Thinking about Evolutionary Theory – Part I: Evolution Isn't a Function of Time

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?

Living fossils

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.

Selection pressure

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.

 

References
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]

9 Comments
  1. @tmkeesey 4 years ago

    Minor correction: our MRCA with gorillas is from 7-10 Mya, and our MRCA with orangutans is from 13-17 Mya. (Dates rounded off from those in TimeTree: http://timetree.org )

    When it comes to measuring morphological change, there aren't any great objective measures. What we subjectively perceive as stasis may in fact include a great deal of change. (For this reason a lot of researchers cringe at the term "living fossil".) That said, the point that rates differ across lineages certainly holds!

    • Profile photo of Andrew
      Andrew 4 years ago

      What’s a few million years here or there? :) Thanks.

      Yup, totally agreed on the second point. I tried to qualify it a bit by restricting it to morphological changes. But ultimately, I just tried to shoot for general public palatability over strict accuracy (or researcher stomachability) – by glossing over the underlying genetic component in particular. And then there’s the whole problem of soft tissue fossilization (or lack thereof) AND the lack of 350 MYO DNA.

      Ah well… the 3rd tier nerds are free to hash it out in the comments. A Google Scholar search for articles on “punctuated equilibrium” since 2010 yields 1,600+ results… apparently still plenty of finer points to consider.

  2. David Csonka 4 years ago

    I find it hard to relate concepts of evolution to others in the form of average rates of speciation. But this post certainly makes me think a little bit harder about saying "x years = little evolution"

  3. hbomb9000 4 years ago

    Isn't that the whole issue with the fossil record at large? That everything gets very subjective because we're effectively lining up morphological changes? We have no way of knowing if the lampreys in the past were electrical, but due to some sort of sexual catastrophe, the more electrical ones started electrocuting their mates leading to mating preference changes which cleaned the electrical right on out of the lampreys.

    Even in recent history we've seen that morphologically similar species aren't always the closest relatives to their morphological counterparts. Once genetic sequencing was solidified and complete, we were able to rearrange the whole 'family tree' to be more correct.

    However, I do agree with you that selection pressures are far more important than the passage of time. It's one of those things that really hits upon "CW", and as such is hard to root out, partially because it's not something the majority of people think about every day. Many people will just learn it (or anything) once, never challenge that knowledge, and go through life not realizing their position is wrong.

    • Profile photo of Andrew
      Andrew 4 years ago

      Yes, morphological stasis/similarity leaves out some of the story. However, I'd also toss the idea out there that speciation (or taxonomic comparison) isn't necessarily a better method for measuring total difference. Speciation can occur on a very tight morphological and/or genetic level, whereas some species can demonstrate a huge range of difference yet maintain the ability to reproduce. A good example of this might be comparing the similarity of beetle species versus the wide variety among dog breeds.

      While I don't think simple taxonomic distinction necessarily tells us a lot about total difference, I do agree that direct genetic sequencing [basically] does. Unfortunately, DNA doesn't fossilize so morphology will have to do for our purposes here.

  4. J. Stanton 4 years ago

    Stated in one sentence and lightly oversimplified:

    Amount of evolutionary change = elapsed time * delta(selection pressure) / time per generation

    The delta(selection pressure) is important because only a change in environment will cause evolutionary change, no matter how strong the selection pressure. Strong, static selection pressure tends to cause evolutionary stasis, since deviations from local optima are severely punished.

    JS

    • Profile photo of Andrew
      Andrew 4 years ago

      Nice. The entire time I was writing this I was thinking… "Don't get into the math… Don't get into the math…" …purely because I a lot of people are terrified of it. Your comment seems easy enough for most to follow though.

  5. Aaron Blaisdell 4 years ago

    Nice summary of a difficult subject. There are many deeper levels to delve into, such as the difference between microevolution (the focus of early pioneers such as Sewall Wright) and macroevolution (as studied by paleontologists like George Gaylord Simpson). These scientists focus on different data sets, which biases their particular views of evolutionary processes. But you are right to prod the "paleo diet" community to move away from the undue focus on elapsed time, and focus more on selection and adaptation.

  6. Paul Groky 4 years ago

    I have a real problem with the claim that the Neanderthals are dead. It might be that I got something terribly wrong – so if you know it please let me know. Nevertheles, here are some of my thoughts on why I am confused that the Neanderthals are claimed to be extinct while the term homo sapiens is still applied to most humans on earth: Recent research suggests that 1-5% of the genome of humans living outside Africa are from Neanderthal origin while some groups of humans within africa (like the San/Bushmen) do not carry Neanderthal genes. Considering that Homo Sapiens : Neanderthal ratio was 20:1 at the time the two interbred and that the subsequent reproduction efficency of the carriers of mixed genomes was comparable to 100% homo sapiens it is a logical consequence that the amount of neanderthal alleles would stay constant over time while spreading and merging in the mixed population.
    I am interested in how much of the neanderthal genome could be identified by genotyping the entire humanity. Perhaps the entire Neanderthal genome is spread all over the world and still present.
    As Homo Sapiens and Neanderthals were able interbreed the differences between them must have been very small. this may make it difficult to determine whether stretches of DNA are derived from one or the other.

    using the definition "A species is extinct when the capacity to breed and recover is lost" I am asking myself if it is correct to say that the neanderthals are extinct because parts of their genome form part (1-5%) of most earths inhabitants. It may be more appropiate to say that say that the species have merged.

    On the other hand one could argue that the Homo Sapiens is in danger of extinction. nowadays, full-blooded homo sapiens are only found in Africa within small populations of aboriginal peoples.

    It may be correct to change the affiliation of most humans to Homo Neasapiens (NEAnderthal-Sapiens).
    In summary, I find it acceptable to say that the Neanderthals are extinct under the condition that homo Neasapiens (or whatever) is introduced as a new term to distinguish humans with a mixed homo Sapiens / Neanderthal genome from full blooded homo sapiens.

    Let me know what you think. Did I get something completely wrong? do you agree on some points?

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