Equilibria will occasionally post lightning introductions to our favorite big questions in biology. This is our first, and is in collaboration with Josh Cofsky, a PhD student in Jennifer Doudna and John Kuriyan’s labs at UC Berkeley.
The world teems with an incredibly diverse array of lifeforms, each shaped by millions of years of evolution. Biologists and philosophers have long pondered: was the evolution of the particular varieties of life that we observe on Earth today a predetermined process, or a product of chance? Or, perhaps a bit of both?
The paradigm of evolution most widely understood by the public is the Darwinian “survival-of-the-fittest model”. We were all (hopefully) introduced to this idea in high school – competition between genes, organisms, and populations is fierce in the natural world, and among all the random genetic variation in these biological entities, only the fittest are able to reproduce and continue existing. In this way, nature selects for the entities that are most effectively adapted to their environment – the ones with the genes, bodies, and population structures best suited to the times. Richard Dawkins, a British evolutionary biologist, suggests that, if this paradigm is correct, there should be a net progress towards new adaptations that represent excellent solutions to nature’s challenges. In this context, “progress” is most intuitively understood in terms of organismal complexity. For example, most people would say that the evolutionary path from bacterium to human was a path of progress. So, was the evolution of humans inevitable? Did evolution point to us, and does it continue to point in a certain direction?
Did evolution point to us, and does it continue to point in a certain direction?
Stephen Jay Gould, an American evolutionary biologist, would say “no.” According to him, the illusion of progress is a product of human bias. Gould criticized the idea that evolution is a process of progress on the grounds that the direction of change is dependent on the time scale. Gould, who knew a thing or two about paleontology, divided evolutionary time into three tiers:
- the ecological moment, which operates at timescales we’re able to experience (years to hundreds of years)
- punctuated equilibria (this is “normal” geological time, in millions of years)
- mass extinction events (think “ice ages” or “asteroid collisions”, which happen on the timescale of hundreds of millions of years).
Darwin’s vision of progress describes the first tier very well, encompassing the day-to-day struggle for existence. At this scale, the “output” of the struggle – whether or not an organism, gene or population survives and reproduces – is dependent on the “input”, what exactly the organism, gene, or population is made of. If you inherit particularly useful genes for an environment, you’re pretty likely to survive. Not too much is left to chance.
Because the second tier is composed of several iterations of the first tier (over a much longer period of time), and the third is likewise comprised of several iterations of the second, we might expect the pattern of progress to hold across all of biological history. In fact, the pattern does not extrapolate to the other tiers, and in that discontinuity lies Gould’s “Paradox of the First Tier”. Unforeseen catastrophes, which happen incredibly rarely but carry huge biological consequences, can easily eradicate even the most well-adapted entities, outfitting environments with entirely new sets of challenges that survivors must contend with. Gould points out that several events in the story of life on Earth were entirely contingent, and that these contingencies only become obvious upon examination at the second and third tier. An example: the asteroid that wiped out the dinosaurs surely did not consult Darwin’s Origin in deciding which species to destroy. Mammals were likely only able to gain a competitive foothold because of the demise of the dinosaurs, and if the asteroid had just missed us, I almost certainly would not be around to write about it.
With his Paradox, Gould draws an important distinction between theory and reality. Evolutionary biologists cannot rely exclusively on the principles of Darwinian evolution to explain every twist and turn in life’s path. Yes, progressive evolution can explain many features of modern life, perhaps even most features, but certainly not every feature. For that, we need appreciate that chance events, like asteroids, have a large role to play in history.
All of this leads naturally to some really interesting philosophical questions about determinism and historical directionality. Gould spent his career wrestling with the relative importance of general laws and random contingencies. Discussion of these questions are outside the scope of this post, but I’d recommend the following reading if you’re interested: