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What Is Evolution?

v2.15 ©2006 Laurence A. Moran

ost 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 that I want to define here. What do biologists mean when they refer to biological evolution?

Talk Origins

An earlier version of this essay appears on the TalkOrigins Archive.

It's important to distinguish between the existence of evolution and various theories about the mechanism of evolution. For the time being, I'm not interested in describing evolutionary theory because that's not something that requires a "definition." However, when we refer to the existence of biological evolution we must know what we're talking about. When biologists say that they have observed evolution or that humans and chimps have evolved from a common ancestor they have in mind a scientific definition of evolution. What it it?

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 that 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 that 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..


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


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 that 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 demonstrate. It would be like saying they don't believe in gravity!

Anti-evolutionists often claim that 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 processe, 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 becomes 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.


Evolving Definitions

In 1997 a group of twenty scientists chaired by Douglas J. Futuyma issued a working draft of a "white paper" on Evolution, Science, and Society. The paper was written on behalf of eight scientific societies who wanted to make a statement about evolution. The initial draft defined evolution as,

Biological (or organic) evolution consists of change (modification) in the hereditary characteristics of groups of organisms over the course of generations. Such groups of organisms, termed populations or species, are formed by division of ancestral populations or species, and the descendant groups then change independently. Hence, in a long-term perspective, evolution is the descent, with modification, of different lineages from common ancestors.

This is a pretty good definition. It includes the minimal definition but adds the idea that long-term evolution is descent with modification. The initial draft definition was modified [final draft] so that on the current website it now reads,

Biological evolution consists of change in the hereditary characteristics of groups of organisms over the course of generations. From long-term perspective, evolution is the descent with modification of different lineages from common ancestors. From a short-term perspective, evolution is the ongoing adaptation of organisms to environmental challenges and changes.

This last sentence is really unfortunate. These twenty scientists have now agreed to a definition that specifically mentions the mechanism of adaptation. This is not how one should define evolution. One wonders whether they mean to exclude random genetic drift or whether they simply lost sight of their goal in trying to work out a compromise definition.

The Gene Centrist Objection

Ernst Mayr wrote an entire book on the subject of this little essay. One might expect some insight from one of the original founders of the Modern Synthesis but, unfortunately, we aren't going to get any help from Mayr. On page 157 he says,

Evolution in sexually reproducing organisms consists of genetic changes from generation to generation in populations, from the smallest local deme to the aggregate of interbreeding populations in a biologial species.

Ernst Mayr (2001) What Evolution Is,
Basic Books, New York p.157

This is good stuff. It restricts the changes to genetic changes and it clearly identifies the population as the unit that evolves. There's no mention of any particular mechanism. But—and you knew there was going to be a "but" didn't you?—good things never last. In his chapter on macroevolution Mayr describes the work of his colleagues Rensch and Simpson. These workers were able to study macroevolutionary events without referring to allele frequencies in a population. Mayr coments,

This approach was consistent with the modern definition of evolution as a change in adaptedness and diversity, rather than a change in gene frequencies, as suggested by the reductionists.

Ernst Mary (2001) What Evolution Is,
Basic Books, New York p.189

Consistency is not one of the hallmarks of Ernst Mayr's writings and that's why he can propose two conflicting definitions in the same book a book that's devoted to the topic of defining biological evolution! Nevertheless, he highlights two different objections to the minimal definition that I am defending.

First, Mayr wants a definition that restricts evolution to the mechanism of adaptation. This is a lost battle [see Adaptationism and Pluralism]. There may have been at time in the 20th century when a majority of biologists rejected random genetic drift and other non-adaptationist forms of evolution but that time is long gone. Mayr was one of the last hold-outs. Besides, as I mentioned above, it isn't appropriate to restrict the definition of evolution to a particular mechanism even if you strongly believe that it's the only possible mechanism. That's not how you define something.

Second, Mayr doesn't like reducing evolution to the level of the gene. This charge of reductionism is more interesting. In spite of the fact that Mayr was one of the founders of the Modern Synthesis, he never had much respect for genes and population genetics, or "bean-bag genetics" as he called it. He makes this point very strongly in the preface to his book.

... most treatments of evolution are written in a reductionist manner in which all evolutionary phenomena are reduced to the level of the gene. An attempt is then made to explain the higher-level evolutionary process by "upward" reasoning. This approach invariably fails. Evolution deals with phenotypes of individuals, with populations, with species; it is not a "change in gene frequencies." The two most important units in evolution are the individual, the principle object of selection, and the population, the stage of diversifying evolution.

