What is macroevolution?
n science, macro at the beginning of a word just means "big", and micro at the beginning of a word just means "small" (both from the Greek words). For example, a macrophage means a bigger than normal cell, but it is only a few times bigger than other cells, and not an order of magnitude bigger.
In evolutionary biology today, macroevolution is used to refer to any evolutionary change at or above the level of species. It means the splitting of a species into two (speciation, or cladogenesis, from the Greek meaning "the origin of a branch") or the change of a species over time into another (anagenesis, not nowadays generally used). Any changes that occur at higher levels, such as the evolution of new families, phyla or genera, is also therefore macroevolution, but the term is not restricted to the origin of those higher taxa.
Microevolution refers to any evolutionary change below the level of species, and refers to changes in the frequency within a population or a species of its alleles (alternative genes) and their effects on the form, or phenotype, of organisms that make up that population or species.
Another way to state the difference is that macroevolution is between-species evolution of genes and microevolution is within-species evolution of genes.
There are various kinds of dynamics of macroevolution. Punctuated equilibrium theory proposes that once species have originated, and adapted to the new ecological niches in which they find themselves, they tend to stay pretty much as they are for the rest of their existence. Phyletic gradualism suggests that species continue to adapt to new challenges over the course of their history. Species selection and species sorting theories claim that there are macroevolutionary processes going on that make it more or less likely that certain species will exist for very long before becoming extinct, in a kind of parallel to what happens to genes in microevolution.
The history of the concept of macroevolution
In the "modern synthesis" of neo-Darwinism, which developed in the period from 1930 to 1950 with the reconciliation of evolution by natural selection and modern genetics, macroevolution is thought to be the combined effects of microevolutionary processes. In theories proposing "orthogenetic evolution" (literally, straight line evolution), macroevolution is thought to be of a different calibre and process than microevolution. Nobody has been able to make a good case for orthogenesis since the 1950s, especially since the uncovering of molecular genetics between 1952 and the late 1960s.
Antievolutionists argue that there has been no proof of macroevolutionary processes. However, synthesists claim that the same processes that cause within-species changes of the frequencies of alleles can be extrapolated to between species changes, so this argument fails unless some mechanism for preventing microevolution causing macroevolution is discovered. Since every step of the process has been demonstrated in genetics and the rest of biology, the argument against macroevolution fails.
Non-Darwinian evolutionists think that the processes that cause speciation are of a different kind to those that occur within species. That is, they admit that macroevolution occurs, but think that normal genetic change is restricted by such proposed mechanisms as developmental constraints. This view is associated with the names of Schmalhausen and Waddington, who were often characterised as being non-Darwinians by the modern synthesis theorists.
The terms macroevolution and microevolution were first coined in 1927 by the Russian entomologist Iurii Filipchenko (or Philipchenko, depending on the transliteration), in his German-language work Variabilität und Variation, which was the first attempt to reconcile Mendelian genetics and evolution. Filipchenko was an evolutionist, but as he wrote during the period when Mendelism seemed to have made Darwinism redundant, the so-called "eclipse of Darwinism" (Bowler 1983), he was not a Darwinian, but an orthogeneticist. Moreover Russian biologists of the period had a history of rejecting Darwin's Malthusian mechanism of evolution by competition.
In Theodosius Dobzhansky's Genetics and the Origin of Species, he began by saying that "we are compelled at the present level of knowledge reluctantly to put a sign of equality between the mechanisms of macro- and microevolution" (1937, page 12), thereby introducing the terms into the English-speaking biological community (Alexandrov 1994). Dobzhansky had been Filipchenko's student and regarded him as his mentor. In science, it is difficult to deny a major tenet of one's teachers due to filial loyalty, and Dobzhansky, who effectively started the modern Darwinian synthesis with this book, found it disagreeable to have to deny his teacher's views (Burian 1994).
The term 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. As a result, apart from Dobzhansky, Bernhardt 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 do are generally working within the continental European traditions (as Dobzhansky, Mayr, Rensch, Goldschmidt, and Schindewolf are) and those who don't are generally working within the Anglo-American tradition (such as John Maynard Smith and Richard Dawkins). Hence, the term is sometimes wrongly used as a litmus test of whether the writer is "properly" neo-Darwinian or not (Eldredge 1995: 126-127).
The term has been revived by a number of authors such as Stephen Jay Gould and Niles Eldredge, the authors of punctuated equilibrium theory (see Eldredge's 1992 Macroevolutionary Dynamics ), but there is a tendency in these authors to revert to the orthogenetic view that something other than within-species processes are causing macroevolution, although they disavow the orthogenetic view that evolution is progressing anywhere.
Conclusion
There is no difference between micro- and macroevolution except that genes between species usually diverge, while genes within species usually combine. The same processes that cause within-species evolution are responsible for above-species evolution, except that the processes that cause speciation include things that cannot happen to lesser groups, such as the evolution of different sexual apparatus (because, by definition, once organisms cannot interbreed, they are different species).
The idea that the origin of higher taxa, such as genera (canines versus felines, for example), requires something special is based on the misunderstanding of the way in which new phyla (lineages) arise. The two species that are the origin of canines and felines probably differed very little from their common ancestral species and each other. But once they were reproductively isolated from each other, they evolved more and more differences that they shared but the other lineages didn't. This is true of all lineages back to the first eukaryotic (nuclear) cell. Even the changes in the Cambrian explosion are of this kind, although some (eg, Gould 1989) think that the genomes (gene structures) of these early animals were not as tightly regulated as modern animals, and therefore had more freedom to change.
References
Alexandrov, DA: 1994. Filipchenko and Dobzhansky: Issues in Evolutionary Genetics in the 1920s. In The Evolution of Theodosius Dobzhansky, ed. MB Adams, Princeton University Press.
Bowler, PJ: 1983. The Eclipse of Darwinism, Johns Hopkins University Press
Burian, RM: 1994. Dobzhansky on Evolutionary Dynamics: Some Questions about His Russian Background. In The Evolution of Theodosius Dobzhansky, ed. MB Adams, Princeton University Press.
Dobzhansky, Th: 1937. Genetics and the Origin of Species, Columbia University Press
Eldredge, N: 1992. Macroevolutionary Dynamics: Species, Niches and Adaptive Peaks, McGraw-Hill
Eldredge, N: 1995. Reinventing Darwin: The Great Evolutionary Debate, Weidenfeld and Nicholson
Goldschmidt, R: 1940. The Material Basis of Evolution, Yale University Press
Gould, SJ: 1989. Wonderful Life: The Burgess Shale and the Nature of History, Norton
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