There is probably no other group of scientists among evolutionary biologists that are more concerned with the long-term patterns and processes of evolution than paleontologists. By constantly gazing upon different species that have evolved over the course of millions of years, it is unavoidable to wonder if there are any laws of nature governing the patterns by which some species are replaced by others, or how some anatomical traits seem to reappear in the evolutionary history of some lineages whenever they are faced with similar environmental conditions. Interestingly, sometimes different researchers looking at the same pieces of evidence will eventually reach completely opposite conclusions about those general patterns.
This is the story of the debate between two great paleontologists and their views on the evolution of life and how this led to an array of research programs in evolutionary biology. On one side, we have the concept that the history of life is greatly influenced by historical contingency, put forward by the late invertebrate paleontologist Stephen Jay Gould. On the other side, the idea of evolutionary determinism, strongly defended by another paleontologist, Simon Conway Morris. Gould’s idea (illustrated in his 1989 bestseller book Wonderful Life: The Burgess Shale and the Nature of History) suggested that, just as in any historical process, the history of life is dependent upon the series of events that took place until that moment. Replay the tape of life with slight alterations on any sequence of events and the outcome will be different. However, repeated evolution of similar traits (homoplasy) is widespread in nature, leading Conway Morris to suggest that evolution is not so unpredictable as suggested by Gould, but instead, universal physical and chemical laws will determine very similar best fit solutions under the same environmental conditions.
The debate between contingency vs. predictability in nature and the future of evolution was recently reviewed and extensively dissected by Dr. Jonathan Losos (Washington University in St. Louis) in his book Improbable Destinies: Fate, Chance, and the Future of Evolution. As a student of lizard biology and paleontology, I was familiar with Dr. Losos academic work on the evolution of anole lizards in the Caribbean, and so I was naturally driven to read this book and learn in greater depth his views on long-term evolutionary patterns. In this work, Dr. Losos tells the story of the debate between Gould and Conway Morris, goes through a well detailed account on the history of experimental Evolutionary Biology, provides examples to support both ideas, and lays out his personal insights into the debate. For those interested on learning such subjects, I highly recommend this reading.
In a nutshell, Losos describes how several experiments in nature and in the lab demonstrate how closely related species tend to evolve similar adaptations to similar selection pressures, sometimes over the course of only a couple of generations. This seems to indicate that evolution is indeed predictable, and more than that, foreseeable evolutionary changes may occur extremely fast. Such cases include different species of anole lizards living in different islands in the Caribbean evolving convergent solutions to become better adapted to their habitats (e.g. similar adaptations for species living in the canopy vs. another set of similar features for species living on the ground on different islands). On the other hand, when we look at the diversity of life on the planet today, it is quite evident that the world is filled with examples of unique evolutionary trajectories that are unknown in other regions of the planet or in other periods of time. Those include “evolutionary one-offs”, as described by Losos, such as the platypus, giraffes, elephants, and penguins. Many of such cases can be found on the Australian and South American faunas, which were isolated for dozens of millions of years from other continents. This is certainly the case in the fossil record too, with examples ranging from body plans that were never seen again from the Burgess Shale fauna (such as Anomalocaris and Opabinia), to alien-looking Paleozoic sharks with weird projections from their heads and backs, or flying reptiles from the Mesozoic (pterosaurs) possessing head crests up to five times larger than the remaining area of their skulls.
Importantly, Losos also describes how long-term evolution experiments developed in the last decades using E. coli and other microbes have made it possible to conduct the thought experiment of Gould and replay the tape of life. Those experiments have revealed that populations living in similar conditions tend to evolve quite similarly to each other over hundreds to thousands of generations. However, after thousands of generations, sometimes one of those populations may evolve particularly unique traits unobserved in any of the others (such as the capacity to metabolize citrate in the presence of oxygen in E. coli). In other words, closely related species may quickly develop similar solutions to similar environmental pressures given their similar genetic background. However, give it enough time, and unpredictable and perhaps unique features may evolve and become predominant in that population.
Those remarkable advances in evolutionary biology have provided an experimental approach towards testing hypothesis regarding long-term patterns in evolution and the processes responsible for them. Among such testable hypothesis, the conflict between contingency and predictability in evolution are without question among the most interesting ones. So far, as with everything in biology, there are no straight answers to this conflict, but only probabilities and general rules. Yet, it seems that the conflict between contingency and predictability is a matter of scale. Organisms tend to achieve similar solutions to similar problems, but give it enough time (or a small enough population sizes), and anything is possible.