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Animal Models and Human Evolution

Spring 2006

Class #2: Darwinian Evolution: Impact on science and society and controversies

Reaction to “The Origin of Species”. Most biologists accepted that evolution had occurred and that species were not immutable, mainly because of the large amount of supporting evidence presented by Darwin. However, many doubted whether natural selection was really the most important factor in shaping evolution. Most people, including many biologists, did not accept evolutionary concepts as an explanation for human evolution, however. It was especially difficult for people to accept the concept that the human brain could have arisen through evolution. For example, George Jackson Mivart, a biologist, accepted Darwin’s explanations for the evolution of the human body, but maintained that the human mind and capacity for logical thought was a divine creation.

Darwinian concepts had considerable impact on the ideas of some non-biologists. Herbert Spencer, a philosopher and contemporary of Darwin, was known for the phrase “survival of the fittest”---a clear reference to the process of natural selection. Some people used this concept to justify unfettered capitalism. (This phrase, unfortunately, has led to a widespread misunderstanding of the Darwinian concept of evolution; it is reproduction, rather than survival per se, that is the key to evolution. Furthermore, there is no value judgment here. Biologists do not think of evolution as being either ‘good’ or ‘bad’; rather the concept of evolution is simply an attempt to describe what has happened. Therefore, one must be very cautious in using evolutionary concepts as a guide to choosing the goals that individuals or societies should strive towards, or how they should attempt to achieve those goals.)

Two famous debates about Creation vs. evolution are often mentioned. One of these occurred shortly after publication of “The Origin of Species”; this was a debate involving Bishop Wilberforce and Thomas Henry Huxley, which occurred at Oxford University in 1860. The other occurred about 75 years later in Dayton, Tennessee, during the trial of a public school teacher who violated a state law by teaching evolution. This was the Scopes Trial. Clarence Darrow served as the lead lawyer for the defense, and William Jennings Bryan defended the teaching of Creationism and exclusion of evolution from the curriculum.

Good scientific theories usually not only allow us to explain observations that have already been made, but also help us to make predictions about what we will find if we look further (either by doing experiments or by making observations “in the field”). How has evolutionary theory fared in this respect? We’ll discuss some examples:

  1. How did birds learn to fly?
  2. How did we get our jaws, and what does it have to do with hearing?
  3. How did electric eels and electric rays evolve the ability to generate electrical potentials great enough to stun a person?

It is important to keep in mind that Darwinian evolution does not involve any preconceived direction. That is, evolution is not ‘planning’ for future conditions, even though it may sometimes appear (superficially) to have done so. For example, the limbs of land-living vertebrates (amphibians, reptiles, birds, mammals) evolved from the fins of fishes, but one should not think of fish as evolving their fins as a preparation to invade terrestrial habitats. Evolutionary biologists might speak of fins as a “preadaptation” for becoming terrestrial (because legs were derived from fins), but this term does not imply any sort of intent or plan.

In the same sense, the swim bladder of fishes was a preadaptation for air breathing. The swim bladder is a structure that contains gas that is compressed as the fish swims to greater depth or expands as the fish rises. This helps to regulate buoyancy as the fish varies the depth at which it swims. In most fish, the swim bladder is used only for this function; it is not used for ‘breathing’---a function that is relegated to the gills. But in the transition to terrestrial life, the swim bladder became modified to become the lungs. A transitional situation can be seen in modern day lungfish. These fish live in ponds that sometimes dry out for long periods. Lungfish are able to survive these periods of drought because their swim bladders are modified to serve double duty as lungs. The fish secretes a mucus sac around its body to prevent drying out, goes into a sort of torpid state, and takes in the air it needs via the swim bladder/lungs. Lung-like structures may also have evolved in fish that lived in water with low oxygen content, allowing them to come to the surface and breathe air to supplement the gas exchange via the gills.

Closely related to the concept of “preadaptations” is the concept that anatomical structures, physiological mechanisms and behaviors that currently serve as adaptations for a particular function may not have originally evolved to serve that function. To emphasize this fact, Stephen Jay Gould suggested the term “exaptation” be used to refer to ‘adaptations’ that were originally established by natural selection to serve a function different from that which they serve at present.