The Beginnings of Genetics. The Gap in Darwinian Theory. Mendel’s Peas

The reason why it was so easy to misapply evolutionary theory was that the nature of the hereditary mechanism was not understood in the nineteenth century. Spencer could imagine rapid changes in human behavior, and Galton could imagine improving the race by a quick and easy program of selective breeding, out of an ignorance they shared with biologists generally.

In fact, the lack of understanding of the nature of the hereditary mechanism was the most deplorable weakness of Darwinian theory. Put briefly, the weakness was this: Darwin supposed that there were continual random variations among the young of any species and that some variations would better fit an animal for its environment than would others. The young giraffe who happened to grow the longest neck would be the best fed.

But how could one be certain that the longest neck would be passed on? The giraffe was not likely to seek out a long-necked mate; it was as likely to find a short necked one. All Darwin's experiences with the breeding of animals led him to suppose that there was a blending of characteristics when extremes were crossed so that a long-necked giraffe mated with a short-necked giraffe would give rise to young with medium-length necks.

In other words, all the useful, well-fitting characteristics that were introduced by random variation would average out into an undistinguished middle ground as a result of equally random mating and there would be nothing upon which natural selection could seize to bring about evolutionary changes.

Some biologists made stabs at explaining this away, but without much success. The Swiss botanist, Karl Wilhelm von Nageli (1817-91), was an enthusiastic supporter of Darwinism and recognized the difficulty. He supposed, therefore, that there must be some inner push that drove evolutionary changes in a particular direction.

Thus, the horse, as was known from the fossil record, was descended from a dog-sized creature with four hoofs on each foot. Through the ages the descendants grew continually larger and lost one hoof after another until the modern large, one-hoofed horse was developed. Nageli felt that there was an inner drive that moved the developing horse constantly in the direction of larger size and fewer toes and that this might be continued even to the point of harm so that horses might become too large and clumsy for their own good. Unable to escape from their enemies, they would then decline progressively in numbers and become extinct.

This theory is called "orthogenesis" and it is not accepted by modern biologists. However, its existence in Nageli's mind proved unexpectedly harmful as we shall now see.

Mendel’s Peas. The solution to the problem, one which is now accepted, arose through the work of an Austrian monk and amateur botanist, Gregor Johann Mendel (1822-84). Mendel was interested in both mathematics and botany and, combining the two, studied peas statistically for eight years, beginning in 1857.

Carefully, he self-pollinated various plants, making sure in this way that if any characteristics were inherited, they would be inherited from only a single parent. As carefully, he saved the seeds produced by each self-pollinated pea plant, planted them separately, and studied the new generation.

He found that if he planted seeds from dwarf pea plants, only dwarf pea plants sprouted. The seed produced by this second generation also produced only dwarf pea plants. The dwarf pea plants "bred true."

Seeds from tall pea plants did not always behave in quite this way. Some tall pea plants (about a third of those in his garden) did indeed breed true, producing tall pea plants generation after generation. The rest, however, did not. Some seeds from these other tall plants produced tall plants and others produced dwarf plants. There were always about twice as many tall plants produced by these seeds as dwarf plants. Apparently, then, there were two kinds of tall pea plants, the true breeders and the nontrue breeders.

Mendel then went a step further. He crossbred dwarf plants with true-breeding tall plants and found that every resulting hybrid seed produced a tall plant. The characteristic of dwarfness seemed to have disappeared.

Next, Mendel self-pollinated each hybrid plant and studied the seeds produced. All the hybrid plants proved to be nontrue breeders. About one quarter of their seeds grew into dwarf plants, one quarter into true-breeding tall plants, and the remaining half into nontrue-breeding tall plants.

Mendel explained all this by supposing that each pea plant contained two factors for a particular characteristic such as height. The male portion of the plant contained one and the female portion contained the second. In pollination, the two factors combined and the new generation had a pair (one from each parent if they had been produced by a cross between two plants). Dwarf plants had only "dwarf" factors, and combining these by either cross-pollination or self-pollination, produced only dwarf plants. True-breeding tall plants had only "tall" factors and combinations produced only tall plants.

If a true-breeding tall plant were crossed with a dwarf plant, "tall" factors would be combined with "dwarf" factors, and the next generation would be hybrids. They would all be tall, because tallness was "dominant," drowning out the effect of the "dwarf" factor. The "dwarf" factor would, however, still be there. It would not have vanished.

Figure 3. Diagrammatic explanation of Mendel's work with tall and dwarf pea plants. The top illustration is the crossing of a true tall plant with a dwarf plant, resulting in hybrid (or nontrue-breeding) tall plants. Below, the crossing of hybrid tall plants which results in true tall plants, dwarf plants, and hybrid tall plants, in proportions of 1:1:2.

If such hybrids are either cross-pollinated or self-pollinated, they prove to be nontrue breeding because they possess both factors which can be combined in a variety of ways (dictated by chance alone). A "tall" factor might combine with another "tall" factor to produce a true-breeding tall plant. This would happen one quarter of the time. A "dwarf" factor might combine with another "dwarf" factor to produce a dwarf plant. This would also happen a quarter of the time. The remaining half of the time, a "tall" factor would combine with a "dwarf" factor, or a "dwarf" factor with a "tall" factor, to produce non- true-breeding tall plants.

Mendel went on to show that a similar explanation would account for the manner of inheritance of characteristics other than height. In the case of each set of characteristics he studied, crossing two extremes did not result in a blend into intermediateness. Each extreme retained its identity. If one disappeared in one generation, it showed up in the next.

This was of key importance to the theory of evolution (although Mendel never thought of applying his ideas to that theory), for it meant that random variations produced in species in the course of time did not average out after all but kept appearing and reappearing until natural selection had made full use of them.

The reason why characteristics often seemed to become intermediate after random mating is that most "characteristics" casually observed by breeders of plants and animals are really combinations of characteristics. The different components can be inherited independently and while each is inherited in a yes-or-no manner, the over-all result of some yeses and some noes is to lend an appearance of intermediacy.

Mendel's findings also affected the notions of eugenics. It was not as easy to eradicate an undesirable characteristic as one might think. It might not appear in one generation, and yet would crop up in the next. Selective breeding would have to be more subtle and more prolonged than Galton imagined.

However, the world was not to know of all this just yet. Mendel wrote up the results of his experiments carefully, but, conscious of his own status as an unknown amateur, felt it would be wise to obtain the interest and sponsorship of a well-known botanist. In the early i86os, therefore, he sent his paper to Nageli. Nageli read the paper and commented upon it coldly. He was not impressed by theories based on counting pea plants. He preferred obscure and wordy mysticism, such as his own orthogenesis.

Mendel was disheartened. He published his paper in 1866, but did not continue his research. Moreover, with-out Nageli's sponsorship, the paper lay disregarded and unnoticed. Mendel had founded what we now call genetics (the study of the mechanism of inheritance) but neither he nor anyone else knew it at the time.