Evolution: Hypothesis and Fact
Jeff Mitton
Natural Selections (appeared in the Daily Camera January 1, 2005)
Gravity is a fact. Evolution is a fact. So why do we have theories of gravitation,
and theories of evolution?
Physicists refer to five forces of gravity, and they discuss intricacies that are far
beyond my comprehension. Because they do not have consensus on all of the minute
details of gravitational forces, they call the set of alternate hypotheses the theory of
gravity.
Evolutionary biologists disagree over many aspects of evolution. Some biologists
assert that natural selection drives the majority of evolutionary changes, while others
suspect that new mutations and random changes are more important. Some biologists
believe that speciation takes about a million years, while others assert that species can
arise in less than a century. Some biologists suggest that speciation depends on changes
at thousands of genes, while others contend that changes at just a few genes can drive
speciation.
Evolution is defined as substantial genetic change in populations. Thousands of
examples of changes in size, color, morphology, physiology, and behavior have been
reported in the scientific literature. These are sometimes referred to as examples of
microevolution. For example, in the three decades that the insecticide DDT was used to
control insects, over 400 species of insects evolved resistance to it. When a person
becomes infected with AIDS, the infection usually starts with a single type of virus, but
within two years the virus evolves into numerous forms. Within the last three decades,
strains of tuberculosis, syphilis, and gonorrhea have evolved resistance to the antibiotics
that once controlled them. The companies that produce pesticides, insecticides, and
antibiotics are constantly developing new products, for their target organisms are
constantly evolving resistance.
Populations of the same species can accumulate sufficient differences so that, if
given the opportunity to mate, behavioral or developmental differences prevent them
from having healthy offspring. This evolution of reproductive isolation is called
speciation. Some critics acknowledge micro evolutionary change, but deny speciation.
This is like someone saying they believe in inches and feet, but not miles.
Some specific hypotheses about speciation have been rigorously tested, and the
results were consistent with the hypotheses. Evolutionary biologists studying sunflowers
in Texas (Helianthus annulus tetanus) and live-bearing fishes in the mountains of Mexico
(Poeciliopsis monacha-lucida) hypothesized that these species arose quickly, by
hybridization. They were able to reconstruct these species in the laboratory by crossing
the alleged parental species. Thus, one mode of speciation has been successfully
replicated in the laboratory. Other modes of speciation are slower, so they cannot be
replicated in the laboratory, or within a biologist’s career.
Our own history generates debate. Analyses of mitochondrial DNA sequences
suggest that all humans can trace their lineages back to a population, or metaphorically,
to Mitochondrial Eve, in Southern Africa, 200,000 years ago. But anthropologists
studying fossils insist that modern humans began to emerge in many places, on several
continents, closer to a million years ago. These hypotheses are referred to as
Mitochondrial Eve and Multiregionalism, respectively. Clearly, this debate is about
where and when modern humans evolved, not whether they evolved. It would be a
mistake to present the debate as proof that humans did not evolve.
Ongoing evolutionary changes in insects, plants, and microbes have major
economic impacts on agriculture and public health. Modern technological societies have
an obligation to prepare students and train scientists and doctors to face these challenges.
Evolutionary biologists have spirited debates about evolutionary mechanisms,
evolutionary rates, and which modes of speciation are more common. That is, they debate
the theory of evolution. But no credible biologist denies the fact of evolution.