The River of Evolution

THE typical creationist has only the most infantile concept of evolution, so it’s not surprising to hear them criticizing the theory by claiming that “Darwin says” animals change from one species into another, yet this is never seen to happen, so Darwin was wrong.

It’s amazingly easy — even for a creationist — to show that a wrong idea is wrong; but there’s no triumph in that. How can we get such people to understand the correct idea of evolution? At least they’d know what they’re rejecting.

The concept isn’t difficult to grasp, so it can’t be the case that most creationists are incapable of learning it. Some are the victims of misinformation, so it’s worth the effort to develop a tactic or two which might get the correct idea across to them. (We’re talking here about younger people; adult creationists are almost always beyond reach.)

The creationists are correct when they observe that individual organisms don’t change. No biologist claims otherwise. A creature is what it is. In fact, the entire population of which an individual is a member doesn’t change either. You can corral a herd of zebra and keep them confined all their lives, and not one of them will change into something else. You won’t end up with a corral full of penguins, so in that sense the creationists are right — one species doesn’t literally change into another. Dracula is fiction. There are no sudden leaps of “macro-evolution” — if that’s what the term is supposed to mean.

Okay, we’re all clear on that. So how do changes happen?

What does change is the gene pool. Each act of procreation introduces mutations into the offspring’s genome. Most are harmless, some cause defects, and some are beneficial. Virtually all mutations are so trivial that the mutated offspring, if they grow to adulthood, are still able to mate with others in the population. There won’t be any “lonely new creature” unable to reproduce until “chance” provides him with a mate. From one generation to the next, although each individual has unique mutations, they all constitute a breeding population.

Natural selection acts like an invisible genetic filter. Not every individual that is born will survive long enough to reproduce. In some species the percentage that do so is quite small. The occasional individual born with a beneficial mutation (disease resistance, a more usefully shaped fin or claw, etc.) is likely to survive and produce offspring — which offspring will be similarly advantaged, and that will benefit their progeny, etc. The disproportionate success of those individuals means that the whole species may eventually be endowed with this mutation. At the same time, individuals with disadvantageous mutations will be less likely to survive and pass those mutations on to the next generation.

In this way, the genetic inventory of the population is subtly modified with each new generation. Two points should be noted: (1) although mutations are unpredictable, the selection process isn’t random — given the environment in which a species exists, some mutations have a far greater likelihood of persistence than others; and (2) the process is self-executing — it requires no externally imposed plan or goal.

Now that we have the concept of a species’ continuously modified genetic inventory, let’s attempt an analogy. Like all analogies, ours isn’t perfect, and like most of them, it may not be very good. But we’ll give it a whirl.

Instead of the ever-changing genetic inventory of a species, think of a river. And instead of being miles long in physical space, this is a river in time — it flows through the centuries. It’s millions of years long, but we can’t see that. We can only observe today’s segment. It probably looks like a pond.

Through the unseen millennia, the river develops forks and it branches off in different directions, each one coursing through different terrain. The result is that in the present we see many apparently unconnected bodies of water; but if we could see the entire history of the river, then we’d know that each pond flows from a common source.

Each little pond is analogous to a single species — one breeding population — and its history is invisible to us. That’s the view of our ancestors — a world full of ponds, many quite varied, and all seemingly unrelated. Okay, where do we go from here?

We need a few adjustments to make our analogy work. Imagine that you have a god’s-eye view of the river’s past. And instead of mutations, think of things that fall into the river — landslides, tree trunks, etc. The sandy bottom is always filtering the water, so some new material is always being added, and some other material is being subtracted. Sediment in the water is a simple analogy to a species’ constantly changing genetic inventory.

At any point along the river you can dip in with a cup and obtain a sample of water for analysis. It’s always the same river, but the precise contents of each cup are never exactly the same. Wherever you dip in, there are some minute particles that have been added, and some that have been filtered out. Each cup of water you examine is the cumulative result of the history of the river up to the point when you dip your cup.

If you take samples from adjacent locations along the river bank, they’ll be almost identical, but not quite. Samples taken further up or down river will show greater changes. If the interval between samples is great enough, it might even seem that they came from different rivers — except that you’ve been gifted with the ability to see the entire river at once.

If you lined up a great number of samples from the entire length of the river, you could construct a history of what’s happened along the way. But each separate cup of water is … well, it’s a snapshot of the river’s contents at one precise moment, and that doesn’t change. Yet each cup is “transitional” between the samples taken before and after it.

Now we’ll change our analogy — we’re taking away your special vision so that you can’t see the entire river. But you do have a growing collection of water samples from the past that have been preserved. There are large gaps in your sample collection, but more are being found all the time. Newly-discovered samples readily fit into those gaps and improve your understanding of the river’s history.

That, as you’ve guessed, is the fossil record. We don’t have a god’s-eye view of the river, but we have enough samples to put together a pretty good map.

As we said in The Lessons of Tiktaalik, with enough fossil specimens, each is like a single frame in time-lapse photography, or a page in a flip-book.

Will any creationist be helped by the river analogy? Alas, probably not. But there may be an occasion when it’s worth a try.

Copyright © 2009. The Sensuous Curmudgeon. All rights reserved.

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10 responses to “The River of Evolution

  1. That explains micro-rivers, but what about macro-rivers?

  2. Gabriel Hanna

    This is a very good analogy.

    Another think to think of is that species are defined according to human convenience; and if every single organism that ever lived could be assembled in one place ant one time it would be impossible to classify them into species in anything other than an arbitrary way.

    For example, shrews in Spain can’t interbreed with shrews in East Asia, but shrews in Spain can interbreed with those in France, and those in France can interbreed with those in Germany, and so on until you get to East Asia. So where do you draw the line that divides the shrews into “species”? It’s kind of a stupid argument, it’s like deciding what colors should be considered “black”, “white”, or “grey”, or how to divide men into “bald” and “not-bald” by counting hairs. Anything you choose is a matter of convenience and somewhat arbitrary, just like American law divides people into “children” and “adults” on their 18th birthday.

  3. Gabriel Hanna says: “This is a very good analogy.”

    Thanks. Watch for my next one — Gravity is like a woman.

  4. Because you fall for her every time?

  5. Because the attraction is inversely proportional to distance.

    (Yeah, I know about the inverse-square law, but that didn’t make a good sentence.)

  6. Not too bad, and any faults are more in our human limitations of language than the concept of evolution. But don’t expect much. Creationists are far too fond of slaying their imaginary monster of evolution for the braying of their sheep to ever give it up for something like reality.

  7. SC: time’s river — a brilliant analogy; I understand aspects of evolution and genetics now that I did not. Also thanks to G Hanna for clarifying things.
    Keep up the good work and Merry Christmas to all skeptics.

  8. retiredsciguy

    SC: “Each act of procreation introduces mutations into the offspring’s genome.”

    It’s not mutations so much as the mixing of genes in new combinations that you get with sexual reproduction that drives evolution.

    Sexual reproduction first appeared around the beginning of the Cambrian, right? The Cambrian Explosion is most likely a result, not a coincidence.

  9. retiredsciguy

    Forgot to mention — this is a good analogy, Curmy. When you think of it this way, it’s easy to see that *every* species in the fossil record is a transitional species.

  10. retiredsciguy says:

    … it’s easy to see that *every* species in the fossil record is a transitional species.

    Except for throw-backs like creationists, who are retro-transitioning back into the ooze.