How much of a chance is evolution?

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How much chance is involved in evolution?

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Shutterstock / Lewis Tse
Emilio Rolán Álvarez, University of Vigo and Juan Gefaell Borrás, University of Vigo

Is evolution random or determined? In his well-known book Wonderful lifepaleontologist and evolutionary biologist Stephen Jay Gould proposed a thought experiment that gets to the heart of the matter: imagine if we could go back in time to the beginnings of life on Earth as if it were a (now outdated) VHS tape that we could rewind, and let evolution unfold again from that point on. What would we find?

According to the vision gouldiana from evolution, it is very likely that the organisms we would see evolving in this second parallel history of life would be very different from those of today. In doing so, Gould wanted to highlight the role of chance and random events in the evolution of species.

Gould's thought experiment has been very influential. However, not all evolutionary biologists agree with their interpretation of some episodes in the history of life. Gould's "contrarians" point out that deterministic forces are at least as important as chance. Unfortunately, it is not possible to perform 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 approximations.

According to the scenario that prioritizes chance, when faced with similar environmental challenges, the different lineages of organisms should evolve phenotypically very different adaptations, as a result of the influence of random processes. On the contrary, according to the deterministic scenario, when faced with similar conditions, the different lineages of organisms should evolve very similar adaptations to cope with these conditions.

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Species of actinia drawn by Haeckel. Author provided

In favor of chance: we are mutants

What evidence can be counted in favor of randomness? The main evolutionary factor that introduces randomness is the mutation. Mutations are changes in the hereditary material (the sequence of DNA bases) of an organism, and these are the main reason for its phenotypic variation. When evolutionary biologists state that mutations are random, they do not mean that all possible genetic mutations have the same probability of occurring. At least in the vast majority of cases, these mutations have no direct relation 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 set evolution on a random course. For example, in the long-term evolution experiments in 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 alone has 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 unique genetic mutations whose occurrence is rather rare, which certainly supports the random view of evolution.

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The twelve lineages of Escherichia coli used by Lenski and his team in their experiments. Only strain A-3 evolved the ability to grow aerobically in the presence of citrate. Wikipedia. Wikimedia commos

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 in both the short and long term. Among these forces, the following undoubtedly stand out, natural selection.

There are multiple examples described of how natural selection generates similar traits in the face of similar environmental challenges, leading to parallel and convergent evolutionary processes. For example, research in which our group at the Universidade de Vigo has played a key role has shown how ecotypes Wave (small size and light shell) and Crab (larger 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 European west 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 several species of placental mammals and marsupials) is another support for the deterministic role of natural selection.

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Ecotypes Wave and Crab from Littorina saxatilis from the coasts of Galicia (Spain) show adaptations evolved by natural selection to cope with different environmental factors. Analogous versions of these ecotypes have also evolved by natural selection independently at different locations along the coasts of the United Kingdom and Sweden. Author provided

Let us add fortuitous events such as volcanoes and meteorites.

Thus, we have that evolution can be influenced by random processes, such as mutations, and deterministic processes, such as natural selection. Of course, these are not the only ones in their respective classes. For example, chance events such as los meteoritos o las erupciones volcánicas pueden condicionar el transcurso de la evolución; de modo análogo, los sesgos del desarrollo también limitan ampliamente el abanico de posibilidades evolutivas, pudiendo incluso contribuir a generar fenotipos similares en especies solo lejanamente emparentadas.

La importancia relativa de los procesos descritos en este artículo es algo que todavía está en discusión, pero en nuestra opinión la evidencia actual apoya la idea de que las fuerzas deterministas y aleatorias están irremediablemente entrelazadas. La clásica frase atribuida al filósofo griego Demócrito, que dio título a la conocida obra de Jacques Monod, sigue resultando válida en el contexto de la biología evolutiva actual: “Todo cuanto existe es fruto del azar y la necesidad”.The Conversation

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Árbol de la vida. Evogeneao, Author provided

Emilio Rolán Álvarez, Catedrático de Genética, University of Vigo and Juan Gefaell Borrás, Investigador predoctoral en biología evolutiva, University of Vigo

This article was originally published in The Conversation. read the original.