Why it’s so easy

The more and more I think about it, the more and more I recognize just how easy it is to understand evolution.

Evolution is the change within populations over time. This change is continuous. Take birds, for instance. They are descended from dinosaurs. Depending on who you ask, you might even hear them referred to as dinosaurs. However, for the sake of argument, let’s draw a line in the sand. Let’s say birds are birds and dinosaurs are dinosaurs. There is no single point to which we can point that says “Ah-ha! This is the generation in which birds originated!” Evolution doesn’t work that way.

Life is gradual in the big picture. The lines are blurry as to where one species begins and where another ends. It is simply a matter of convenience that we are able to make the distinctions we make. Ancient birds weren’t great at flying. Less ancient birds were better. Even less ancient birds were even better. And then, sometimes, extant birds are back to being terrible at flying. It just so happens that we’ll never know what every year of every species was like. If we ever do, we won’t be able to say “Such-and-such is Species X and this other example is Species Y”. We’ll be looking at Species X.1, X.11, X.111, X.112, etc.

To be this all another way, mother birds (or bats or monkeys or humans or bears or prarie dogs) only give birth to daughter birds (or bats or monkeys or…etc). But over time, small changes accumulate. Think of how much you probably resemble your father in some way. Now think of how much you resemble your grandfather. Odds are, you resemble him less than you resemble your father. Go back further and you’ll see more changes. And that’s just on a phenotypic (for purposes here, “physical”) level. Go to a genotypic (genetic) level and there’s no questioning the facts. You are more similar to your close relatives than to your distant ones.

Now we have to extend this concept over time. We have plenty of it. Evolution has been playing out for nearly 4 billion years. Think about that for a moment. You’ll live around 80 years. If you’re lucky, you might hit 100. A tremendously long human lifespan would be another 20 years on top of that. It’s all a blip on the timescale of Life on Earth.

So here’s what you should be thinking. Every generation is similar to the previous generation. It doesn’t matter what species we want to specify. It’s always true. But the further back we go, the fewer similarities we see. But importantly, we still see similarities.

Take the bones in the wing of a bat. They are easily matched with the bones in the hand of a human or the paw of a cat. They are the same bones but shaped vastly differently. It isn’t simply a huge (convenient) coincidence that this is so. Bats share a common ancestor with other mammals. This common ancestor, being that it is found deeply in time, would hold notable similarities with all extant mammals, but it obviously wouldn’t visually match with every single organism (or even a majority).

But the visual match isn’t all with which we need to concern ourselves. That common ancestor wouldn’t be able to breed with anything alive today. The changes have been far, far too considerable since its time.

I’m breaking stride for a moment because I want to note something. The changes which occur over time in a species are what cause it to be considered a new species. In other words, when two populations cease to be able to breed and produce fertile offspring, we have a two separate species (which one we want to call the “new” one is somewhat subjective, but usually it’s the one least resembling the common ancestor). As I said, there is no single point where speciation happens, but imagine for a moment approximately the time where two populations cease to be able to breed. It won’t be one defined generation where it occurs, but there will be some generation somewhere where some members of a population cannot successfully produce fertile offspring with members of another population.

On the face of it, this sounds like I’m contradicting what I’ve been saying all along about not being able to pinpoint one generation. I’m not.

Remember I talked about the lines being blurry. While some members of one population probably won’t be able to breed with some members of another population, that won’t be true for all members of both groups. Much breeding will still be possible. With time, those possibilities dwindle. Eventually, the line begins to come into focus. That is how evolution works: It’s gradual.

So again, the more and more I contemplate evolution, the more and more it makes so much sense. Of course, the evidence is crystal clear and I don’t need this contemplation to confirm the theory. However, it is through this focus that I’m forced to wonder why we had to wait until Charles Darwin and Alfred Wallace to really recognize how new species come into existence. The truth is that changes occur in populations over vast expanses of time. Then most of the world’s populations either continue to evolve or, more likely, go extinct. In hindsight, we observe definitive periods of stasis, almost leading one to believe in static, unchanging species. Almost.

Forelimb morphology

19 Responses

  1. Life is gradual in the big picture. The lines are blurry as to where one species begins and where another ends.

    Can you give us an example of two species that are linked by one of these “blurry lines”?

  2. @ Grant Dexter

    http://en.wikipedia.org/wiki/List_of_transitional_fossils

    Take your pick, there are many, and the lines get less blurry every day.

  3. Not really what I was looking for, Keri. Try reading the question again .. :)

  4. So .. let me get this straight. You’re trying to provide evidence of this blurry line between species that shows evolution and the best you have are pairs of birds and snails?

    Let me set something of a standard for you to attain. You believe giraffes and camels have a common ancestor, right? What creatures form the blurry line between those two species (there’s no way they could ever reproduce).

    Instead of pointing to snails and birds that can reproduce show us an example of two distinct species that is linked by a “blurry line” of intermediate species. Quit looking for the missing link in the fossil record and show us the myriad that should be alive today.

