The minimum evidence necessary to demonstrate evolution

Each week in Wellesley 207x I will be providing my students with a “thought question for the weekend” related to that week’s course content. Students are invited to provide their responses on the discussion forums. These responses are not graded, but they can be viewed and commented on by others in the class. I have been very happy to see that the first question has prompted responses numbering in the hundreds (maybe thousands), with lots of back and forth comments.

The first question was: What is the minimum evidence necessary to demonstrate evolution?

I have to admit, the question is intentionally vague. I want my students to think about each word in that question (i.e. “minimum,” “demonstrate,” “evolution”) and respond according to their own interpretation of the question. Again, I have been happy to see a variety of responses reflecting a variety of readings of my question.

From my view, there are a number of ways this question can be approached, much of it hinging on how the word “evolution” is interpreted. The definition I provided for evolution in Week 1 is:

Evolution is heritable change in a population through time (or across generations)

How would you demonstrate that? That depends, in part, on what you already know.

If you know that DNA is the primary mechanism of inheritance, it makes sense to start with DNA. And indeed, we have a convenient theoretical model for looking at genetic observations and determining if evolutionary forces are responsible for the pattern of variation we see: Hardy-Weinberg equilibrium. H-W lessons are foundational to understanding evolution and are nearly ubiquitous in introductory textbooks…but seldom is the principle’s significance adequately conveyed to students. H-W is based on our understanding of how genetic variants (alleles) are transferred from one generation to the next, in a particulate fashion (discovered first by Gregor Mendel). A typical H-W equation assumes the simplest scenario of two alleles, represented in frequency by p and q, and provides us with an expectation for their relative frequency in a population. The reason H-W works is because it is a model that assumes no evolution is taking place. Therefore, since there is no heritable genetic change, underlying allelic frequencies should not change. So the H-W equation gives us an expectation of the frequency of variation in a population. In other words, the H-W equation gives us a null model we can test, the rejection of which tells us evolution has happened. A set of genetic observations that are not at H-W equilibrium must have undergone some kind of evolutionary change (mutation, selection, drift, gene flow, etc…).

So one answer to my question, pointed out by several students, is simply a demonstration that a population is not at H-W equilibrium. And indeed, this relationship is foundational to modern population genetics. But as an answer to my question, it assumes we know the mechanism of inheritance, how that mechanism functions (broadly), and that we are looking at genetic variation to demonstrate evolution.

What if instead we are looking at living organisms and their distribution and variation in the natural world (much like Darwin, himself)? What if, additionally, we don’t have a solid understanding of the mechanism of inheritance (again, like Darwin)?

In this case, we really only need to demonstrate that things vary across time and space, and that some of this variation is passed on from parents to offspring. That is it, really. This is a very preliminary demonstration of evolution, however. Darwin was interested not only in demonstrating that things change over time, but that this process of change over time can lead to the formation of new kinds of things (i.e. species). In other words, that the natural processes of change over time give rise, gradually, to new kinds of species and therefore the diversity of life on the planet. In this case, we need to add a few steps to our required evidence.

Now we need to not only demonstrate that organisms vary across time and space, but that the inherited variation is associated with greater reproductive and/or survival benefits, or in other words, higher evolutionary fitness. This is harder to demonstrate, because it requires working across multiple generations of an organism (though in organisms with short life cycles, like bacteria or fruit flies, this is not nearly as onerous). Alternatively, we might bypass the need to observe organisms across multiple generations by observing that across a whole host of environments, organisms seem to “fit,” or are “adapted” to their environments. A logical interpretation of this observation is the process of natural selection, an interpretation realized by both Darwin and Alfred Russell Wallace. Note that in this case we have expanded upon our understanding of evolution to not only include heritable change over time, but also the origin of species. We have also focused our interest in adaptive change, or change brought about by the actions of natural selection (not mutation, drift, or gene flow).

It is harder to observe, because it does not lend itself to a logical explanation as readily, but we could also demonstrate evolution in natural populations by simply observing random change over time via genetic drift. Say we have a flock of 100 sheep that we maintain at exactly that size, allowing every female and male to reproduce equally. In this case, there is no (or almost no) selection occurring, but genetic drift will lead to changes in our population over time. This might seem minor, but it is in fact evolution, and it is possible to imagine this scenario leading to the potential development of new species (particularly if my neighbor’s flock is isolated from mine). I say that drift would be harder to observe because it is random with respect to outcome, and therefore is not as readily observable as a pattern. Traits that are adaptations, traits that “fit” their environment, often are striking to us because of the patterns they depict.

Finally, we might try to demonstrate evolution via the fossil record. Here, the presence of a continuous lineage over time, or perhaps even bifurcating into two lineages, gets us a long way to demonstrating evolution. We have thousands of examples of fossil lineages showing change over time, with intermediate or “transitional” specimens. This observation, however, still requires a theoretical explanation of how evolution takes place. So the theoretical process, again our gift from Darwin and Wallace (built on the backbone of numerous earlier work by 19th, 18th, and even earlier naturalists) is still necessary in this case.

In short, there are a lot of potential ways to demonstrate evolution, even minimally. Depending on how broad a definition of evolution you would like to encompass and the kind of observational data you have available to you, you answer my question in a host of different ways.

About Adam Van Arsdale

I am biological anthropologist with a specialization in paleoanthropology. My research focuses on the pattern of evolutionary change in humans over the past two million years, with an emphasis on the early evolution and dispersal of our genus, Homo. My work spans a number of areas including comparative anatomy, genetics and demography.
This entry was posted in Evolution, Teaching and tagged , , . Bookmark the permalink.