Drawing On Science
This column was mistakenly truncated when it was printed two weeks ago. We are reprinting it in its entirety this week.
In 1831 the "HMS Beagle" departed from Plymouth, England on a voyage to South America, around Cape Horn north to the Galapagos Islands, west to Australia, around the Cape of Good Hope and back to England. The vessel stopped at many locations throughout the voyage for the purposes of mapping and collecting plant and animal specimens. Aboard the "Beagle" was a 21-year old naturalist, Charles Darwin. Darwin's job was to collect, record, and study the animal and plant specimens found throughout the voyage. Over the years his studies of the specimens, particularly those from the Galapagos Islands off South America, convinced Darwin that species are not static.
He concluded that they gradually change or evolve over a period of time and that survival of the species depends on adaptation to a changing environment, complexity of the individual, and diversity within the population of the species. He called this phenomenon
natural selection and he was not alone in drawing this conclusion. Another naturalist, Alfred Wallace, came to the same conclusions in his studies of variations in species. Wallace and Darwin agreed to write a joint paper with the long title "On the Tendency of Species to form Varieties; and on the Perpetuation of Varieties and Species by Natural Means of Selection." The paper was read to the Linnaean Society on July 1, 1858 and was published in its Journal of the Proceedings of the Linnaean Society. A year later Darwin published his findings in his On the Origin of Species by Natural Selection. Wallace fell into the shadow of Darwin over the following years, but his contribution to the Theory of Evolution is still recognized today. In some cases the theory is referred to as the Darwin-Wallace Theory of Evolution.
Evolution, as Darwin explained, is supported by overproduction of offspring within species, fossils of longextinct plants and animals, comparative anatomy, and comparative embryology.
There is a tendency for organisms to
over-produce offspring and in any given population of organisms there are variations among individuals. If a particular variation makes an individual more adaptable to its environment, then it will have a better chance of survival than those not having the favorable variation. It will survive and ultimately reproduce offspring having the same variation. In time the betteradapted offspring will become a major part of the population. After many generations the species may look entirely different from the original population. A classic example of this change was documented in a study of dark- and light-colored moths living on the lightcolored barks of trees. The darker moths against the lighter bark were more easily seen and eaten by birds. This resulted in the darker-colored moths making up a minor part of the population. After the industrial revolution, soot from coal-burning furnaces covered the barks giving the darkercolored moths the advantage. The population is now dominated by the darker moths.
Through the study of fossils it has been determined that many species of organisms living today had ancestors in the past which were completely different in structure, appearance and behavior. A fossil is evidence of a longextinct animal or plant. Some examples are a cast or mold filled in with minerals producing an exact replica of the organism; an imprint in sedimentary rock; a mold or empty space in rock showing exactly what the organism looked like; or an amber-encased specimen. Fossil discoveries have shown that whales were once fourlegged, land mammals; a group of dinosaurs are now represented as birds; primitive, bony fish developed limbs giving rise to amphibians; modern sharks evolved from primitive forerunners; and camels were once small-bodied, cat-sized, four-toed animals with low-crowned teeth.
The study of anatomy reveals evolutionary relationships. The limbs of a crocodile, fins of a whale and wings of a bird are homologous structures. Such structures differ in outward appearance and function, but are similar in skeletal structure. This is evidence of a common origin. The wings of a bird and the wings of an insect are
analogous structures. They are not similar in any way. This is evident of independent origins and evolution. Man's appendix is a vestigial structure as are the eyes of sightless molerats and cave fish, and the rudimentary hind limbs of whales. Some snakes such as boas have vestigial pelvises and tiny legs. Why do flightless beetles have vestigial wings under sealed wing covers and why do male mammals have nipples? Such structures have reduced or no function at all as a result of species change in form and behavior over a very long period of time.
There is no problem telling the difference among adult fish, amphibians, reptiles, birds and mammals. But in the earliest stages of embryonic development it is more difficult to make the distinction in some cases. Embryos of a striped bass, chicken, rabbit and human all show gills and tails in early stages. This supports the fossil record showing that aquatic, gillbearing vertebrates came before airbreathing vertebrates. It also suggests evolution of these species from a common ancestor.
Darwin sketched a "tree of life" to show the relationships among living things. He drew at its bottom a common ancestor with a line rising up and randomly branching out. At the tip of each branch was implied a new species. This evolutionary tree as Darwin called it was his idea of the relationships among all plants and animals. At the top of the drawing he wrote, "I think." Since his, and Wallace's, theory was proposed it has been accepted by an overwhelming number of scientists and continues to be confirmed today.
Next month: Mechanisms of Evolution
Questions/comments? E-mail Steve: Drawingonscience@aol.com.