Evolusi

What Is Evolution?

Most non-scientists seem to be quite confused about precise definitions of biological evolution. Part of the confusion is because the word "evolution" has many different meanings, depending on the context. When we talk about biology we are thinking about biological evolution and that's the term I want to define here. What do biologists mean when they refer to biological evolution?

One of the most respected evolutionary biologists has recently defined biological evolution as follows:

Biological (or organic) evolution is change in the properties of populations of organisms or groups of such populations, over the course of generations. The development, or ontogeny, of an individual organism is not considered evolution: individual organisms do not evolve. The changes in populations that are considered evolutionary are those that are ‘heritable' via the genetic material from one generation to the next. Biological evolution may be slight or substantial; it embraces everything from slight changes in the proportions of different forms of a gene within a population, such as the alleles that determine the different human blood types, to the alterations that led from the earliest organisms to dinosaurs, bees, snapdragons, and humans.

Douglas J. Futuyma (1998) Evolutionary Biology 3rd ed., Sinauer Associates Inc. Sunderland MA p.4

Note that biological evolution refers to populations and not to individuals. In other words, populations evolve but individuals do not. This is a very important point. It distinguishes biological evolution from other forms of evolution in science (e.g., stellar evolution). Another important point is that the changes must be genetic, or heritable—they must be passed on to the next generation. Evolution is the process by which this occurs and it is spread out over many generations. Thus, the short minimal definition of biological evolution is,

Evolution is a process that results in heritable changes in a population spread over many generations.

This is a good working scientific definition of evolution; one that can be used to distinguish between evolution and similar changes that are not evolution. Another common short definition of evolution can be found in many textbooks:

In fact, evolution can be precisely defined as any change in the frequency of alleles within a gene pool from one generation to the next.

Helena Curtis and N. Sue Barnes, Biology, 5th ed. 1989 Worth Publishers, p.974

One can quibble about the accuracy of such a definition, but it also conveys the essence of what evolution really is. When biologists say they have observed evolution, they mean that they have detected a change in the frequency of genetic variants (alleles) in a population. (Often the genetic change is inferred from phenotypic changes.) When biologists say that humans and chimps have evolved from a common ancestor, they mean there have been successive heritable changes in the two separated populations since they became isolated.

Unfortunately, outside of the scientific community, the common definitions of evolution are quite different. For example, in the Oxford Concise Science Dictionary we find the following definition:

evolution: The gradual process by which the present diversity of plant and animal life arose from the earliest and most primitive organisms, which is believed to have been continuing for the past 3000 million years

This is inexcusable for a dictionary that's supposed to be a dictionary of science. Not only does this definition exclude prokaryotes, protozoa, and fungi, but it specifically includes a term "gradual process" that should not be part of the definition. More importantly the definition seems to refer more to the history of evolution than to evolution itself. Using this definition it is possible to debate whether evolution is still occurring, but the definition provides no easy way of distinguishing evolution from other processes. For example, is the increase in height among Europeans over the past several hundred years an example of evolution? Are the color changes in peppered moth populations examples of evolution? The definition of evolution in the Oxford Concise Science Dictionary is not a proper scientific definition of evolution.

Standard dictionaries are even worse.

evolution: ...the doctrine according to which higher forms of life have gradually arisen out of lower.. (Chambers)

evolution: ...the development of a species, organism, or organ from its original or primitive state to its present or specialized state; phylogeny or ontogeny (Webster's)

These definitions are simply wrong. The problem is that it's common for non-scientists to enter into a discussion about evolution with such a definition in mind. This often leads to fruitless debate since the experts are thinking about evolution from a different perspective. When someone claims they don't believe in evolution they cannot be referring to an acceptable scientific definition of biological evolution because that would be denying something that is easy to prove. It would be like saying they don't believe in gravity!

Anti-evolutionists often claim scientists are being dishonest when they talk about evolution. The anti-evolutionists believe that evolution is being misrepresented to the public. The real problem is that the public in general, and anti-evolutionists in particular, do not understand what evolution is all about. Their definition of evolution is very different from the common scientific definition and, as a consequence, they are unable to understand what evolutionary biology really means. Scientist are not trying to confuse the general public by using a rigorous definition of evolution. Quite the contrary, saying that evolution is simply "a process that results in heritable changes in a population spread over many generations" is a way of simplifying discussions about evolution.

Note that I have described the minimal scientific definition of biological evolution. Nobody believes that this is all there is to evolution. There are other processes, such as speciation for example, that are clearly important parts of the process of evolution. [Macroevolution]

Objections to the Minimal Definition

Some people, including some scientists, are uncomfortable with this minimal definition because they think it excludes some important parts of evolutionary biology. I'll try and discuss the various objections in a short while but first let me explain why we need a strict minimal definition in the first place.

I've already alluded to one of the classic questions that a proper definition can answer—the increased height of Europeans over the past five centuries. Armed with a good definition of biological evolution we can focus on one of the key requirements; namely, heritable change. It turns out that the increase in height is due to a better diet and not to genetic changes. Therefore, this is not evolution according to the scientific definition.

We can also ask whether the development of antibiotic resistance in bacteria is a valid example of biological evolution. In this case the answer is "yes" because a new antibiotic resistance allele has arisen by mutation and subsequently became fixed in the population. Anyone who wants to offer an alternative minimal definition of evolution will have to make sure that it will help answer questions such as these.

Sometimes it's convenient to refer to evolution as "descent with modification." This conveys a different impression of evolution than the minimal definition. Descent with modification refers to the long-term consequences of short-term changes within a population. It incorporates additional concepts such as speciation, which is an important part of macroevolution. Paleontologists are one group of scientists who aren't directly concerned with the minimal definition of evolution since they are mostly interested in the history of life. They have to deduce that evolution, in the sense of the minimal definition, has taken place from evidence of phenotypic change in the fossil record.

The bad thing about "descent with modification" is that it's not a very rigorous definition. It doesn't rule out modifications that are not genetic in origin and it doesn't rule out individuals evolving—as opposed to populations.

Many people are confused about the difference between a definition and an explanation. That's why we often see incorrect "definitions" that describe how natural selection works. This is wrong. In order to be useful, a definition has to enable us to distinguish examples of evolution from non-evolution but the definition should be neutral with respect to how evolution occurs. It should not distinguish, for example, between Lamarckian evolution and Darwinian evolution even though we know that one of these explanations is incorrect.

Attempts to define evolution in terms of natural selection are not only logically flawed but scientifically flawed as well. They exclude change due to random genetic drift when every evolutionary biologist agrees that drift is a mechanism of evolution.