The evolution of sexual reproduction

StoryImage( ‘/Images/Story//Auto-img-116421753632262.jpg’, ‘Photo by Dr. Robert A. Hedeen (of a statue in the Jardine des Plantes, LaRochelle, France)’, ‘The mythological Greek lovers Hero and Leander obey a fundamental biological law of nature.’);

“Nature” long ago recognized the need for the combination of the genetic material of two individuals to form a new entity was absolutely necessary for the preservation of the species and to provide the raw material for individual variations necessary for evolution.

Early zoologists watched through their primitive microscopes as one-celled animals reproduced by simply dividing into two new individuals (asexual reproduction). But, every now and then, the microscopist would be amazed to observe two individuals come together and exchange nuclear material (sexual reproduction). The nuclear material was, of course, DNA, but these observations were made long before DNA and genes were understood. Much later, scientists were astounded to watch the most primitive of organisms, the bacteria, engage in sexual reproduction. Males and females were not differentiated as the partners in these single-celled organisms and were simply designated as Strains + and – .

As we progress up both of the phylogenetic trees of the Animal and Plant kingdoms, we quickly see the appearance of both male and female sex organs. In the jellyfish and their kin, though they frequently reproduce asexually, individuals form an ovary to produce eggs and a testis to produce sperm cells. Though the sperm of an individual may occasionally fertilize an egg from the same individual, the male cells most often fertilize the eggs of a different organism (cross fertilization rather than self-fertilization). Self-fertilization is the equivalent of asexual reproduction and a biological no-no.

As animals became more complex in their evolutionary ascendancy, sexual reproduction dominated the asexual method. In the flatworms, for example, we find the development of hermaphroditism, where a single individual possesses both ovaries and testes. Again, cross fertilization is assured as the ovaries and testes produce their reproductive cells at different times. The tapeworm, however, is a flat worm that routinely produces new tapeworms asexually by producing buds that develop into an adult parasite.

A feature of a variety of animals that is used for sexual reproduction is called protandrous (first male) hermaphroditism. These creatures develop an undifferentiated sex organ called a gonad that is neither testis nor ovary. As the animal matures, the gonad functions as a testis and produces sperm cells for a while. Then, it does an about face and becomes an ovary, and produces egg cells. As sperm and eggs are not produced at the same time, cross fertilization is assured. Many invertebrate (without a backbone) animals use this method of reproduction as do some primitive vertebrates (with a backbone).

The real biological value of cross fertilization was not realized until Gregor Mendel’s 2nd law was rediscovered. Mendel, “the father of genetics,” called this the Law of Independent Assortment, and he was referring to the determiners we now call genes. This law shows how a new individual can be produced sexually that shows traits not possessed by either of its parents. Variations produced by independent assortment are the raw material of evolution. When we arrive at the top of the animal evolutionary tree, we find no asexual reproduction among the birds and mammals

An exception to this is the asexual production of a mammal, or any other living creature by artificial cloning. It has long been known that each cell of the body of an organism possesses the full genetic complement necessary to produce a complete new individual. But it was not until the mid-1970s that Gurdon in England was able to successfully clone a species of frog from a single cell taken from its gut. Since then, a wide variety of higher animals have been cloned, including clandestine claims of the successful cloning of a human. It is only a matter of time until a documented cloning of a human is made known. Cloning, of course, is artificial asexual reproduction.

There is the one supposedly documented case of asexual reproduction in a human that is an enigma to most biologists. It involves a virgin German girl who was so traumatized by constant bombing by Allied planes that she became pregnant and eventually gave birth to a normal child. If true, this would be another Immaculate Conception. Definite scientific proof of the accuracy of the claim has not been forthcoming.

Meantime, “Vive la Difference!”

Dr. Robert Hedeen is a former resident of Maryland’s eastern shore and resided in the Chicago area from 1960 to 1971. He is a retired professor emeritus of biological sciences in the University of Maryland system. He has published more than 30 scientific papers, has written numerous magazine articles, and is the author of two books on the natural history of the Chesapeake Bay.

From the Nov. 22-28, 2006, issue

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