The notion that macroevolution is distinct from microevolution seems to
have arisen within biology around the turn of the 20th century. There is
no clear definitional borderline between macro- and microevolution, but
most theorists have meant something like the following: macroevolution
is "evolution above the species level" or "evolution of the higher taxa"
(in other words, evolution that leads to the creation of new genera,
families, orders, etc.), whereas microevolution refers to changes due
to natural selection occurring within populations (in other words,
evolution that leads to the creation of new varieties, races,
subspecies, and species). In essence, macroevolution is thought of as
evolution leading to the creation of new organs and anatomical
patterns--the kinds of change that serve to distinguish the higher taxa.
According to Mayr (1982), before the synthesis of genetics with
evolutionary biology during the 1930s:
"Only very few paleontologists were strict Darwinians, accepting natural
selection as the dominant agent in evolution. Most paleontologists
believed in either saltationism or in some form of finalistic
autogenesis. Macroevolutionary processes and causations were generally
considered to be of a special kind, quite different from the
populational phenomena studied by geneticists and students of
speciation." (p. 607)
Thus, most palentologists and comparative anatomists believed that
natural selection was not able to explain macroevolution. They had some
good reasons for concluding this. As Rensch (1954) stated:
"the progressive evolution of many characteristics suggests the
existence of special, not fully understood, phylogenetic agents.
Moreover, it was the regularity of the phylogenetic course in evolution
that led some paleontologists to the assumption of such unknown agents.
One of the regularities of evolution has been claimed to be an explosive
formation of many types at the beginning of a new family or order, then
a slowly decreasing speciation that parallels increasing specialization,
and finally a degeneration before the extinction of species. These facts
were interpreted as due to autonomous factors, creative principles...,
or even as an expression of a 'will to individual and free development'
...." (p. 1)
The evolutionary synthesis (the integration of Mendelian and population
genetics with "Darwinism") of the 1930s and 1940s changed everything.
The work coming out of the evolutionary synthesis was able to explain
macroevolution as the result of the same processes as microevolution:
"The first task of the Darwinian macroevolutionists was to refute the
claim of the anti-Darwinians that there are macroevolutionary phenomena
which are in conflict with the formula 'genetic variation and natural
selection.' This refutation was successfully carried out by [Bernhard]
Rensch [summarized in his _Evolution above the species level_, first
published in 1954] and [George Gaylord] Simpson [in his _Tempo and mode
in evolution_, published in 1944]. Both of them, and also Julian Huxley
[in a wonderful book entitled _Evolution: The modern synthesis_
published in 1942], showed that there is no need to invoke a mysterious
autogenetic factor to explain evolutionary trends but that increases in
the size of the entire body, changes in the proportions of individual
structures (such as teeth), the reduction of certain parts (for example,
toes in horses, eyes in cave animals), and other long-continued
evolutionary regularities can readily be explained by natural
selection." (pp. 608-090
Of course, those who don't wish to accept natural selection acting upon
genetic variants as a good explanation for these phenomena (such as the
Christian writer referred to by Jim Guinee) can point to one apparent
limitation mentioned by Mayr: "this conclusion had to be based on
inference, consisting of morphological, taxonomic, and distributional
evidence, since higher taxa were at that time--and, except for molecular
evidence, are still today--inaccessible to genetic analysis" (p. 607).
This is only an apparent limitation however: with any complex set of
phenomena (as macroevolutionary events surely are), one looks at a large
amount of information and, if there is an explanation that can tie it
all together better than any other--an explanation for which there
already is a good deal of empirical support (as is true with natural
selection and genetics)--one accepts provisionally that explanation. The
inability for us to go back and observe historical events or to create
macroevolutionary events in the laboratory is not a good reason for us
to attribute the cause of those events to some supernatural and (thus)
ultimately unknowable factor, especially when a testable (and tested)
natural explanation is available that can explain these events.
Jeff
References:
Mayr, E. (1982). _The growth of biological thought: Diversity,
evolution, and inheritance_. Cambridge: Belknap.
Rensch, B. (1954). _Evolution above the species level_. New York:
Columbia University Press.
--
Jeffry P. Ricker, Ph.D. Office Phone: (480) 423-6213
9000 E. Chaparral Rd. FAX Number: (480) 423-6298
Psychology Department [EMAIL PROTECTED]
Scottsdale Community College
Scottsdale, AZ 85256-2626
"The truth is rare and never simple."
Oscar Wilde
"No one can accept the fundamental hypotheses of scientific psychology
and be in the least mystical."
Knight Dunlap
