Wednesday, February 12, 2020

We All Love Our Hypotheses: A Story of Charles Darwin

On this day in 1809, both Abraham Lincoln and Charles Darwin were born. Here’s to the next 211 years of freedom and science!

And here is a little story about Charles Darwin.

All of us, especially scientists, love our hypotheses. But what scientists try to do, and often succeed at doing, is to test the hypotheses and then let go of those that do not pass the test. This is why we consider ourselves “objective” (rather than subjective), and this is one of the major ways in which scientific thinking differs from non-scientific thinking.

But, as individuals, scientists often find it difficult to let go of cherished ideas. This is why science is a community; one scientist might cling to a useless hypothesis, but the other scientists can challenge him or her.

One good example of a scientist clinging to a cherished hypothesis was Charles Darwin. I do not refer to evolution; he was right about that. But at the time Darwin wrote the Origin of Species, and for the rest of his life, Darwin did not know how traits could be passed on from one generation to another. That is, neither he nor anybody else understood heredity. Well, one man did: Gregor Mendel. But Mendel did not know that he knew. He died, not knowing that every biology student in the world would learn his name.

Here is why this was such a problem for Darwin. A scholar named Fleeming Jenkin wrote a criticism of natural selection. Jenkin invited us to imagine a situation where a new, superior, but rare hereditary variation was introduced into a population. No matter how good it was, it would get swamped out by the rest of the population before natural selection could save it. Jenkin used a violently racist example which was, we now regret, common in his day.

If heredity acts like a paint pot, then Jenkin’s argument cannot be answered. A drop of white paint will disappear in red paint and is lost forever. But Mendel discovered, and Darwin believed, that traits were not like paint. They were more like (to use my analogy) marbles. White marbles can mix in with red marbles, but retain their individuality, and can someday show up again. Rare traits retain their individuality and can become common. But how?

Darwin, in the course of writing a big book Variation in Domesticated Plants and Animals, came up with an explanation. He believed that all parts of the body produced what he called gemmules, and these gemmules found their way, presumably through the blood, to the reproductive organs, where they were passed on to the offspring. The circumstances of life can cause organs to change the kinds of gemmules they release. This is how acquired traits can be inherited, in Darwin’s theory of pangenesis.

Darwin was not the only person who was excited about pangenesis. So was his younger cousin Francis Galton, often called the childless father of eugenics. Galton knew more about heredity than Darwin; in fact, Galton gathered lots of inheritance data for his book Hereditary Genius. He also contributed to the early development of statistics. He invented the correlation coefficient in statistical regression. And he was ready to subject pangenesis to an experimental test, which he was sure would confirm the theory.

Galton used different breeds of rabbits, with recognizably different coat colors. He took blood from one kind of rabbit and injected it into the bloodstream of a different kind. He was careful to not harm the rabbits, even though he transfused up to one-third of their blood volume. Galton expected that this blood, full of gemmules, would cause the rabbits to have offspring with the characteristics not of their parents but of their blood donors, at least sometimes.

All of the rabbit-blood experiments failed. Galton was not sure what to do next. Darwin was, however, sure. Darwin simply made a post-hoc rationalization. Triumphantly, Darwin said that the gemmules must find their way to the generative organs through some medium other than blood. This assertion led to no further experiments. Pangenesis theory declined into oblivion, and today we all learn about Mendelian genetics.

Darwin was more objective than almost every other scientist of or since his day. But, like any human, he was imperfect. Objectivity failed him in the matter of his beloved pangenesis theory.

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