Those who still dispute the existence of biological evolution sometimes use the argument that it is "just a theory", implying that even scientists don't agree that evolution occurs. Those employing this argument do not have a clear understanding both of how scientific investigation and evolution work. The term evolution merely means change, and biological change is observable not only in the geologic record, but also in the world around us. The aspect of evolution that scientist still investigate and debate is not whether or not this biological change actually occurs, but rather, the specific mode of evolution, what is driving the change.
* Is Evolution "Just" a Theory? *
Non-biologists discussing the theory of evolution typically confuse two separate aspects of evolution:
1. Does evolution occur?
2. How does evolution occur (i.e. what is the mechanism)?
~ Does Evolution Occur?: The first is a question of whether modern organisms evolved from ancestral organisms and are continuing to change over time. Opponents of evolutionary theory often state that scientists don't agree that evolution even occurs. Biologists consider the existence of evolution to be a fact. It can be demonstrated today, and the historical evidence for its occurrence in the past is overwhelming, and is informed by the geologic record, fossils and nucleic acid (DNA and RNA) genetic sequencing.
~ How Does Evolution Occur?: Beyond the fact that organisms do change over time, there is the important question of how this change happens; the specific mechanism. Although natural selection is considered to be the primary driver of biological change, scientists are still hammering out the specifics. Disagreement and exploration of possibilities is what science is all about; a cycle of debate and discovery. Saying that a scientific theory is invalid, due to the fact that all scientist don't agree, is nonsense.
* Evolution as Fact & Theory *
The late Stephen J. Gould, one of the great thinkers in evolution theory, explained the confusion as such: "Facts and theories are different things. Facts are the world's data. Theories explain and interpret facts". Darwin continually emphasized the difference between his two great, and separate, accomplishments: establishing the fact of evolution, and proposing a theory-natural selection-to explain how evolution occurs.
* How Does Evolution Work? *
Evolution can be defined as inheritable change in a population that ultimately results from the interaction of individuals with their environment, and this action takes place over very large stretches of time. Natural selection of the individuals best adapted to their environment addresses how new species emerge, how those less "fit" individuals are eliminated, and ultimately how the world has become populated with a vast array of very different types of organisms. Today's species are related to each other through common decent (ancestors that they share) and are products of evolution over billions of years.
Key concepts of evolution:
* Any change must be inheritable (able to be passed on to the next generation).
* The characteristics that persist, and those that disappear, are regulated by natural selection.
* Natural Selection *
Natural selection is the primary theorized mechanism of evolution. It is the process in nature by which the organisms that are best adapted to their environment tend to survive and transmit their genes to the next generation in higher numbers than do the less fit individuals. The idea of evolution by natural selection can be difficult to comprehend, due to the huge expanse of time required for most species to show large changes in the characteristics of their population. Here is an example of how environmental pressure can change a population that reproduces very quickly; the antibiotic resistance of bacteria.
* Selection and Antibiotic Resistance *
As is true for any group of organisms that belong to the same species, a population of bacteria contains individuals that are a little genetically different from each other. When those bacteria are exposed to the right antibiotic, most of the population will die. This is what happens when you take antibiotics for illness. A few of the bacteria, however, have genetic differences from the rest of the population that protect them from being killed by the antibiotic. These surviving bacteria can sometimes repopulate, generating a new population of bacteria that is more difficult to kill with that same antibiotic.
In conclusion, the scientific discussion that surrounds evolutionary theory, at least among scientist, has nothing to do with the IF, and everything to do with HOW of evolution.
* Sources and Resources *
Public Broadcasting Service (PBS) Evolution Website
Moran, L. (1993) Evolution is a Fact and a Theory.
Gould, S. J. (1981) Evolution as Fact and Theory. Discover, May issue.Campbell, N.A. (1990) Biology 2nd ed., Benjamin Cummings, p. 434.