Darwin’s dilemma – a monk to the rescue

EPS 106

Sept: 28^th, 1999

Darwin theory had a serious weak point. Natural selection happened, but why? If two parents were slightly different in some way, shouldn’t the offspring be intermediate between the two? And if this were the case, then any variations should average out over the generations, until all organisms looked the same?

One way out was to have a large jump. Radical idea? Not really. It is seen in artificial selection. Farmers would have offspring that were different. They were scared, believing it was divine intervention, to be viewed with suspicious and distrust. They were called ‘monsters’ from the Latin “to warn”. Another term is “sport”.

In 1791 a male lamb from the flock of Seth Wright in Massachusetts was born with short legs. Wright bred it, creating a herd of short-legged sheep (they couldn’t jump the fence). Later, the same characteristic appeared in Norway.

A Mr. Karl Wilhelm Von Nägeli, a swiss botanist, suggesting in print that evolution occurred as a series of jumps. Nägeli went so far as to say that there was a drive within a species for these jumps to keep it varying in the same direction. Once started, it continued faster than the ordinary mating level could even out. This explains the horses. It might even be that it overshoots – grows larger than desirable. The concept was ‘biological intertia’ and called orthogenesis.

Starting in 1886 Hugo De Vries started studying wild American evening primrose plants. He noticed that they sometimes underwent mutations (the new plant was quite different from the parents). Mutants is from the Latin ‘to change’. By 1900, he was ready to publish. It turns out that two other botanists had arrived at the same conclusions and were also getting read to publish. They were Erich Tschermak and Carl Erich Correns.

All three, working independently, did a thorough background search of the literature and found an obscure paper in an obscure journal by an unknown scientist that exactly described what they had found. Not only that by it had been published in 1866.

In The Proceedings of the Natural History Society of Brünn, by an amateur gardener of the name of Gregor Mendel! The three scientists gave Mendel all the credit, and it is still attributed to him.

In the 1860’s Mendel taught natural history at the monastery school in Brünn, Czechoslovakia) and tended the gardens. He amused himself by carefully crossing plants and observing the exact results.

Working with peas, he chose seven distinct characteristics. For example red and white peas. The red peas always produced red peas, and the whites always white. What if he crossed them. Would he get pink? Half red, half white?

He got all red upon crossing. Not one white flower. The white flower had disappeared? Then he crossed this new generation and found that there now were some white offspring. It turned out that exactly one-quarter were white.

Now, what would happen if he crossed these new whites amongst themselves. Only white offspring.

And what about the third generation reds? Some produced only red offspring, others produced one-quarter white offspring.

The white flowered plants always ‘bred true’, the reds sometimes ‘bred true’ and sometimes did not.

Mendel devised a theory which stated that each plant contained two factors which controlled a particular characteristic, such as color of the flowers. Today, we call this a gene from the greek word to produce.

A red plant would have a RR gene, and a white plant a WW gene. Mendel went further and figured out that only one of the genes from each parent would be transmitted to the offspring. Thus a RR would transmit one R gene, and a WW would transmit one W gene. Obviously RR crossed with RR would produce only RR offspring. Likewise, WW crossed with WW would produce only WW offspring.

If an RR was crossed with a WW plant, all offspring would be RW (they would get the R from the RR parent and the W from the WW parent). Yet they are all red, convincing Mendel that the R gene is dominant. The W gene (or allele, as one of two genes that governs a particular characteristic) is recessive.

But when we now cross the RW with itself there are four possibilities. Each parent could give either a R or a W gene. (Half the pollen grains are W and the other half are R). The four possibilities are

Pollen grain	Ovule	Offspring	Color of flowers
R	R	RR	red
R	W	RW	red
W	R	WR	red
W	W	WW	white

So ¼ should be white, as was observed by Mendel. In addition to flower color, he looked at wrinkled vs. smooth seeds, yellow vs. green seeds, tall stems vs. short stems. All of the characteristics followed those of the flowers, but each was independent from the others. They were inherited independently!

These results were a great help in explaining Darwin’s theory. Variations did not run the entire gamut of infinitely small changes. There were discreet jumps that existed. Second, there was no blending of inheritance. Red and white flowers, when crossed did not produce pink flowers. And finally, even when recessive, a gene was still there, unharmed and unmodified, to appear in a later generation.

Many characteristics are far more complicated than a single gene. For example, blood types may be controlled by as many as eight alleles (genes that control this characteristic feature). Mendel was fortunate that he chose a simple system. (Or he was smart enough to pick simple characteristics!).

Mendel wrote up his results and sent them to Nägeli. Nägeli was not impressed because he thought that Mendel was just counting plants instead of working on some fundamental new scheme like his own orthogenesis.

Bad break, for Mendel’s theory was of fundamental importance, while Nägeli’s was wortheless. But Nägeli had the reputation, Mendel did not. His work remained unknown and he himself unhonored.

Darwin died in 1882, never knowing that the greatest weakness in his theory had been saved. Mendel died in 1884, never suspecting that he was destined for fame. And Nägeli died in 1891, never dreaming what a terrible mistake he had made.

We can explain evolutionary changes in terms of preferential inheritance of certain genes that benefit a creature’s survival. But the changes are not always obviously beneficial. Man’s continuing practice of eating cooked meat has led to a weakening of his jaw with time. A more obvious example is the animals that have returned to live in caves. There is no light there, so they have lost their pigment and eyesight. This is a ‘step backwards’ for a light-loving animal, but of no disadvantage to a cave-dweller.

Simplification also occurs in parasites, who have a ‘cushy’ life. They never need to search for food, and is protected in its host (unless the host can expel or kill it). Because of its simple life, it has evolved away from having the complex paraphernalia associated with having to search out food and survive from predators. A tapeworm, living in the intestines of its host, has dispensed with most of these things. It has small hooks, with which to hold onto the intestine lining, a body surface, through which it can absorb predigested food and a full-blown reproductive system, by which it can produce eggs and eggs and eggs.