Ernst Mary (2001) What Evolution Is,
Basic Books, New York p.xiv

I happen to agree with some of those who criticize the extreme reductionist views of scientists like Richard Dawkins but in this case Mayr has it all wrong. When we define evolution as a change in the heritable characteristics of a population we are not reducing evolution to the level of the gene. We are merely stating that populations don't evolve unless they undergo genetic changes. This is not controversial in spite of Mayr's objection. He is confused about the difference between a definition of evolution and a proposed mechanism of change—as was obvious by his attempt to include adaptation. This is a remarkable error in a book called "What Evolution Is."

The Minimal Definition and Macroevolution

The minimal definition of evolution is not inconsistent with [Hierarchical Theory] and a focus on [macroevolution] as opposed to microevolution. This point is worth emphasizing since the minimal definition has often been criticized for excluding lots of evolution that takes place at higher levels. Stephen Jay Gould—no fan of reductionism and no stranger to hierarchical theory—addressed this problem in his last anthology.

The Darwinian principle of natural selection yields temporal change—"evolution" in the biological definition—by a twofold process of generating copious and undirected variation within a population, and then passing only a biased (selected) portion of this variation to the next generation. In this manner, the variation within a population at any moment can be converted into differences in mean values (such as average size or average braininess) among successive populations through time.

Gould, S.J. (2002) "What Does the Dreaded
'E' Word Mean Anyway?" in I HAVE LANDED
Harmony Books, New York p. 246

The purpose of his essay was to point out the fundamental difference between this biological definition and the common vernacular meaning of the word "evolution." (I wish he hadn't used the word "selected" in his definition since it implies natural selection and Gould knows that there are other mechanisms.) Gould points out that other sciences, such as astronomy, use the word "evolution" in a very different sense—one that is actually closer to the original nineteenth century meaning. The vernacular meaning carries an implication of purpose and direction that is entirely absent from the biological definition of changes in the heritable characteristics of a population over time. This is why Darwin never used the dreaded "E" word.

Gould argues that an understanding of the true importance of the biological definition is absolutely essential to understanding why the general public is confused. He is especially concerned about emphasizing the lack of progress and direction in the definition of biological evolution. He advocates that scientists owe it to the general public to teach the biological definition.

I don't mention these differences to lament, or complain, or to criticize astronomical usage. After all, their concept of 'evolution' remains more faithful to etymology and the original English definition; whereas our Darwinian reconstruction has virtually reversed the original meaning. In this case, since neither side will or should give up its understanding of "evolution"—astonomers because they have retained an original and etymologically correct meaning, evolutionists because their redefinition expresses the very heart of their central and revolutionary concept of life's history—our best solution lies simply in exposing and understanding the legitimate differences, and in explaining the good reasons behind the disparity of use.

In this way, at least, we may avoid confusion and the special frustration generated when prolonged wrangles arise from mis-understandings about words, rather than genuine disputes about things and causes in nature. Evolutionary biologists must remain especially sensitive to this issue, because we still face considerable opposition, based on conventional hopes and fears, to our emphasis on an unpredictable history of life evolving in no inherently determined direction. Since astronomical 'evolution' upholds both contrary positions—predictability and directionality—evolutionary biologists need to emphasize their own distinctive meaning, especially since the general public feels much more comfortable with the astronomical sense—and will therefore impose this more congenial definition upon the history of life if we do not clearly explain the logic, the evidence, and the sheer fascination of our challenging conclusion.

(ibid p. 250-252)

I agree with Gould. That's why I think it's important to explain the real biological definition of evolution as a change in the heritable characteristics of a population over time. We can explain that this is a minimal definition, and that there's more to evolution than this, but we shouldn't back away from the real meaning of the term since it conveys some important messages. If we cave into pressure from the general public to make evolution into something they can understand, with all their biases, then we will have lost the battle before we even begin.

The amazing thing about the minimal definition of biological evolution is that it doesn't carry any baggage concerning the history of life or its future. As soon as we try to define evolution in terms of the historical record, we run into all kinds of problems because we confuse evolution as a process with evolution as a history of life. The scientific definition attempts to describe the minimum thing that might be called evolution. We know that the history of life is more complicated than this and we know that evolutionary theory encompasses other things such as the formation and extinction of populations. There is no conflict between the minimal definition of evolution as a change in the genetic composition of populations and macroevolution. Gould understands this.