Why I am an evolutionist, part 2

Mendel: how heredity works

Charles Darwin published On the Origin of Species By Means of Natural Selection in 1859. His theory of evolution generated enormous controversy, even within the scientific community.

The problem of dilution

One of Darwin’s critics, Fleeming Jenkin, threw down a challenge that Darwin could not answer. Jenkin argued that a single variation (i.e., a novel and potentially beneficial trait) would disappear in just a few generations.

If Jenkin was right, Darwin’s theory could not be true. The theory presupposes that variations persist generation after generation, providing the raw material upon which natural selection can work.

To illustrate the force of Jenkin’s argument, imagine yourself with a pitcher of red juice and a pitcher of water. Mix them together and what do you get? Pink juice. That was how Jenkin understood heritability. When a baby is made, it represents a blending of the traits it inherits from its two parents.

Now imagine one pitcher of red juice among 999 pitchers of water. The red juice represents a variation — a unique trait — in a given population of animals. But only one animal (or perhaps a few) possesses the innovation. In subsequent generations, the trait would be diluted to such an extent that it would disappear.

To return to our analogy, you wouldn’t be able to discern any red at all after mixing one pitcher of red juice with 999 pitchers of water.

Jenkins cited the hypothetical example of a white man cast away on a tropical island populated only by black people. No matter how industriously the castaway impregnated the native women, he pointed out, the population would never become white. Indeed, his presence there would, in a few generations, be obliterated.1

Darwin was stumped by Jenkin’s argument. The problem was, he didn’t know how variation worked (as I pointed out in my previous post on evolution).

During Darwin’s lifetime, cytology — the branch of biology that deals with the formation, structure, and function of cells — was barely in its infancy. Darwin knew nothing about DNA, chromosomes, or genes; neither did anyone else of his era.

Mendel’s discovery

Only six years after The Origin of Species was published, Gregor Mendel made a key discovery about how heredity works. But scientists didn’t “get it”; they completely overlooked Mendel’s research at the time. Darwin died without knowing what Mendel had discovered. It would have supplied him with a response to Jenkin’s challenge.

Mendel demonstrated that variations persist, just as Darwin’s theory presupposed; variations are not blended and diluted.

Let’s revisit our thought experiment. Imagine yourself with six pitchers of red juice and six pitchers of water. You mix them together and — much to your amazement — you end up with twelve pitchers of red juice. You don’t understand how it is possible, but the juice has not been diluted.

Now you pour the twelve pitchers of juice into a big bowl, before pouring it back into the pitchers. This time — unbelievably! — you end up with nine pitchers of red juice and three pitchers of water. This is the very thing that happened in Gregor Mendel’s experiment.

Mendel carefully bred yellow peas with green peas. Every one of the next generation of plants was yellow. The green was there (in the plants’ genes) but it could not be seen.

Then he bred those apparently yellow peas with one another. The result:  in the second generation, three quarters of the plants were yellow; the other quarter, green.

Genes do not blend; they are not diluted. Once a variation is introduced, it persists in the population — even if it is hidden.

