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.
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.