Macroevolution
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Macroevolution is the concept that evolution of species and higher taxa is the result of large-scale changes in gene-frequencies over time.
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Overview
There are a number of views regarding macroevolution. Some evolutionary biologists, particularly Charles Darwin and those ascribing to the modern synthesis, see the only difference between microevolution and macroevolution as being one of scale. Other evolutionary biologists, including Gould, Schmalhausen, and Waddington, hold that microevolution and macroevolution are fundamentally different processes. Essentially the question is - how important are sudden developmental "spurts" to the overall process of evolution? Are they the driving force, or is gradual change the more important process?
Macroevolution is controversial outside the scientific community, and aspects of it are disputed by many movements such as creationism, intelligent design, and panspermia. Generally speaking, these groups differentiate between microevolution and macroevolution, asserting that the former is an observable phenomena, but that the latter is not. They have proposed a number of limits beyond which they believe evolution cannot occur. Proponents of Intelligent design and panspermia argue that the mechanisms of evolution are incapable of giving rise to instances of specified complexity and irreducible complexity. Proponents of creation biology assert that life was originally created in a finite number of discrete created kinds beyond which and between which no evolution can occur, and that the dominant source of biological change is population isolation and genetic drift, or the loss of the diversity of the original kinds, rather than an increase of genetic diversity through mutation.
Research questions
Outstanding questions in macroevolution include;
- What caused the major groups of animals suddenly appear in the fossil record (known as the Cambrian Explosion)? This is likely because earlier such species were soft-bodied and left no fossils when they died. Therefore when organisms developed more robust shells, we would see a large increased in preserved fossils, even though the actual number of creatures had not increased.
- Does evolution occur in spurts, with many species undergoing long periods of stasis with little evolutionary change, or do gaps in the fossil record simply give that impression (punctuated equilibrium)?
- What processes lead to speciation and maintain the barriers between species?
Proposed mechanisms
There 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.
Speciation
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.
History of macroevolution
The debate over the relationship between macroevolution and microevolution has been going on since the 1860s, when evolution first became a widely accepted idea following the publication of Charles Darwin's The Origin of Species.
The first theory of macroevolution, Lamarckism, developed by biologist Jean-Baptiste Pierre Antoine de Monet, Chevalier de Lamarck, asserted that individuals develop traits they use and lose traits they do not use, and that individuals pass the traits the acquired traits onto their offspring. Lamarck asserted that when environmental changes changed the "needs" of a species, which caused it to develop different traits, leading to the transmutation of species.
Gregor Mendel, an Austrian monk, popularly known as the "father of modern genetics" for his discovery of the laws of genetic variation in his study of natural variation in plants, believed that the laws of inheritance provided no grounds for macroevolution. In a lecture on March 8, 1865, Mendel noted that his research described the mechanism of microevolution, but gave no grounds for belief in macroevolution, saying "No one will seriously maintain that in the open country the development of plants is ruled by other laws than in the garden bed. Here, as there, changes of type must take place if the conditions of life be altered, and the species possesses the capacity of fitting itself to its new environment. [However,] nothing justifies the assumption that the tendency to form varieties increases so extraordinarily that the species speedily lose all stability, and their offspring diverge into an endless series of extremely variable forms." To the contrary, he said, the tendency is toward stability, with variation being the exception, not the rule. (Henig, 141)
Darwin, on the other hand, saw no fundamental difference between microevolution and macroevolution. He asserted that "Certainly no clear line of demarcation has as yet been drawn between species and sub-species—that is, the forms which in the opinion of some naturalists come very near to, but do not quite arrive at, the rank of species: or, again, between subspecies and well-marked varieties, or between lesser varieties and individual differences. These differences blend into each other by an insensible series; and a series impresses the mind with the idea of an actual passage." (Darwin, 77)
Although Mendel's laws of inheritance were published as early as 1866, his theory was generally overlooked until the early twentieth century, in part because it was published in an obscure journal and by someone from outside the mainstream scientific community. Darwin himself never read of Mendel's work, and his own proposed mechanism for inherited traits, pangenesis, was more useful for statisticians of the biometric school than it was for biologists. Darwin had discovered a variation ratio of 2.4:1 in a study of snapdragons which he published in 1868, similar to the 3:1 ratio that led Mendel to discover the laws of genetic variation. However, Darwin was not sure of its ultimate meaning. (Henig, 143) After the rediscovery of Mendel's laws in 1900, the statisticians and biologists argued against each other until they were reconciled by the work of R.A. Fisher in the 1930s.