further explanation to follow

~~~~~~~~~~~~~

1Blueprints: Solving the Mystery of Evolution, by Maitland Edey and Donald Johanson, p. 98.

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5 Comments (+add yours?)

  1. reportcard
    Nov 29, 2006 @ 16:09:00

    A fantastic explanation…Thank you,

    I’ve never heard Mendel’s experiment illustrated so clearly before. Looking forward to further details.

    Reply

  2. Jamie
    Dec 02, 2006 @ 19:05:07

    I agree with reportcard; that was a great illustration of Mendel’s experiment.

    I think the big issue with Mendel is he only proves that variations remain in the gene pool even if they are hidden for several generations. What he doesn’t show is how new information could get there in the first place. And Darwin doesn’t show that either. Of course there are random variations in genetic material, but the problem is showing that these variations result in an increase in information/variation rather than a decrease. Mendel provides no support to evolutionary theory unless someone can show that there is actually new genetic information being introduced into the gene pool.

    Reply

  3. Stephen
    Dec 03, 2006 @ 17:05:05

    • Reportcard:
    Thanks, but I can’t take all of the credit. The book I’ve been citing, Blueprints, uses a similar illustration. But I tweaked it more than a little, so some of credit is legitimately mine.

    • Jamie:
    The problem is showing that these variations result in an increase in information / variation rather than a decrease. Mendel provides no support to evolutionary theory unless someone can show that there is actually new genetic information being introduced into the gene pool.

    I’m not entirely sure what you mean by increase and decrease, but I’ll respond as best I can.

    There is no such thing as new genetic information. DNA consists of four nucleotides — that’s it. (Presumably you know this already.) The four nucleotides, “read” in combinations of three, instruct for the construction of twenty amino acids. The amino acids then join together to make protein molecules, which are the building blocks of all life. (So we’re told.)

    It defies belief, but all the vast variability of life arises from the same four nucleotides, combined in different sequences. That data is perfectly consistent with Darwin’s claim that species are mutable and we have all descended from a common ancestor. Darwin knew nothing about genetics, but as it turns out, we’re all built from identical components.

    I could speculate further, but I’ll stop there. If I’ve completely missed the point of your question, presumably you’ll return to clarify it.

    Reply

  4. Jamie
    Dec 03, 2006 @ 21:47:34

    I realize that all variation arises from just four nucleotides, but not every combination of those nucleotides and amino acides makes for information.

    It’s similar to the idea that every sentence I write is composed of the same 26 letters of the alphabet, but not every combination of letters makes sense. zvbnm qewrui fj jhshdkq. Case in point. This current sentence constitutes information (because it communicates something meaningful), but that other gobbldygook is not information (because it has no meaning).

    And this is where creationists object. Random variation can produce new goobldygook, but can it produce new and increasingly complex information?

    Reply

  5. Stephen
    Dec 04, 2006 @ 09:52:13

    • Jamie:
    To carry on a dialogue built on metaphors is a doubtful way to proceed. Any metaphor can be pushed too far. But I’ll continue in the direction you indicate to make a small point.

    Not many decades ago, if I had written “deoxyribonucleic acid”, it would have constituted gobbledygook. Now it has meaning, as in DNA. That combination of letters is now deemed meaningful because it is useful to biologists.

    I think genetic variation is similar. Many times, mutations are harmful. Most of the time, they are neutral — conferring neither an advantage nor a disadvantage. Very, very occasionally, a mutation may confer an advantage. When that happens, the mutuation may prosper and come into widespread use.

    Based on your recent comment to Juggling Mother, I think I understand where your argument is going. You accept adaptation, but you deny wholesale evolution. So, for example, if woodpeckers gain a significant advantage from having longer beaks, we might expect natural selection to produce woodpeckers with longer beaks over time. Darwin amassed lots of evidence of that sort — but it doesn’t prove evolution.

    The question is whether one species can change into another; whether one can begin with a single-celled, prokaryotic organism (lacking a nucleus), and proceed step-by-step all the way to homo sapiens. We are so complex as to possess consciousness — surely a fundamentally different animal! (you would argue).

    Here I am at least sympathetic to your position. I intend to offer evidence in support of the sort of change you’re asking about before this series is done. But I continue to doubt that naturalism is an adequate explanation of the cosmos and the life within it.

    I have two quotes that you may find useful:

    “Three of the greatest perplexities are these. First, why is there something rather than nothing? How is it possible that there is anything at all? Second, how is it possible that among the stuff that exists there is life? Third, how is it possible that some living things are conscious?” (Owen Flanagan — himself a naturalist — The Science of the Mind, 2nd ed., chapter 8.)

    I tend to emphasize those three crucial innovations: from nothing to something; from inanimate matter to life; and from instinct to consciousness. With respect to those “perplexities”, naturalists are carried into the realm of speculation and even faith: whereas there is some evidence to support the transition from one species to another.

    “The most recent research has shown that the closer neuroscientists analyse the functions of the brain, the less they actually understand, in light of the usual models, central aspects of the consciousness. So, the prophesied explanation of the relation between brain and consciousness is not, now many claim, to be expected at all. …

    “Brain research offers, at this time, no empirically provable theory about the coherence of spirit and brain, of consciousness and nervous system.” (Hans Kung, quoted on Chris Tilling‘s blog.)

    Reply

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