertebrate]] limbs are a favorite example of ''homologous structures'', organs on two organisms that share a basic structure that had existed in the last common ancestor of the organisms. The current dominant theory of evolution is known as the "[[modern evolutionary synthesis]]" (or simply "modern synthesis"), referring to the synthesis of Darwin's theory of evolution by natural selection and [[Gregor Mendel|Mendel]]'s theory of the [[gene]]. According to this theory, the fundamental event of speciation is the genetic isolation of two populations, which allows their gene pools to diverge.

Further evidence of the universal ancestry of life is that [[abiogenesis]] has never been observed under controlled conditions, indicating that the [[origin of life]] from non-life, is either very rare or only happens under conditions that are not at all like those of modern earth.

===The emergence of novel traits===

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====Microevolution and Macroevolution====

[[Microevolution]] refers to small-scale changes in gene-frequencies in a population over a few generations ([[population genetics]] is the branch of biology that provides the mathematical structure for the study of the process of microevolution). These changes may be due to a number of processes: [[mutation]], [[gene flow]], [[genetic drift]], as well as [[natural selection]]. [[Macroevolution]] refers to large-scale changes in gene-frequencies in a population over a long period of time (and may culminate in the evolution of new [[species]]). The difference between the two is hard to distinguish because, over time, successive tiny mutations like those evidenced in microevolution could build up in isolated populations and eventually create entirely new species, which is known as macroevolution. TheseWhile twomicroevolution termshas arebeen oftendemostrated usedin bythe certainlaboratory [[creationism|creationists]]to the satisfaction of most observers, whomacroevolution claimhas thatto microevolutionbe caninferred andfrom doesthe happen[[fossil butrecord]], thatand macroevolutiontheories cannot.concerning its Thisprecise ismechanisms basedare oncontroversial. the suppositionSome critics hold that while microevolution may occur with an existing gene pool, whereas macroevolution requires the introduction of newly-evolved genes. These newly-evolved genes would represent beneficial mutations; creationistsit generallyis argueargued that such mutations are observed to be almost always detrimental.

The studyTheories of macroevolution addressesseek to address such questions such as;

* Why did the major groups of animals suddenly appear in the [[fossil record]] (known as the [[Cambrian]] Explosion)?

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* What process leads to [[speciation]]?

There are two main ways in which "macroevolution" maymight occur. The first way is through the extrapolation of microevolutionary processes. Tiny microevolutions, over sufficient time, add up and accumulate in isolated populations and eventually result in new species. The second way in which "macroevolution" occursis believed to occur is through sudden and rapid changes. This theory, [[punctuated equilibrium]], put forth by [[Stephen Jay Gould]], is based on the fact that there are critical genes (such as the [[homeobox]]) in all living organisms, and a small change in them could cause drastic changes in the organism, resulting in a new species quite rapidly.

Single small mutations are sometimes the main difference between one species and another. Scientists have discovered very important genes, such as the [[homeobox]], which regulate the growth of animals in their embryonic state. Scientists have managed to create new species of fly by irradiating the homeobox gene, causing a radical mutation in the development of the segments of the body. The fly may grow an extra thorax, or grow legs out of its eyestalks, all due to a single base pair alteration!. The additional information needed for these structures did not arise from the mutation, of course, but existed elsewhere in the animal's DNA and was replicated at the novel ___location. It has been proposed that centipedes and millipedes originated from insect precursors, but their homeobox gene mutated and they ended up growing dozens of body segments instead of just one. A very small change, and an entire species is formed.

It must be noted that many mutations are common and unexpressed, particularly when it involves toggling of the third base sequence in a [[codon]]. Most ''deleterious'' mutations are not seen simply because they do not result in viable reproduction.

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Mutations of the homeobox and other critical genes are sometimes called [[macromutations]], which cause the addition of body segments among the [[Arthropoda]]. One major problem lies in the scales of resolution offered by biological techniques. The fossil record cannot record events that happened in less than a million years, which allows it to clearly show slow speciation events that are the result of accumulated mutations over a long time, but records sudden "jumps" in species that are most likely the result of mutations in the critical regulatory genes in only a few generations. Macromutations are probably the best explanation of the [[Cambrian Explosion]] that occurred 550 million years ago.

Some proponents of [[creationism]]critics accept that microevolution occurs in the short term, whereas macroevolution, specifically leading to speciation, is expressly rejected. They claim that known sources of variation can only account for variation within species, and can not account for the variation between larger taxonomic groups, thus making [[macroevolution]] impossible.

Microevolution can easily be demonstrated in the laboratory to the satisfaction of most observers. Whilst speciation events have been demonstrated in the laboratory and observed in the field, really dramatic differences between species do not usually occur in directly observable timescales (it occurs too quickly for the process to be shown in the fossil record.) SomeIt creationists haveis argued that, since macroevolution can not be confirmed by a [[controlled experiment]], it cannot be considered to be part of a scientific theory. However, evolutionists counter that astronomy, geology, archaeology and the other historical sciences, like macroevolution, have to check hypotheses through [[natural experiment]]s. They confirm hypotheses by finding out if they conform or fit with the physical or observational evidence and can make valid predictions. In this way, macroevolution is testable and falsifiable.

ScientistsMost scientists consider large gaps between taxonomic groups to be explainable by ecological/evolutionary factors, such as extinctions, population bottlenecks, and the emergence of unoccupied ecological niches. Macroevolution is simply the result of microevolution over a longer period of time. According to the [[modern synthesis]], no distinction needs to be drawn between different kinds of evolution because all are caused by the same factors.

====Creation versus Evolution====