Posted by: Dan | May 13, 2009

Defining Evolution

As Larry Moran noted with his question 2+ years ago, “What is Evolution?”, there’s a lot of confusion in the general public about what evolution is, and most people who object to it cannot define it. Perhaps the most definitive definition is that offered by biologist and author of the authoritative textbook on evolution Douglas Futuyma:

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

More generally speaking, evolution most simply refers to change over time. Since Darwin, evolution has always been a property of the population, not the individual. And, as alluded to by attempts to define evolution as changes in allele frequencies of populations, evolution of populations revolves around the notion of a collective set of heritable units that determine traits in the individuals carrying those units (i.e. genes).

But definitions of evolution centered on individual populations are limited to phyletic gradualism, a.k.a. ‘microevolution.’ What of evolution above the level of the individual population? How do we include the isolation of incipient species and interspecific competition – processes collectively referred to as ‘macroevolution’ – in our definition of evolution?

In the end, the best definition that I’ve come across, acknowledging both intra- and inter-population dynamics of biological evolution, is from the early draft of a “white paper” on Evolution, Science, and Society, chaired by Futuyma. The paper was written on behalf of eight scientific societies who wanted to make a statement about evolution, and appears to tersely sum up the view of evolution from across the dearth of sub-disciplines in biology.

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 says nothing of mechanisms, and is written as a distinct theory of biology from those of Natural Selection, Sexual Selection, Neutral Theory (Genetic Drift, basically), and modes of speciation and clade sorting, which it is. So I would have to say that, as a definition of evolution, this initial draft of the ‘White Paper’ is precise and satisfactory.

What do you think?


Responses

  1. How about simplifying it even further:
    Evolution is the descent of different lineages, through independent modification, from common ancestors.
    Although that really describes branching evolution. A single lineage can also undergo significant evolution, so how about:
    Evolution is the modification of a lineage over time, or the descent of different lineages, through independent modification, from a common ancestor.

  2. Bipedal,
    “Lineages,” or “populations”?

  3. I was trying to stick as closely to Futuyma’s wording as I could. In his last sentence he uses “lineage” for an arbitrary group size, be it population or species.

  4. Right. And I think that lineages is intentionally more vague, referring to a couple things at the same time – populations, species, families, phyla, etc., making it more appropriate in one sense.

    In another sense though, many biologists rightly say that evolution occurs at the level of population(s), gene pool(s), etc.

    Semantics, semantics… ;-)

  5. I think that mitochondria are a good example of evolution. Scientists believe mitochondria inhabiting the body was the result of a microbial invasion. These little buggers became pretty useful to us and can even help us track our ancestors through the maternal DNA! Small change, big result. Good article!

  6. As I have recently taken on critical thinking to question whether a thing is true instead of seeking to confirm something I have believed blindly, I think it would help to carefully define mutations of different kinds. In critical thinking we learn that we need to fist define a problem. You are trying to do this I think with a very broad scope. It might help to be more specific. I think if you look at my blog you might like my attempt. It is the types of mutation we should be labeling more clearly. There may actually be several ways to do this.

  7. The problem with your current definition is that several types of lineage growth can, according to evolution, occur. Is it a lineage of one species or many? Is it only a lineage within a specific sub-species? Clearly the type of mutation must be better defined.

  8. I’ll respond to you on mutations on your blog as you request.

    For lineages, it’s generally meant in the same sense as it is used in genealogy (for obvious reasons). So when you trace your lineage from an ancestor 8 generations ago (great-times-6 grandfather), there is a straight line that you can draw. This same ancestor may have a lot of other descendants besides you also – making that ancestor a common ancestor to you and your other extant (still existing) distant relatives. The same is true for a given taxon (taxonomic group) of biological organisms.

    You can also look at the extant descendants and study inter-relatedness (how related they are between each other). For example, for a genealogy you can see that first-cousins share more hereditary characteristics (and thus genes) than, say, fifth-cousins. I’m actually part of an ongoing study now with an early American immigrant who I think might be my gggggg-grandfather. A few dozen people across the PA-OH region with the same last name all took this genetic test, and with most of them it turns out that my first cousin and I matched except for one small mutation, whereas someone who it seems is our fourth cousin has three such mutations, but otherwise is also almost identical. So we’re pretty sure that the three of us share a common gggg-grandfather in this lineage to the gggggg-grandfather in question. In this extremely simple example, you can see that descendants diverge more and more over time, but still are obviously more related than people not descended from the same common ancestor.

    In this sense, lineage is used to trace “lines” back to ancestors and observe divergence to help understand inter-relatedness.

    The same terms and approaches are used in studying the divergence and inter-relatedness of taxons as they descend from common ancestors.

  9. Mike,
    Just got done responding to your blog post. Obviously, you wrote that before reading this blog post, because you obviously still cannot tell evolution from whatever that nonsense is that you describe.


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