Of particular importance to understanding
evolution is differentiating fact from theory.
Observations - facts - are explained by theories. Evolutionary
theory explains the fact of evolution. To illustrate this point
consider Isaac Newton and gravity. In Newton’s time, it
was already well-established that objects will fall to the ground
unless acted upon by some other force. Throw an object into the
air and it will not continue on that trajectory indefinitely;
it will eventually fall back to the earth. We can call this the
fact of gravity. Newton wanted to know why; what made objects
fall? He theorized
that all objects with mass possess an attractive force, a force
whose strength is dependent on the mass of the object and its
distance from other objects[#](Giancoli 2001).
We can call this the theory of gravity[#](Newton 1687),
an explanation of why objects fall to earth (and what
keeps planets in their orbits).
For our purposes, we must distinguish the fact of evolution from
the theories proposed to explain that fact[#](Gould 1981). We know that evolution
is a real phenomenon - as
real as lightning or gravity or atoms. We know this because we’ve
observed it. We’ve observed it in field studies of birds[#](Grant 1999)(Barrowclough 1992),
mice[#](Grant 1999)(Stanley 1979),
and lizards[#](Losos, et al. 1997).
We’ve observed it in field and laboratory studies of fruit flies and other insects[#](Dobzhansky & Pavlovsky 1971)(Barker & Starmer 1982)(Turner & Hahn 2007)(Kettlewell 1973)(Majerus 1998).
We've seen it in flowering and non-flowering plants[#](Mosquin 1967)(Rabe & Haufler 1992). And in strains of bacteria
that are no longer harmed by even the strongest of antibiotics[#](Palumbi 2001).
Life can and does change - evolve - over time.
Much of this change results simply in adaptationChange in a species resulting from natural selection; a structure
which is the result of such selection.
by a population
to its environment. But evolution also creates
new species.
After extensive (and unrelated) trips collecting fossils and
specimens, Charles Darwin and Alfred Russel Wallace were both
impressed by the large amount of variation among individuals of
the same species, by the similarities among species, and by the
sheer number of species. Why were there so many species
and how did they come to be? How does evolution lead
to the formation of new species? This is where theory comes in.
Darwin and Wallace each independently concluded that evolution
was the answer and proposed essentially the same explanation (theory):
not only did populations evolve and become better adapted to their
environment (as was known and accepted at the time), but new
species evolved from other species[#](Darwin & Wallace 1858).
They asserted that evolution
occurs by a process Darwin called natural
selection, as adaptations
to different habitats accumulate in each population. We now know
that natural selection is not
the only cause of evolution. Modern
evolutionary theory includes these other explanations as well.
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