Macroevolution

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Macroevolution is evolution on a scale of separated gene pools.^[1] Macroevolutionary studies focus on change that occurs at or above the level of species, in contrast with microevolution,^[2] which refers to smaller evolutionary changes (typically described as changes in allele frequencies) within a species or population.^[3] Contrary to claims by creationists, macro and microevolution describe fundamentally identical processes on different time scales.^[1][4]

The process of speciation may fall within the purview of either, depending on the forces thought to drive it. Paleontology, evolutionary developmental biology, comparative genomics and genomic phylostratigraphy contribute most of the evidence for the patterns and processes that can be classified as macroevolution. An example of macroevolution is the appearance of feathers during the evolution of birds from theropod dinosaurs.

The evolutionary course of Equidae (wide family including all horses and related animals) is often viewed as a typical example of macroevolution. The earliest known genus, Hyracotherium (now reclassified as a palaeothere), was a herbivore animal resembling a dog that lived in the early Cenozoic. As its habitat transformed into an open arid grassland, selective pressure required that the animal become a fast grazer. Thus elongation of legs and head as well as reduction of toes gradually occurred, producing the only extant genus of Equidae, Equus.^[5]

Origin of the term

Russian entomologist Yuri Filipchenko first coined the terms "macroevolution" and "microevolution" in 1927 in his German language work, "Variabilität und Variation". Since the inception of the two terms, their meanings have been revised several times and the term macroevolution fell into limited disfavour when it was taken over by such writers as the geneticist Richard Goldschmidt (1940) and the paleontologist Otto Schindewolf to describe their orthogenetic theories.^[6]

A more practical definition of the term describes it as changes occurring on geological time scales, in contrast to microevolution, which occurs on the timescale of human lifetimes.^[7] This definition reflects the spectrum between micro- and macro-evolution, whilst leaving a clear difference between the terms: because the geological record rarely has a resolution better than 10,000 years, and humans rarely live longer than 100 years, "meso-evolution" is never observed.^[7]

As a result, apart from Dobzhansky, Bernhard Rensch and Ernst Mayr, very few neo-Darwinian writers used the term, preferring instead to talk of evolution as changes in allele frequencies without mention of the level of the changes (above species level or below). Those who did were generally working within the continental European traditions (as Dobzhansky, Ernst Mayr, Bernhard Rensch, Richard Goldschmidt, and Otto Schindewolf were) and those who didn't were generally working within the Anglo-American tradition (such as John Maynard Smith and Richard Dawkins). Hence, use of the term "macroevolution" is sometimes wrongly used as a litmus test of whether the writer is "properly" neo-Darwinian or not.^[6]

Macroevolution and the modern evolutionary synthesis

Within the Modern Synthesis school of thought, macroevolution is thought of as the compounded effects of microevolution.^[8] Thus, the distinction between micro- and macroevolution is not a fundamental one – the only difference between them is of time and scale. As Ernst W. Mayr observes, "transspecific evolution is nothing but an extrapolation and magnification of the events that take place within populations and species...it is misleading to make a distinction between the causes of micro- and macroevolution”.^[8] However, time is not a necessary distinguishing factor – macroevolution can happen without gradual compounding of small changes; whole-genome duplication can result in speciation occurring over a single generation - this is especially common in plants.^[9]

Changes in the genes regulating development have also been proposed as being important in producing speciation through large and relatively sudden changes in animals' morphology.^[10][11]

Research topics

Some examples of subjects whose study falls within the realm of macroevolution:

• Adaptive radiations such as The Cambrian Explosion.
• Changes in biodiversity through time.
• Genome evolution, like horizontal gene transfer, genome fusions in endosymbioses, and adaptive changes in genome size.
• Mass extinctions.
• Speciation and extinction rates.
• The debate between punctuated equilibrium and gradualism.
• The role of development in shaping evolution, particularly such topics as heterochrony and phenotypic plasticity.
• At present-day there is developing an interdisciplinary approach which aims at considering macroevolution at a transdisciplinary scale, and concentrates on the comparison between biological and social macroevolution as this gives new significant possibilities to understand peculiarities of each of the two types of macroevolution.^[12]

Abrupt transformations from one biologic system to another, for example the passing of life from water into land or the transition from invertebrates to vertebrates, are rare. Few major biological types have emerged during the evolutionary history of life and most of them survive till today. When lifeforms take such giant leaps, they meet little to no competition and are able to exploit a plethora of available niches, following a pattern of adaptive radiation. This can lead to convergent evolution, where unrelated populations display similar adaptations.^[5]

Recently, Persi & Horn^[13] have argued that macroevolutionary events along the history of an extant species may leave a stamp on its proteome: the existence of compositionally ordered (CO) sections of proteins. Introducing a universal measure that counts an effective CO vocabulary they have shown that it distinguishes between major eukaryotic clades.

Misuse

The term "macroevolution" frequently arises within the context of the evolution/creation debate, usually used by creationists alleging a significant difference between the evolutionary changes observed in field and laboratory studies and the larger scale macroevolutionary changes that scientists believe to have taken thousands or millions of years to occur. They accept that evolutionary change is possible within what they call "kinds" ("microevolution"), but deny that one "kind" can evolve into another ("macroevolution").^[14] Contrary to this belief among the anti-evolution movement proponents, evolution of life forms beyond the species level (i.e. speciation) has indeed been observed multiple times under both controlled laboratory conditions and in nature.^[15] In creation science, creationists accepted speciation as occurring within a "created kind" or "baramin", but objected to what they called "third level-macroevolution" of a new genus or higher rank in taxonomy. Generally, there is ambiguity as to where they draw a line on "species", "created kinds", etc. and what events and lineages fall within the rubric of microevolution or macroevolution.^[16] The claim that macroevolution does not occur, or is impossible, is not supported by the scientific community.

Such claims are rejected by the scientific community on the basis of ample evidence that macroevolution is an active process both presently and in the past.^[6][17] The terms macroevolution and microevolution relate to the same processes operating at different scales, but creationist claims misuse the terms in a vaguely defined way which does not accurately reflect scientific usage, acknowledging well observed evolution as "microevolution" and denying that "macroevolution" takes place.^[6][18] Evolutionary theory (including macroevolutionary change) remains the dominant scientific paradigm for explaining the origins of Earth's biodiversity. Its occurrence is not disputed within the scientific community.^[19] While details of macroevolution are continuously studied by the scientific community, the overall theory behind macroevolution (i.e. common descent) has been overwhelmingly consistent with empirical data. Predictions of empirical data from the theory of common descent have been so consistent that biologists often refer to it as the "fact of evolution".^[20][21]

Describing the fundamental similarity between Macro and Microevolution in his authoritative textbook "Evolutionary Biology," biologist Douglas Futuyma writes,

Nicholas Matzke and Paul R. Gross have accused creationists of using "strategically elastic" definitions of micro- and macroevolution when discussing the topic.^[1] The actual definition of macroevolution accepted by the vast majority of^[22] scientists is "any change at the species level or above" (phyla, group, etc.) and microevolution is "any change below the level of species." Matzke and Gross state that many creationist critics define macroevolution as something that cannot be attained, as these critics dismiss any observed evolutionary change as "just microevolution".^[1]

See also

• List of transitional fossils
• Transitional fossil
• Microevolution
• Speciation

References

External links

• Introduction to macroevolution
• Macroevolution as the common descent of all life
• Macroevolution in the 21st century Macroevolution as an independent discipline.
• Macroevolution FAQ

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