From Genesis to Genetics: The Case of Evolution and Creationism

5. Twentieth-Century Evidence

F I V E Twentieth-Century Evidence Darwin's magisterial assembling of the indirect evidence for evolution was convincing to many scientists almost from the publication of the Origin,and by the end of a decade to nearly all. However, his hypothesis for the mechanism of evolutionary change, namely, inherited variations acted upon by natural selection in a finite environment, was not so convincing. A major reason for this was the near total lack of information in Darwin's time about the origin of variation and the workings of inheritance. THE RULES OF INHERITANCE In the last half of the nineteenth century it was not clear whether or not inheritance is a constant and repeatable phenomenon. Some observations suggested that there might be rules of some sort for inheritance , but equally persuasive evidence suggested a fickleness at best. It was obvious, of course, that the offspring of human beings were human beings and that this principle applied to all known life-the species of animals and plants "breed true." It was equally obvious that children of the same human parents were far from identical except in the rare cases of identical twins. Sometimes parental characteristics Twentieth-CenturyEvidence / I 17 seemed to be inherited, but in other cases, not. One of the most confusing of all was that a parental feature would not be expressed in the children but would reappear in the grandchildren. And then there was that most astonishing difference of all: offspring of the same parents can be either males or females, which differ greatly in structure, physiology , behavior, and reproduction. In most speciesthat reproduce sexually , about half the offspring are females and the others males. It is obviously convenient that approximately equal numbers of males and females are born, but convenienceis not a mechanism-so what could possibly account for this phenomenon? Darwin's theory required a pool of inherited variations among individuals of the same speciesfrom which natural selection couldchoose those that were more fit for survival and reproduction. When Darwin published the Origin, he believed that inheritance must have a definite biological basis and that, whatever it was, it had to provide offspring like the parents but with minor variations. There were no data to explain how this was possible during the last half of the nineteenth century. In subsequent editions of the Otr'@n Darwin was driven more and more to the belief that the variations he saw in nature were due in some degree to the direct action of the environment. This had been the suggestion of Lamarck back at the beginning of the century, and by the century's end a belief in Lamarckism had become common among Darwin's contemporaries. The modern understanding of inherited variations began in 1900, more than a decade after Darwin's death. In that year, experiments on inheritance in garden peas conducted in the 1860s by an Austrian monk and amateur naturalist, Gregor Mendel, were rediscovered and made known to the scientific world. Mendel had crossed varieties of garden peas that differed in characteristics such as the shape and color of the seeds or the length of the stems. He observed patterns of inheritance in the second and subsequent generations of peas, and he made sense of these patterns by recognizing that each characteristic he studied came in two versions. Today, we would call the versions of a I I 8 / Twentieth-Century Evidence given gene responsible for this kind of variation alleles. For example, a gene for seed color in Mendel's peas had two alleles-one for yellow and the other for green. When both alleles were green, the seeds in the first and second generations were green. When both alleles were yellow, the seeds in the first and second generations were yellow. But they were also yellow if one allele was yellow and the other green. That is, the yellow allele was dominant, and the green one was recessive . Today we would say that when together the yellow allele was "expressed," while the green one was not. When an organism has two dflerent alleles for a given characteristic, say, yellow and green, the organism is heterozygous for that characteristic . When an organism has two identical alleles for a given characteristic , we say the organism is homozygous for that characteristic. In Mendel's experiments, when two pea plants that were heterozygous for seed color were crossed, three-quarters of the offspring produced...