What in the heck is Convergent evolution? An explanation, and how scientists study it.

Growing up I was always obsessed with the universe and the life it birthed. Atoms forged and fired from dying stars at the outer reaches of the cosmos travelling eons to coalesce into a cloud of universal dust. Gravity taking hold; pressurizing atoms until rock is formed; then Earth. A single pale blue dot orbiting a 4.6 billion year old nuclear fusion reaction we call the “Sun”; It’s awesome. The baron rock we call Earth slowly morphing into the one we know today. Lush and full of life. But how did life get there? The answer is deceptively simple; Evolution, baby!

A Pikachu evolving into Raichu after encountering a thunderstone. Credit

Have you ever noticed unrelated animals share the same traits?

Did I say deceptively simple? What so deceptive about it? Animals have traits, and those traits are selected for by the environment and passed down to offspring. Not very deceptive at all you might say; until you think about Bats, Birds, and Butterflies. Mammals, aves, and insects respectfully. How is it that each of these three organisms all share a common trait so distinguishable as wings and yet be completely different in every other way? Surely long ago an ancient ancestor of each of these animals evolved wings right? What if told you that Birds, Bats, and butterflies each evolved wings independently of one another? This is what is known as convergent evolution and its something that researchers led by Pedro Simoes out of Portugal decided to elucidate through their published journal article.

What are these Scientists actually researching?

So lets look at this incredible image I have made to better understand what it is these researchers are looking for. The orange background is the environment, the circles are the generations of species A and species B; the colour changes represent new adaptations that have been selected for. So what do we see between species A and species B? Well they both stem from different coloured circles (which means both species have different traits to begin with) and as they evolve, they begin to differ more and more, until! Aha! In the top right corner, can you see it? Even though the starting circle In each box was a different colour, both boxes have ended with a circle that is yellow! This is convergent evolution in action! Two separate organisms developing the same trait independently of one another.

That’s great and all but how does this relate to the research by Pedro Simoes?

Well with the knowledge you now have on convergent evolution I’m sure you’ve already put two and two together. Since we can make an environment homogenous (make completely the same), could we then breed animals in that environment for 100s of generations, add new organisms, and then predict the traits the new organisms will develop based off the 100s of older generations? Well this is exactly what Pedro Simoes’ team set out to discover!

They took flies and recorded their fecundity traits (the number of eggs laid, the age of first reproduction, peak number of eggs laid) over the generations and compared that to the results of the older generations of flies that had been living in the lab for a much longer time. If the traits of the new flies became similar (converged) to the older flies, then that would mean we could theoretically predict how the flies would evolve when placed in the lab! How cool is that?

Here’s what they found!

Image source
Moving clockwise starting on the far left we have: The difference to the control, The different populations of flies, the traits they studied, and the number of generations. The 0 represents identical traits to the older generations of flies that have lived int he lab previously. The dotted lines represent the smaller number of generations, and the solid line represents the larger number of generations
It should also be mentioned that early fecundity is the number of eggs laid in the first week, and peak fecundity is the number of eggs laid after that week.

In order to get this data Simoes et al caught flies from separate regions in Portugal, brought them back to the lab, bred them, and after two generations began collecting data (this is to help standardize the data, which is done to decrease variables in the experiment). To collect data Simoes et al performed phenotypic assays of the flies in each generation, by transferring mated flies to fresh media every day, and counting the number of eggs laid in the first week of life (early fecundity), the number of eggs laid between days 8 and 12 (peak fecundity), and the number of days before the first eggs were laid (age of first reproduction).

How did they make these graphs?

In order to normalize the data and plot it on this graph some pretty heavy statistical methods were used.

Source: Giphy

In interest of transparency i couldn’t accurately explain it to you, so instead i will give you a very simple explanation that gives you the gist. The data was averaged, and then a linear regression of the data was performed, mix that in with some statistics that go way over my head and bingo bango, you’ve got this graph.

In the graph, we can see that as time goes on the traits begin to converge (as seen by the dotted lines, and solid lines moving towards the 0 mark) but then as more time passes (more generations are born) they begin to diverge again e.g., move away from the 0 line (as seen in the very late generations; specifically in the NARA and TW populations). Some traits don’t even converge at all! (looking at you; peak fecundity for the NARA population) The writers of this article have stated there is “an overall theme of convergence” which is true. Overall some convergence is occurring. Unfortunately, due to the transient divergence seen between the early, and late generations of the populations; there is no way you could accurately predict the fly’s evolutionary trajectory.

So all in all, as cool as it would be to be able to predict traits in animals, there is just so much variety when it comes to evolution that it leaves us struggling to predict the trajectory animals will take along their evolutionary path. Perhaps with time, and more generations, the traits of the new flies will converge in a manner more predictable. Until then we will just have to remain ignorant of our evolutionary trajectory. Despite the results. I hope you can walk away from your computer screen knowing you’ve learnt something about convergent evolution, and how experimental evolution can shed light on how this phenomena arises.

Source Material

Simoes, P., Fragata, I., Santos, J., Santos, M. A., Santos, M., Rose, M. R., Matos, M. (2019) How phenotypic convergence arises in experimental evolution. Evolution, 73(9), 1839-1849.  doi: 10.1111/evo.13806

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9 Responses to What in the heck is Convergent evolution? An explanation, and how scientists study it.

  1. jcturnbullnz says:

    Bingo bango! Great effort at translating those graphs into laymans terms. I enjoyed your introduction to the topic for the Simoes et al. team’s work.

  2. msbdavies says:

    Hi Liam

    I like that you didn’t try to pull more from the paper than there was, and left it with an honest ending of uncertainty. A fun read.

  3. aearnshaw9 says:

    Hiya Liam,

    Appreciate the effort that went into explaining this in simple terms (and laughed at the statistical gif). I was wondering if evolving the flies in different media would change the gradient/how quickly convergent evolution would occur? As in, does the environment chosen affect if animals would evolve similar traits?

    Thank you,
    Alyssa

    • lshewson says:

      Hey Alyssa,

      That’s an interesting question! I lean towards thinking it would have an effect. It would make sense that the more stringent the environment is towards survival, then the least number of possible outcomes a species could evolve to.

      So hypothetically by choosing a more stringent environment then it would be possible that convergent evolution could occur more rapidly. Of course, this would be much more evident in bacteria, as in all likelihood at the multicellular level, such a stringent environment would more commonly result in death.

      Thanks for the question,
      Liam

  4. annabehlingnz says:

    I really enjoyed reading this, Liam.
    It would be interesting to see some experimental evolution-based convergent evolution experiments in animals other than Drosophila (and perhaps in taxa from different kingdoms) to investigate if there were any taxon-specific influences on the results these researchers obtained.

    • lshewson says:

      I couldn’t agree more, Anna.

      I would definitely like to see bacteria of different species/genera evolving similar traits and seeing just how similar you could make two different species. What if you could produce an environment that selects for traits that the two species’ common ancestor had. Could you theoretically cause the bacteria to “evolve backward”? 🙂

      😀 😀 😀 😀 😀 😀
      Liam

      • annabehlingnz says:

        Fascinating idea, one would think with many potential (albeit very far off at this stage) implications for the ‘recreation’ of extinct species.
        😀

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