One of the fundamental problems facing biologists was the question of how organisms become adapted to their environment. When the creation model was accepted, the answer was simple: God arranged it that way, by miracle. Once evolution was accepted another solution became evident: natural selection molded the organisms. This lecture highlights the differences in these two approaches.
Adaptations are the unique features of an organism which are specializations for their environment & their way of life. e.g. the beaks of birds are highly specialized for the foods they eat.
How do organisms adapt?
Argument by Intelligent Design.
This argument was developed by the priest William Paley with his watch argument. He said if you were to find a watch in a forest and open it you would see that all of its workings could not have developed by chance—it would be seen as having been designed. That means there was a designer.
Arguing by analogy—we look at organisms and see they have a design--distinct parts that perform particular functions. Hence they must have a designer and that designer is God. The design is perfect since God made it. Any given trait is designed for a purpose.
The purpose is predetermined.
The design is perfect.
Problems with the argument:
Organisms are not perfect. E.g. human eyes have many defects. E.g. Most species are extinct and clearly were not perfect.
Organisms have vestigial organs which have no function—this isn’t perfection. E.g. cave animals have non-functioning eyes.
Organisms have embryological structures that have no function. E.g. human embryos have tails which are made then reabsorbed.
Creationists say that structures like eyes are irreducibly complex—they cannot function except if all of their parts are created together; there are no intermediate forms that function. This is incorrect as many intermediate simple eyes exist.
God of the Gaps argument (or Argument from Ignorance): Creationists argue that if you can’t explain something it is proof of God’s existence. Obviously, this runs counter to the way scientists work—we study things and eventually find out their explanation.
Theological argument: if we argue that God must have created an adaptation by miracle, and then find that we can explain it scientifically, this weakens God’s supposed powers.
Legal/political argument: In the public classroom, to argue that God created adaptations by miracle falls afoul of the US Constitution, this calls for separation of church and state.
There is no way to test the argument that God did it by miracle except to provide a scientific answer to the puzzle, which is evolution.
This is just a restatement of the steps of evolution by natural selection: Variation in individuals occurs; these variations must be inherited, struggle for existence (natural selection of most favored traits). This means that those characteristics which help the organism survive will be retained when the organism survives and passes on those traits to their offspring. Natural Selection is the designer.
We can explain the presence of vestigial traits in adults and odd embryological vestiges by simply stating that in past ancestors those structures did have a function.
Divergent Evolution expresses the observation that the longer that two populations are separated the more distinct they tend to become. Still when we look at them closely we can see that they have basically many of the same structures although altered. E.g. the bone structure of the limbs of vertebrates are basically the same. We say these structures arehomologous
Adaptive Radiation is the situation when many species have come from a common ancestor and then diverged rapidly specializing for different life styles.
Convergent Evolution expresses the fact that sometimes organisms with very different ancestral histories can become more similar in certain characteristics; this is typically because they are subject to similar environments. E.g. both whales and sharks have dorsal fins even though they have evolved these independently. We say such structures are analogous or are homoplasies.
How do adaptations develop?
Pre-adaptation (exaptation)—traits evolved for one purpose become useful for a new purpose in another environment. E.g. fins of fishes used for swimming become useful for walking and convert into limbs; and in birds and bats they become useful for flight.
De novo—traits develop from scratch. E.g. horns develop from skin and bones of head. This typically would involve mutations that occur and are advantageous.
How are traits lost?
If a trait is no longer beneficial then the selective pressure is removed; it becomes more variable. There is a cost to building and maintaining it, so there is a selective pressure to lose it.
Red Queen Principle—This principle states that since the environment is always changing, organisms cannot ever fully adapt to their environment. Like the Red Queen in Alice in Wonderland—organisms must keep running just to stay in place.
Not all traits are adaptations—specializations that improve the chances of survival. They may be present because they are due to
Genetic drift—i.e. have no survival value but developed by chance
Pleiotropy—i.e. are present because the gene(s) causing it are coding for other characteristics and the apparent “adaptive trait is a byproduct.
Sexual selection—a trait chosen by females because it is “attractive.”
Vestigial organ—it was an adaptation but no longer is.
Two views of evolution:
Phyletic Gradualism—this is the idea that natural selection proceeds gradually and slowly over time to produce its effects.
Punctuated Equilibrium—this is the idea that evolution proceeds by fits and spurts—rapidly sometimes and slowly of not at all on other occasions.
Can you answer these questions? How does the argument by design differ from the evolutionary argument to explain how organisms develop adaptations?
Give three reasons why the Intelligent Design argument should not be taught in public schools as an explanation of the origin of adaptations.
Why does the irreducible complexity argument fail as a serious challenge to eye evolution?
Knowing what you do know about vertebrate fossils and their sequence, why can we be sure that the whale and shark dorsal fins are homoplasies and not homologies?
Bird feathers are believed to have originally evolved to function in temperature regulation, then used for sexual display and then for flight. Their anatomical structure changed along the way. What term should be applied for this shift in function and structure? Explain.
The appendix in humans is said to be a vestigial structure even though it does still play a role in the immune system. Explain the reasoning.
One of the lungs in snakes is shriveled and non-functional. Explain how this could have occurred if their ancestors had two functional lungs.
Explain how pleiotropy can cause confusion in our interpretation of a presumed adaptive structure.
Consider the two major patterns of evolution—phyletic gradualism and punctuated equilibrium. Natural selection must be acting in very different ways in these two situations, mustn’t it?