In light of my recent, totally flawed and unscientific survey, which indicated a sad state of affairs regarding the understanding of evolution among a segment of the population that partially relies on evolution for its framework (read: paleo), I’ve decided to jot down a few thoughts from the world of evolutionary theory for your consideration.
It’s tempting to think about evolution as a function of time. This makes some intuitive sense because evolution happens over time, and longer periods of time theoretically allows for more mutations, which theoretically allows for more adaptations. However, it is misleading to use time as a heuristic for thinking about evolution in an individual species, or when making comparisons between species.
Before we dive in too far, here are some obvious qualifiers: Of course, evolution literally happens over time, and is bound by time to some extent. Thus, time isn’t completely irrelevant in the function of evolution. And again, more time generally means more mutations, which means more fodder for adaptation via selection pressures. So yes, time is a factor, but it’s not really that important when thinking about evolution. In fact, there are examples species that haven’t undergone any noticeable evolutionary change. But why not?
The fossil record does include examples of organisms changing gradually over time and undergoing speciation. However, the fossil record also includes examples of near stasis, and examples of rapid evolutionary change. No lesser names than Richard Dawkins and Stephen Jay Gould have disagreed on the rapid vs. gradual debate in evolutionary biolog. While it’s currently understood that gradual and rapid change simultaneously fit into the neo-Darwinian synthesis, debates have raged on the issue even within the last decade.
As it pertains to humans, much of the discussion of evolutionary change orbits around the ~ 6 million years separating humans from our most recent common ancestor (MRCA) with chimps/bonobos. Also commonly mentioned are our MRCA with other Great Apes (~10 MYA) and the earliest primates (~65 MYA). How are these misleading in terms of context?
For our purposes, we’ll roughly take “living fossils” to mean species still living today that haven’t changed much in physical structure over long periods of time (morphological stasis). There remain many examples of living fossils. At more than double the time between humans and MRCA, we find Colombian fish that haven’t noticeably undergone evolutionary change in 15 million years (Lundberg, et al. 1986). With these fish as a backdrop, if we were thinking about things in terms of time, we’d have to say that humans, chimps, gorillas, and even orangutans aren’t simply like humans, we’d have to say that they’re the same as humans. If that isn’t enough of an illustration, some species of lamprey haven’t really changed in 350 million years. That’s only a couple hundred million years before the “Jurassic” part of Jurassic Park. Using this period of stasis as analogy, if we were to compare evolution in terms of time, that would make humans the same as dogs, cats, horses, and pretty much all other mammals.
If we wanted to dig deeper into this, we could drum up mathematical models about mutation rates over time. We’d factor in things like the number of chromosomes and genes and crunch that into a formula that took into the account the time each generation takes to reach reproductive age, how long they remain in reproductive age, how many offspring they have, how many survive, et cetera. To make comparisons between species, we’d normalize all of those things across species and try to make predictions about how much evolution should have occurred. Yet, even with models taking species specific mutation loads and reproductive potential into consideration, knowing the amount of time would leave us unable to reliably predict morphological change. The main thing all the modeling and calculation and extrapolation would tell us is a probabilistic potential for evolution. But we’re still missing the crucial component.
The selection pressure placed on an organism by its environment is the crucial component for thinking about evolution. More accurately, the interaction of multiple selection pressures of varying strength within an organisms total ecology over time is the crucial component for thinking about evolution. Some Colombian fish and some species of lamprey have been reproducing for millions of generations. Each generation across that time has seen a semi-reliable and reasonably predictable mutation load. Yet, we see virtually no change. Without selection pressure from the environment, the genetic material underlying phenotypic expression can simply be averaged-out over time.
Note: this isn’t to say that morphological change can’t happen over time absent selection pressure. Indeed, genetic drift can cause significant change, but it’s rather unpredictable, and doesn’t necessitate change. Further, if a species is already well adapted to its environment, negative selection may keep genetic drift in check, preventing significant change.
Let’s look at it the other way around. Think about the difference between humans and chimps that have evolved over the last ~ 6 MYA. Knowing that mutations in generations of lamprey have been happening for 350 million years, what would we expect to see in the lamprey if we multiplied the difference between humans and chimps over 350 MY? Not only would lamprey have developed electric defense mechanisms, but perhaps also the ability to fly, play Guitar Hero, and lay golden eggs.
Another way to think about the irrelevance of time is to consider a scenario in which a highly contagious and fatal pathogen sweeps through a human population. If there is a genetic immune system variance in 17% of that population that renders individuals resistant to the pathogen, you can have strong selection that may act almost instantaneously in a single generation. If the pathogen manages to survive for longer than 9 months, it will likely further exert selection pressure on the next generation – strengthening the inheritance component of the adaptation. Between this near instantaneous (in terms of evolutionary time) adaptation, and that of the 350 million years of (relative) stasis in lamprey, we have enough to throw time out the window as a relevant factor in evolutionary heuristic thinking.
Humans aren’t cats. Neanderthal were more closely related to humans than chimps, yet Neanderthals are dead, and chimps aren’t. Lamprey don’t have superpowers. When thinking about evolutionary theory, you’ll get better mileage by thinking about how a species fits in its environment (or range of environments) than you will by thinking about time. Corollary: Think about how it doesn’t fit into its environment.
Also, it’s wise to beware of those trying to “prove” similarity or dissimilarity simply by reciting evolutionary time separating them, and without the complex context of their respective environments over that time.
Dawkins, R. (1986). The Blind Watchmaker. Society. [link]
Gess, R. W., Coates, M. I., & Rubidge, B. S. (2006). A lamprey from the Devonian period of South Africa. Nature, 443(7114), 981-984. [abstract]
Lundberg, J. G., Machado-Allison, A., & Kay, R. F. (1986). Miocene Characid Fishes from Colombia: Evolutionary Stasis and Extirpation . Science , 234 (4773 ), 208-209. [abstract]