image by Mike Rosulek
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It’s a classic question: if Charles Darwin had known about Gregor Mendel‘s genetic research, would Darwin have realized it was the missing piece he needed to explain how individual variation was inherited and selected? Was it simply bad luck that Darwin never stumbled on the right experiments? Or was Darwin so constrained by his own perspective on inheritance that he couldn’t have seen the importance of Mendel’s work, even if he had known about it?
Jonathan Howard has written an intriguing overview of this question. He argues that Darwin was on the brink of discovering Mendel’s Laws several times – if he had only been looking for them. Darwin, it turns out, did have data much like Mendel’s, from Darwin’s own plant breeding experiments:
In one especially poignant case , working with the recessive character of radially symmetrical (peloric) flowers of Antirrhinum (Figure 4), Darwin came close to the kind of result that might have ended with a law of segregation. He crossed pure-breeding peloric plants with pure breeding wld types, noting the dominance of the wild type in the F1 progeny. He then established the F2 generation and obtained wild-type and peloric plants in a ratio (88:37) that Mendel (and now we) would effortlessly accept as representing 3:1. However, Darwin had other priorities and was in no way programmed to see the critical meaning in these numbers. He cites them within a sentence and they receive no further comment.
According to Howard, Darwin didn’t see the Antirrhinum results as particularly relevant to natural selection. He was more concerned with continuous, quantitative variable traits – height, skin color, intelligence, strength, etc. He saw subtle differences in those quantitative traits as the fodder for natural selection, as attested by one of his most famous passages from The Origin of Species:
There is a simple grandeur in this view of life . . . that from so simple an origin, through the process of gradual selection of infinitesimal changes, endless forms most beautiful and most wonderful have been evolved. (emphasis added).
“But wait,” you might say, “that’s not the quote I know!” You’re right – this is the 1842 version of the passage. The better-known 1859 version is simpler, more elegant, and more rhythmic (I always sense echoes of the King James Bible in Darwin’s work):
There is a grandeur in this view of life. . . that. . . from so simple a beginning endless forms most beautiful and most wonderful have been, and are being, evolved.
Although the passage evolved (heh heh), it’s worth remembering that when Darwin wrote it, he was thinking of “infinitesimal changes” in continuously variable traits as the basis for those “endless forms most beautiful.” It was quantitative traits he saw as the key to natural selection – not discontinuous, qualitative traits like flower shape or pea color. As Howard puts it, “although artificial selection of such anomalous variants could provide an analogy to evolution by natural selection, this was not the real thing.”
Darwin had a point. The traits geneticists struggle to understand today – intelligence, personality traits, risk for diseases like hypertension – are not simple dominant/recessive characters, but complex traits, to which many alleles make small individual contributions in conjunction with environmental factors (and gene x environment interactions). Complex traits are not the exception, but the rule.
But these traits are also the most refractive to genetic analysis – so challenging that genome-wide association studies on thousands of individuals may be required to tease out the tiny contributions of different loci. Complex traits made terrible subjects for experiments to understand the basic mechanisms of inheritance: it would be like trying to learn the principles of electrical circuitry by taking your laptop apart.
The argument Howard makes – that Darwin’s personal philosophical bias prevented him from recognizing the genetic principles to which he was so tantalizingly close – is plausible but hard to sew up. After all, it wasn’t just Darwin who failed to make that leap. His peers didn’t either. Mendel’s work languished in obscurity for years, until the genetic paradigm shifted around the turn of the century. As Howard points out, Darwin’s brilliant cousin Francis Galton (another pea-breeder) was at one point practically on top of Mendel’s Laws:
While Darwin was wrestling with pangenesis Galton was pioneering the analysis of the inheritance of quantitative characters and he documented with extraordinary insight the properties of such inheritance. In particular, he documented the phenomenon of regression to the mean in the context of the inheritance of quantitative characters such as height or intelligence. Regression to the mean records the interaction between control of the character by multiple polymorphic loci of small effect and, of course, multiple environmental effects. Galton approached a correct genetic interpretation of this phenomenon while Darwin confused it with blending inheritance. He also took a lively interest in pangenesis (which he eventually rejected, to Darwin’s chagrin) and corresponded extensively with Darwin about it. In a somewhat gnomic response to a query from Darwin, Galton replied “If there were two gemmules only, each of which might be white or black, then in a large number of cases one-quarter would always be quite white, one-quarter quite black, and one half would be grey”.
(Getting warmer! Warmer! You are SO CLOSE RIGHT NOW!!!)
Were both Galton and Darwin so set on quantitative traits that they couldn’t see the laws revealed by qualitative traits? It’s hard to imagine that such brilliant men, thinking so hard about the mechanisms of inheritance, could both miss something so simple. But perhaps it’s hard to imagine because, for modern geneticists, Mendel’s Laws are child’s play (I always wanted to decorate a nursery in Punnett Squares – they’d lend themselves quite nicely to quilts). Mendel was intelligent, a sharp observer, and, given what we now know of molecular genetics, extremely lucky to select well-defined single-gene traits that sorted independently (so lucky, some people have accused him of massaging his data). We almost can’t help underestimating how revolutionary Mendel’s observations were, and how even the most subtle bias could have thrown another researcher off the track. Who knows what fundamental observations are out there right now, obscured by a researcher’s preconceptions?
Figure 4: Wild-type and peloric flowers of Antirrhinum majus
(photo by Enrico Cohen)
Reference: Jonathan C. Howard. Why didn’t Darwin discover Mendel’s Laws? Journal of Biology 2009; 8:15. (free registration required)