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Home , August Weismann, Blending inheritance

Objections. . .

History of Evolutionary Thought • By about 1870, the idea of through some kind of natural process, which science could study objectively, was accepted by almost all scientists Part VI: Objections Sustained? who had studied the matter. • BUT. . . Darwin’s idea of was not widely accepted for about another 50-60 years. – (Thomas Henry’s grandson, and a great BIOL 4415: Evolution biologist in his own right) called this “the eclipse of Dr. Ben Waggoner ” – So why the eclipse?

Objections sustained. . .  (1833-1885) • The Earth couldn’t be old enough for slow, plodding natural selection to have produced the A Scottish engineer and entire diverse spectrum of living things. economist—he invented • Favorable variants couldn’t be selected for, because supply and demand curves, interbreeding would “wash out” variation in the and helped develop population. underwater telegraph cables • Darwin had no explanation for where variation —Jenkin also pointed out a comes from, or how it can be inherited. serious flaw in the theory of • Natural selection didn’t seem like a good scientific natural selection, coming law with predictive value—it was “the law of from what he called higgledy-piggledy”, as one detractor called it. “”. Sir William Thomson, Lord Kelvin (1824-1907) was The “blending inheritance” problem a rather more formidable opponent. . .

• In a population of organisms, “favorable variants” do sometimes occur—but whom do they mate with? The limitation of • They can only mate with normal individuals—and their geological periods offspring will be a blend of parental characteristics, and imposed by physical won’t completely show the favorable trait that their science. . . does seem parent had. sufficient to disprove the • Over generations, the favorable trait will become fainter and fainter (like a drop of red paint mixed into a bucket doctrine that transmutation of white paint). has taken place through • Any favorable variation will be “swamped out” by ‘descent with modification interbreeding with members of the population that don’t by natural selection.’ have it.

The Earth must have solidified only about 20 million Kelvin’s reasoning went like this: years ago—and must be even younger.

• The Earth has internal heat (as shown by volcanoes, the heat in deep mines, etc.— But I think we may with much temperature increases with depth by 1 °F/ 70 ft). probability say that the consolidation [of the earth] cannot have taken place • A warm body in cold space must radiate that heat less than 20,000,000 years ago, or we away to space, at a rate governed by the laws of should have more underground heat thermodynamic physics (which Kelvin had than we actually have, nor more than worked out). 400,000,000 years ago, or we should not • For the Earth to be at its current temperature and have so much as the least observable losing heat at its current rate, it must have been underground increment of temperature. molten recently—too recently for Darwinian —“On the Secular Cooling of the evolution to have had any effect. Earth”, 1864 Huxley defended the possibility of a much older Earth. Jenkin’s, Kelvin’s, and others’ objections Still, Kelvin’s words carried a lot of weight. supported alternative evolutionary theories that could take place on shorter time scales.

Mathematics may be compared • directed variation—natural selection may work, but to a mill of exquisite workmanship, organisms don’t vary randomly. Something (God? some which grinds your stuff to any law of nature? Both?) causes purposeful change over time degree of fineness; but, nevertheless, what you get out depends on what • Neo-Lamarckianism—a revival of Lamarck’s ideas about you put in; and as the grandest mill the inheritance of acquired characters in the world will not extract wheat • — are not formed gradually, but in sudden flour from peascod, so pages of bursts of change—possibly in only one generation formulae will not get a definite • —species evolve because of some sort of result out of loose data. “internal drive” that has nothing to do with the —“Geological Reform”, 1869 environment

A number of scientists—including ! Huxley argued that species might change by sudden, —accepted evolution by natural selection up to a certain discontinuous changes in one generation, not slowly limit, but invoked something else to explain major and gradually—a theory known as saltation. changes, or human uniqueness. . .

