Gemmules and Blended Inheritance

GEMMULES

One of the biggest problems Darwin had with his theory was how the various changes occurred and how they were inherited. With no knowledge of genes, mutations, and all of what we call genetics, he had great trouble in explaining things.

He could not explain how new variations arose or how they were inherited. He seems to have tended to believe in a form of Lamarckian evolution: that continued use or disuse of parts would in some manner affect how they were inherited. Since he believed in “blended inheritance” he could not see how the origin of favorable variation in a single organism could spread throughout the species without becoming so diluted with the original form as to lose its identity.

What is really amazing about Darwin, is how with all the theoretical reasons against natural selection, he still believed it to be the correct reason for evolution. He believed it because of what he had observed in the real world. He seems to have been capable of remembering much of what he observed and identifying patterns. Practical experience overruled theoretical objections. He didn’t know how inheritance worked, only that it did.

When Darwin returned to England after his around-the-world voyage on the H.M.S. Beagle he set out to research some of the ideas he had developed on the voyage. One of the things he did was to talk to animal breeders, zoo keepers, game wardens, and anyone who had experience with animals and plants. He himself took up pigeon breeding. He soon learned from personal experience that animals varied in any number of traits. Each individual was not exactly like all the other individuals of the same species. Many of the differences were “normal” variation—different size or body conformations that were within a range of expected variability. However some variations were outside the normal range. They were new, or at least, never seen before. Darwin (and others) thought that these “new” traits might be “reversions” to a primitive, ancestral form. Darwin could not explain where these traits came from, how they occurred. He knew from personal observation that they did appear. He also knew that, at least some of them, were inherited. The appearance of a new character in a single individual could be transmitted to its offspring not as some diluted “blended” form, but in its entirety. Theory predicted that a new trait would combine with the original form of the trait to produce an intermediate form, which over the generations would become so diluted to effectively disappear. His (and others) practical experience showed that somehow reality didn’t follow theoretical predictions. A single individual with a particular trait could be crossed with an individual without the trait, those offspring crossed, the trait “fixed” and an entire breed that consistently bred true for the new trait developed.

Without knowing how genetics worked, without an understanding of genetics, Darwin was able to see how it functioned in the real world and extrapolate the greater consequences of it.

Darwin thought that the theory of pangenesis might be correct. The theory was that each cell in an organism produced “gemmules.” These contained information about that particular cell. Gemmules drifted about the body, collecting in the reproductive cells (eggs or sperm depending on the sex of the organism). During reproduction the gemmules passed to the newly fertilized egg and influenced the development of the embryo, somehow “telling” the appropriate cells just what to do. The mature offspring in turn produced their own gemmules to influence their own offspring. Darwin, if I remember correctly, referred to them as some sort of “information atom.”

Interesting idea, wrong of course, but interesting especially if you “reversed it.” Imagine if the fertilized cell contained “gemmules” that, as the embryo developed entered every cell and “told” each one how to develop and some of the gemmules gathered in the future sex cells to be passed on to the next generation, to influence the development of that generation. Sounds a lot like genes and DNA to me.

BLENDING INHERITANCE

Fleeming Jenkins attacked Darwin”s theory because any new trait would blended with the old trait during reproduction and would quickly disappear. Suppose that you were growing a garden of plants that had white flowers and among these flowers there appeared one identical in all aspects except that it had red flowers. Darwin didn’t know where this new characteristic came from. He talked of “reversion to a primitive type,” or maybe subtle differences in temperature, soil, etc. as a cause. He only new for sure that organisms were capable of producing new characteristics. According to the concept of blending inheritance, crossing that red-flowered plant with a white-flowered plant would produce a pink-flowered plant. Crossing that pink-flowered plant with another white-flowered plant would produce a pale pink-flowered plant. This would continue for generations producing even paler pink flowers until they were virtually white. The new character would be blended with the original character and since the original character existed in such large numbers relative to the new character, the new character would become so attenuated over the succeeding generations to virtually disappear. Even if you crossed a pink-flowered plant with another pink-flowered plant all that you would get is a pink flower and only slow down the inevitable.

This works with animals far better than with plants since plants can self-fertilize (in most cases) so the red-flowered plant could cross with itself to produce more re-flowered plants. Animals are obligated to “cross” with another individual. Theoretically with plants that can self-fertilize a single plant with a new characteristic could cross with itself and its decedents cross with themselves (or their siblings) thus forming a new variety without “blending” with any other individual with the original characteristic. However given the large number of plants with the original characteristic, the chances are that the red-flowered plant and its offspring would cross with others that were white-flowered far more often than with a red-flowered, or more likely a pink-flowered plant. It would just take a few generations more before the blending eliminated the new character. This was part of the problems Darwin had with understanding how a species could evolve. He didn’t know how a new character could arise and with the concept of blending inheritance he could not see hoe a new character arising in one organism, especially an animal, could possibly spread throughout the population without being diluted beyond effect. He only knew that new characters could appear and that they could be inherited with undiminished effect to the decedents of the organism. He had seen it happen.

It was not random chance that picked red and white flowers of a plant as an example. I picked it deliberately. Gregor Mendel did the first experiments in genetics and he used the common garden pea. He focused his attention on seven traits, one of which was flower color—red or white. The traits he chose may have been chosen because they gave clear results. We now know that all seven traits1 are examples of pairs of dominant and recessive alleles. In peas, red is dominant over white. Had Mendel chosen flower color in snapdragons he would have gotten a different result.

Had he crossed a red-flowered snapdragon with a white-flowered snapdragon he would have gotten a pink-flowered snapdragon (as the blending theory predicted). Beyond this he would have gotten (as those who have done this experiment have) different results than blending theory predicts. Red × red always yields red, white × white always yields white, and red × white yields pink. So far, so good. Red × pink yielded half red and half pink. White × pink yields half white and half pink, and pink × pink yields, and pink × pink yields ¼ red, ¼white, and ½ pink. The pinks were not darker or paler than the original pink. There was a blending of the two colors but whatever carried the flower color did not blend, it remained separate to be passed on to the next generation unchanged (which is also one of Mendel’s conclusions). Only their effect blended. In snapdragons, the alleles for flower color are co-dominant.

1 Round vs wrinkled seeds, yellow vs green seeds, green vs yellow pods, inflated vs constricted pods, long vs short stems, axial vs terminal flowers, and red vs white flowers.

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