In the late 1930s, evolutionary biologist Theodosius Dobzhansky devised the Modern evolutionary synthesis. In bringing macroevolution and microevolution to the English language, wrote "we are compelled at the present level of knowledge reluctantly to put a sign of equality between the mechanisms of macro- and microevolution" (Dobzhansky, 12). Some have argued that he was reluctant to equate macro- and microevolution was that it went against the beliefs of his mentor, Filipchenko, who was an orthogenetist, and of the opinion that micro- and macroevolution were of a different mechanism and calibre. (Burian, 1994). From the writings of Dobzhansky, the modern synthesis view of evolution grew to its present prominence.
With the discovery of the structure of DNA and genes, genetic mutation gained acceptance as the mechanism of variance in the 1960s, in what is called the Williams revolution. This developing theory of evolution was then called the modern evolutionary synthesis, which remains prominent today. The synthetic model of evolution equated microevolution and macroevolution, asserting that the only difference between them was one of time and scale.
A few non-Darwinian evolutionists remained, however, including Schmalhausen and Waddington, who argued that the processes of macroevolution are different from those of microevolution. According to these scientists, macroevolution occurs, but is restricted by such proposed mechanisms as developmental constraints. The concept can be summarized in: "Schmalhausen's Law," which holds that "When organisms are living within their normal range of environment, perturbations in the conditions of life and most genetic differences between individuals have little or no effect on their manifest physiology and development, but that under severe and unusual general stress conditions even small environmental and genetic differences have major effects." Non-Darwinian evolution points to evidence of great changes in population under conditions of stress; however, it is generally rejected by the scientific community because it provides no mechanism for larger changes at a genetic level under those circumstances. For a discussion of Schmalhausen's theory of "canalization," see this article (http://www.med.harvard.edu/chge/course/infectious/biodiverse/cns.pdf).
In the late 1970's, Stephen J. Gould challenged the synthesis model of evolution, and proposed a punctuated equilibrium model, whereby major evolutionary changes took place in limited gene pools after radical climate changes. He said, "I well remember how the synthetic theory [of evolution] beguiled me with its unifying power when I was a graduate student in the mid-1960's. Since then I have been watching it slowly unravel as a universal description of evolution.....I have been reluctant to admit it—since beguiling is often forever—but if Mayr's characterization of the synthetic theory is accurate, then that theory, as a general proposition, is effectively dead, despite its persistence as textbook orthodoxy." (Paleobiology, Vol.6, 1980, p. 120).
Despite his rejection of the synthetic theory, however, he asserted that "Evolutionary theory is now enjoying this uncommon vigor. Yet amidst all this turmoil no biologist has been led to doubt the fact that evolution occurred; we are debating how it happened. We are all trying to explain the same thing: the tree of evolutionary descent linking all organisms by ties of genealogy. Creationists pervert and caricature this debate by conveniently neglecting the common conviction that underlies it, and by falsely suggesting that evolutionists now doubt the very phenomenon we are struggling to understand."
Today, the synthetic model of evolution and punctuated equilibrium are alternative models for the theory of evolution.
References
- Darwin, Origin of Species, New York: Modern Library, 1998.
- Dobzhansky, Th: 1937. Genetics and the Origin of Species, Columbia University Press
- Henig, The Monk in the Garden: The Lost and Found Genius of Gregor Mendel, the Father of Genetics, Boston: Houghton Mifflin Company, 2000.
See also
External links
- Macroevolution as an independent discipline: Macroevolution in the 21st Century (http://www.nhm.ac.uk/hosted_sites/paleonet/paleo21/mevolution.html)
- Macroevolution as the common descent of all life: "29+ Evidences for Macroevolution" (http://www.talkorigins.org/faqs/comdesc/)
- "A Critique of 29+ Evidences for Macroevolution" (http://www.trueorigin.org/theobald1a.asp)
- A reply to the critique of "29+ Evidences" (http://www.talkorigins.org/faqs/comdesc/camp.html)
| Basic topics in evolutionary biology |
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| Processes of evolution: macroevolution - microevolution - speciation |
| Mechanisms: selection - genetic drift - gene flow - mutation |
| Modes: anagenesis - catagenesis - cladogenesis |
| History: Charles Darwin - The Origin of Species - modern evolutionary synthesis |
| Subfields: population genetics - ecological genetics - human evolution - molecular evolution - phylogenetics - systematics - evo-devo |
| List of evolutionary biology topics | Timeline of evolution |