What are the Facts and Theories of Evolution?
As I’ve been writing this blog, you’ve probably been noticing some common threads weaving in and out of various posts: the relationships between animal groups, the nature of evolution, and the modern evidence of evolution.
To start with, in any discussion of evolution, you need to define a theory. In popular usage, a theory is basically a “best guess” as to why something happened; in science, however, a theory is a well supported supposition, explaining observed facts.
The next definition you need is for evolution. In modern biology, evolution is simply the name given to the fact that species change over time, and that similar species are likely to be closely related genetically.
These two definitions make it clear that there is no one “theory of evolution.” Evolution is an observed fact; there are theories designed to explain the fact, and there is a great deal of debate among biologists about those theories.
So what are some of the theories which have been put out there to explain evolution? Well, I am not a biologist, but I do like the field and try to read up on it, so here is my take on some theories of evolution:
1) The most venerable, of course, is Darwin’s theory of natural selection. Darwin’s model describes how, when individual organisms act in their own best interest (defined as reproductive success; leaving the most surviving offspring), they will tend to pass favorable features on to succeeding generations. As a result, populations will change over time.
2) Theories of genetic contraints on development. These get complex. Basically, in a strictly Darwinian natural selection, organisms vary randomly, and the fortuitously best adapted will survive and reproduce. However, organisms are not truly random with respect to variation. As complex systems in their own right, organisms are constrained in how they can change by their genetic code and body structure. These constraints can guide the paths of evolution.
3) Punctuated Equilibrium is a theory about the pattern of evolution. It takes the fossil record literally, and predicts that gradual change is not to be expected. Rather, that species are in stasis for most of their history, and have bursts of evolutionary change at the beginning and end of stasis periods. These bursts are incredibly long by human standards, 10,000 to 100,000+ years, but minute specks of time compared to the 500 million year history of animal life on Earth.
As you can see, one fact, evolution, can spur people to develop numerous theories. These burgeoning theories do not mean that the fact is wrong; only that it is complex, with far reaching implications. A fact like evolution, which predicts that all life arose from a single common ancestory, meets that definition.
