How random is evolution?

Evolution, is it random or is it determined? In his well-known book wonderful life, the paleontologist and evolutionary biologist Stephen Jay Gould proposed a thought experiment that allows us to understand the bottom of the matter: imagine that we could temporarily go back to the beginning of life on Earth as if this were a (now outdated) VHS tape that we could rewind, and let evolution unfold again from that moment. What would we find?

according to the vision gouldiana of evolution, it is very likely that the organisms that we saw evolve in this second parallel history of life were very different from the current ones. With this, Gould wanted to highlight the role of chance and random events in the evolution of species.

Gould’s thought experiment has been highly influential. However, not all evolutionary biologists agree with his interpretation of some episodes in the history of life. Gould’s “opponents” point out that deterministic forces are at least as important as chance. Unfortunately, it is not possible to carry out Gould’s thought experiment in reality, so it is difficult to know which of these two approaches is correct.

One way to approach the question is to establish what predictions are derived from each of these two alternative scenarios and try to find examples in nature or small-scale experimental approaches.

According to the scenario that prioritizes chance, faced with similar environmental challenges, the different lineages of organisms should evolve adaptations that are very different phenotypically speaking, as a result of the influence of random processes. On the contrary, according to the deterministic scenario, given similar conditions, the different lineages of organisms should evolve very similar adaptations to each other to face those conditions.

In favor of chance: we are mutants

What evidences can be counted in favor of chance? The main evolutionary factor that introduces randomness is mutation. Mutations are changes in the hereditary material (the DNA base sequence) of an organism, and they are the main reason for its phenotypic variation. When evolutionary biologists say that mutations are random, they do not mean that all possible genetic mutations are equally likely to occur. At least in the vast majority of cases, these mutations are not directly related to the adaptive value they provide to the organism that carries them. Whether a genetic mutation occurs depends on unpredictable processes of a subatomic nature.

Some evidence indicates that mutations can impart a random course to evolution. For example, in long-term evolution experiments on Escherichia coli led by Richard Lenski, a microbiologist at Michigan State University, it has been found that the evolution of the ability to grow aerobically on citrate has only evolved in a single lineage of the twelve identical ones that make up these experiments over several tens of thousands of generations. . The molecular basis of this evolutionary phenomenon seems to depend on a combination of single genetic mutations whose occurrence is rather rare, which certainly supports the chance view of evolution.

In favor of determinism

But not everything seems random in evolution. In the evolutionary process there are forces that operate in a strongly deterministic sense, constraining the phenotypic change of organisms both in the short and long term. Among these forces stands out, without a doubt, natural selection.

There are multiple examples described of how natural selection generates similar traits in the face of similar environmental challenges, giving rise to parallel and convergent evolutionary processes. For example, research in which our group at the University of Vigo has played a key role has shown how ecotypes Wave (small size and light shell) and crab (larger size and more resistant shell) of the marine snail Littorina saxatilisadapted to different microhabitats of the rocky intertidal ecosystem, have evolved by natural selection in parallel and partially independently in different latitudes of the western European coast (Spain, United Kingdom and Sweden).

Similarly, the convergence of different lineages towards similar morphologies in response to the same environmental challenges (something observed in various species of placental and marsupial mammals) is another support for the deterministic role of natural selection.

Let’s add random events like volcanoes and meteorites

Thus, we have that random processes, such as mutations, and deterministic processes, such as natural selection, can affect evolution. Of course, these are not the only ones in their respective classes. For example, fortuitous events such as meteorites or volcanic eruptions can condition the course of evolution; Similarly, developmental biases also greatly limit the range of evolutionary possibilities, and may even contribute to generating similar phenotypes in only distantly related species.

The relative importance of the processes described in this article is still under discussion, but in our opinion the current evidence supports the idea that deterministic and random forces are inextricably intertwined. The classic phrase attributed to the Greek philosopher Democritus, which gave the title to the well-known work of Jacques Monod, continues to be valid in the context of current evolutionary biology: “Everything that exists is the result of chance and necessity.”