  5. We aren’t talking about pairs of anything. Evolution happens to populations (with natural selection acting on the individual). But I digress.

    I’m talking about blurry lines. As I explained above, that means that a mother [insert Species X] will always give birth to a daughter or son [insert Species X]. In other words, a mother wolf will always give birth to a daughter wolf.

    Going with that specific example, the genes in that mother wolf become less and less like the genes of her descendants as time passes. In theory, the mother wolf, or any mother wolf from her generation, would be able to mate with a descendant, say, two generations away, assuming everyone was alive to do so. We can say this with an extremely high degree of certainty. But what of three generations? Still, certainly. Four? Eight? 100? 1,000? 10,000? 100,000? At some point, a mother wolf will not be able even in theory to mate with her descendants. The differences accumulated will be much too great. As it happens, this is the case with modern day dogs, though probably in fewer than 100,000 generations.

    Okay, so let’s examine further. Where can we say that the mother wolf cannot mate, in theory, with her descendants anymore? Is it precisely 10,049 generations down the road? If so, what about the rest of her generation? Some wolves will be able to mate (again, in theory) with their descendants at that point while others can not. The lines aren’t clear except in hindsight.

    So the snails offer a current example of this blurry line. In essence, they are two species because they basically never interbreed. However, there may be some crossover. And even if there isn’t, the potential for it is still fairly high. No one can say, “Okay, these are the same species on Tuesday, but come Friday we have two species.” The concept isn’t so clear-cut.

  6. So you cannot answer my question?

  7. The whole point I’m making is that evolution is gradual. It is only in hindsight and through a forever incomplete fossil record that we can point to two organisms and deem them separate species. So when you ask me to show you the blurry line between two random examples, you’re entirely missing the point. Please. Reflect on that for a moment. I know you do not understand what is being said here.

  8. Why is it only in hindsight? Why is evolution not happening today? Surely you can point to one of these “blurry lines” between two species that are more diverged than a pair of snails that can interbreed.

  9. Evolution is happening today. It only stops when a species goes extinct. I’m saying that determining what is and isn’t a species is done in the convenience of hindsight. The reason is that if we watched the replay of the entire evolution of any given species, we wouldn’t be able to point to a single generation and say “Here’s a dinosaur and THERE! Now they’re birds”. We’d need to look at wide expanses of time, which is what the fossil record gives us (and what we should expect it to give to us).

    If you’d like another line, even though those snails illustrate my point best, go mate a horse and a donkey.

  10. Mules are your next evidence?

    I’d have stuck with snails. At least they can continue their line…

  11. That’s the very point. The line between horses and donkeys is somewhat blurry because they can still breed. But that offspring is not fertile. There’s clearly a close relationship, but gradual change has added distance. I’d wager that it would take just a few thousand more years for donkeys and horses to have no ability to bear mules.

  12. So where is an example of two species that have crossed this thousand year bridge and can no longer interbreed?

  13. I imagine ring species satisfy this question. This is where several closely related populations, often birds, are able to interbreed, but the two bookend populations cannot.

    Wikipedia has a sufficient example.

    A classic example of ring species is the Larus gulls circumpolar species “ring”. The range of these gulls forms a ring around the North Pole.

    The Herring Gull L. argentatus, which lives primarily in Great Britain and Ireland, can hybridize with the American Herring Gull L. smithsonianus, (living in North America), which can also hybridize with the Vega or East Siberian Herring Gull L. vegae, the western subspecies of which, Birula’s Gull L. vegae birulai, can hybridize with Heuglin’s gull L. heuglini, which in turn can hybridize with the Siberian Lesser Black-backed Gull L. fuscus. All four of these live across the north of Siberia. The last is the eastern representative of the Lesser Black-backed Gulls back in north-western Europe, including Great Britain.

    The Lesser Black-backed Gulls and Herring Gulls are sufficiently different that they do not normally hybridize; thus the group of gulls forms a continuum except where the two lineages meet in Europe.

  14. they do not normally hybridize

    Fail.

  15. That fact is precisely why this example works, Grant. These different species are still closely related enough where they can interbreed. That is, until one reaches a certain point.

    It goes A>B>C>D, but not A>D. The lines are close between A and B and B and C, and even A and C may be able to interbreed, but stretch that line just a little further and A and D cannot produce offspring. The evolution of A and D is too different to allow interbreeding any longer.

  16. The example you give is of two species that can still interbreed. I asked for an example where interbreeding was a definite no-go (like the giraffe and the camel) that you were willing to point to.

  17. I’m not sure you’re getting what I’m saying. At the very least, I’m not sure I get what you’re saying.

    Do you want a complete fossil record linking two non-breeding species? If so, the horse fossil record is one of the best and links modern horses with their ancestors (with which they could not interbreed if we could bring them back to life). Otherwise, I don’t know what you want.

  18. Two living species that form either extreme of the blurry line you’ve described.

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