Thursday, September 5, 2019
Discovery in the Theories of Evolution
Discovery in the Theories of Evolution Prologue Throughout his childhood and adult life, Siddhartha Mukherjee, physician and author of The Gene: An Ultimate History, has been troubled by his family history of mental illnesses. His two uncles, Rajesh and Jagu, have suffered from mental illness, and so does his cousin Moni. If mental illness was genetic as recent studies showed, could he be a carrier as well and pass the illness to his two daughters? If his cousins mental illness was genetic, then why had this father and sister been spared? How much of the mental illness arose from nature (i.e., genes that predisposed to mental illness) versus nurture (environmental triggers such as upheaval, discord, and trauma)? Similar unresolved questions arose in his scientific work as a cancer biologist. Many forms of cancer arise from gene mutations that occur during a persons lifetime. Should we be concerned if we have a family history of such diseases? And could we pass these diseases to our children? Genetic engineering has advanced so much that we can treat some of these diseases by using gene therapies. Imagine that if technologies were available to change our genetic codes, resulting in altered identity or sexuality or behavior. Who would control such a technology, and who would ensure their safety for our society, our children, and ourselves? This chapter chronicles the fascinating history of discovery in the theories of evolutions and heredity from early Greek philosophers to the theories developed at the end of the 19th century. It also covers the history of eugenic movements in the late 19th and early 20th centuries. Key concepts covered: Darwins theory of evolution explains why species change. Mendels laws of heredity explain why species stayed the same. Eugenic movements in the late 19th and early 20th centuries were based on a faulty understanding of genetic science. They used phenotypes (physical or mental attributes) as genetic traits. Early Ideas about Heredity and Evolution Since the earliest times, human has recognized the influence of heredity and has applied its principles to improve crops and domestic animals. In around 530 BC, Pythagoras (570-495 BC), the Greek scholar, proposed one of the earliest theories to explain the similarity between parents and their children. The core of his theory was that male semen carried all the hereditary information. Once inside the womb, semen matured into a fetus via nourishment provided by the mother. A century after Pythagorass death, Aristotle (384-322 BC), A Greek philosopher, rejected the notion that heredity was carried solely in male semen. In around 350 BC, he proposed that male semen carried the instructions to build a child while female contributed the physical raw material for the fetus. The transmission of heredity was essentially the transmission of information. Aristotle was wrong in his partitioning of male and female contributions into message and material, but he had captured one of the essential truths about heredity. No new ideas were introduced in the next two millennia until the 18th century when the idea of preformation was introduced. Using microscopes, scientists imagined that they could see miniature replicas of humans inside human sperms. In 1809, Jean-Baptiste Lamarck (1744-1829), a French biologist, introduced the inheritance of acquired characters as a model for evolution. According to Lamarck, organisms evolve due to two forces: (1) Simple organisms emerge and then evolve to become more complex; and (2) Organisms adapt to their environments by changing their characteristics. He believed giraffes developed long necks because, over many generations, they had to keep stretching their necks to reach higher foliage. Lamarckism fell from favor after August Weismann (1834-1914), a German embryologist, performed an experiment in 1883 showing that changes from use and disuse were not heritable. In that experiment, Weismann had cut off the tails of five generations of mice, then bred the mice to find out if the babies would be tailless. But the babies were all born with their tails intact, not even marginally shorter.
Posted by g at 7:38 AM