I see you are inclined to I hold that there was a subsequent advocate the possibility of act of creation, a giving to man, considerable "saltus" [jumps] when he had emerged from his on the part of Dame Nature in ape-like ancestry, of a spirit or her variations. I always took soul. Nothing in evolution can the same view, much to Mr. account for the soul of man. The Darwin's disgust, and we used difference between man and the often to debate it. other animals is unbridgeable. —Letter to , —Interview, 1910 1894 Darwin himself never ruled out Lamarckian evolution, and accepted a greater role for it in his later years, in response to the criticism that natural selection by itself was too slow to explain all change.

Variability is governed by many unknown laws. . . Something, but how much we do not know, may be attributed to the definite action of the conditions of life. Some, perhaps a great, effect may be attributed to the increased use or disuse of parts. The final result is thus rendered infinitely complex. —Origin of Species, 6th ed., 1872

Natural radioactivity in the Earth causes about 80% of the Earth’s heat. The Earth isn’t cooling down, as Kelvin had thought, because it has an internal source of heat! Beginning in 1895, physicists discovered and began to study a strange new phenomenon: radioactivity. Here are Marie and Pierre Curie with their great discovery: the new, rare, and highly radioactive element radium. As for “blending inheritance”: the answer had been (1834-1914) was a German biologist worked out by this man—but nobody paid much who argued strongly against neo-Lamarckian ideas. . . attention at the time.

. . . the constant characters which • Weismann hypothesized the appear in the several varieties of a existence of “”— group of plants may be obtained some sort of physical in all the associations which are material of inheritance possible according to the laws of • “Continuity of germ plasm”: combination, by means of The hereditary material in the repeated artificial fertilization. is set apart from the — “Experiments in Plant rest of the body (Weismann Hybridization”, 1865 barrier). Lamarckian evolution isn’t possible.

In 1873, Friedrich Schneider discovered — and by 1885, Weismann and others had concluded that the chromosomes were the “germ plasm”. The Dutch botanist (1848-1935), who’d been impressed by Origin of Species as a student, began experimenting with breeding and crossing plants (unknowingly repeating “. . . there is an hereditary substance, a material bearer of some of Mendel’s work). hereditary tendencies. . . contained in the nucleus of the germ- cells, and in that part of it which forms the nuclear thread, which at certain periods appears in the form of looops or rods.” — Weismann, 1887 Working with a species of The Great Coincidence flower known as the evening primrose, De Vries noticed • In the first half of 1900, within a few months, three what he later called scientists independently rediscovered Mendel’s work: —sudden changes Hugo de Vries, Carl Correns, and Hugo Tschermak. in color or shape from one • The idea of “” as independently assorting generation to the next. He particles ultimately disproved Jenkin’s “blending argued that traits must be inheritance” inherited as units (which he – At first it wasn’t clear how genes could be responsible for called pangenes), rather continuously variable traits. There was a long controversy than as blends. . . between the Mendelians and the “biometricians”.

De Vries explained In the early 1900s, there was much dispute between mutations as changes in the Mendelians (who attributed everything to single the “genes” in his book, discrete genes) and the biometricians (who Die Mutationstheorie. De emphasized continuous traits). Vries was not a supporter of natural selection, Opposed to the Biometrical army is the Mendelian. More however—he was a recent in origin, less martial in organisation, but very saltationist, and thought vigorous, the Mendelian army has already turned the that new species flanks and pierced the centre of the older one opposed to spontaneously appeared it. For signs of surrender on one wing, and or retreat, very by mutations in a single skilfully covered, on the other, are visible in the generation. (As we’ll see biometrical ranks. later, he wasn’t entirely —Anonymous, The Mendel Journal, 1909 wrong. . .) Skipping over a great deal of complex controversies . . . and to The American and history. . . by about 1930, Mendelian had complete the link biologist Thomas joined with Darwinian natural selection to produce a with Hunt Morgan and powerful theory known as “Neodarwinism” — or radioactivity, H. his students, J. Muller showed working with in the 1920s that fruit flies in the mutations could 1910s, went on to be caused by clinch the case radiation—they that genes are resulted from carried on physical damage chromosomes. . . to a .