Tuesday, March 17, 2009

A monk in his garden

Imagine a monastery in Moravia, and in the monastery a garden, and in the garden a monk. The monk is busy handling pea plants in pots, wrapping the flowers in paper bags after carefully dusting them with pollen from a different variety of the pea plant. He is no ordinary monk. He has studied mathematics and science. In a few years’ time he will be elected abbot of the monastery --which will force him to abandon his scientific work.

       He is Gregor Mendel, the father of the science of genetics, and his experiments with pea plants will provide the missing link to Charles Darwin’s theory of evolution by natural selection. But not before both Darwin and Mendel are dead and gone.

       Darwin will die in 1882, still plagued by the mystery of the mechanism of inheritance. The theory of evolution by natural selection requires that heredity work in such a way that mutations --or fortuitous variations in the hereditary makeup of an organism-- are passed on intact to offspring. This would guarantee the conservation of advantageous mutations (a longer neck in giraffes, a change in pigmentation in moths living in soot-covered trees in central England); whereas the alternative mechanism of blending inheritance --whereby offspring simply strike  an average between the characteristics of their parents-- would cut mutations in half with each generation, rapidly diluting their effect, advantageous or otherwise.

       Mendel will die in 1884 in total obscurity. The revolutionary nature of his experiments will only be recognized in 1904, when three European botanists will independently rediscover his work.

       Ironically, Darwin’s would-be savior was already working with his pea plants when the great scientist published The Origin of Species, in 1859. Mendel’s method of “hybridization” was straightforward. First he would open a pea flower before it was fully developed, removing the anthers (or male sexual organs) with tweezers to avoid self-pollinization. Then he would dust the flower’s stigma with pollen from the selected variety, immediately wrappping the flower in a paper bag to keep away other pollen. Finally, he would wait patiently for the plant to produce seeds and for the seeds to produce the next generation of plants. Mendel then recorded the results.

       The monk chose pea plants because they have traits (such as blossom color and plant height) that are easily distinguishable and that breed true. Thus, he crossed six-foot plants with one-foot plants, and plants with purple blossoms with plants with white blossoms. Would the result be three-foot plants with mauve-colored blossoms, as the popular theory of blending inheritance dictated?

       In contrast with other botanists who had performed hybridization experiments before him, Mendel had studied mathematics and was an able statistician. He found that when he crossed six-footers with the short variety the first-generation hybrids were all six-footers. No intermediate-sized plants were produced. However, when these first-generation hybrids were allowed to self-pollinize, the result was astonishing --the second generation included both tall plants and short plants, and in an approximately three-to-one ratio. A similar result was obtained for six other contrasting traits.

       Mendel’s conclusion was that, contrary to popular belief, the parent’s traits are not blended in the offspring. Inheritable characteristics are determined by units of inheritance that are segregated rather than blended in the offspring, with certain traits dominating over their “recessive” opposites (i.e. long stem versus short stem). Today we call these units “genes.” Mendelian genetics meshed perfectly well with natural selection. In the first decades of this century, genetics and evolution became integrated in what is known as the synthetic theory of evolution.

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