MacroevolutionMacroevolution is the occurrence of large-scale changes in the characteristics of life -- in effect, the evolution of species and higher taxa. It is distinct from microevolution, which is the sequence of changes that occur within a single population.
OverviewAccording to Charles Darwin and also the modern synthesis, this distinction is relative and purely a matter of scale. The theory further maintains that species are a statistical rather than ontological phenomena. Neo-Darwinian theory maintains that all changes in gene-frequencies, regardless of scale, are explained by the same observable, natural forces. While microevolution has been demonstrated in the laboratory to the satisfaction of most observers, macroevolution has to be inferred from the fossil record, and its precise mechanisms are an active topic of discussion amongst scientists. Some critics hold that while microevolution may occur with an existing gene pool, macroevolution requires the introduction of newly-evolved genes. These newly-evolved genes would represent beneficial mutations; it has been argued that such mutations are observed to be almost always detrimental. The recent discovery of extensive genome wide gene duplication in many organisms, however, allows for the preservation of an existing functional gene copy leaving other gene copies free to accumulate mutations, some of which may be beneficial. Research questionsOutstanding questions in macroevolution include;
Proposed mechanismsThere are two proposed mechanisms for macroevolution. 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" is 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. 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.) It is 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 experiments. 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. Most 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. SpeciationMutations 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. Macroevolution and creationistsSome critics, mostly creationists, 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. In the creationist hypothesis of intelligent design this distinction is absolute and central. Intelligent design advocates argue that microevolution may be explained by constant, observable, natural forces, but that macroevolution must be explained by other forces. See alsoExternal links
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