1. How exactly can natural selection work?
Natural selection requires four conditions in order to operate:
- Reproduction: Organisms must reproduce themselves, creating a new generation of individuals.
- Heredity: The characteristics of the parents must tend to be present in the offspring.
- Variation: Different individuals in the population must be different in at least some characteristics from each other.
- Differential fitness: Some individuals must be more fit (that is, more likely to survive long enough to reproduce) than others, based on their varying characteristics. (Ridley 1993)
By definition, living things must reproduce themselves. Thus condition #1 is fulfilled.
As Mendel first proved in the 19th century, parental characteristics are passed to the offspring. While the lack of a known mechanism for heredity was a primary weakness of Darwin's theory when it was first proposed (Ridley 1993), today the mechanisms of gene transfer and expression are well understood. Thus condition #2 is fulfilled.
Darwin recognized that different individuals of the same species vary from each other in their characteristics (Darwin 1964). Today we can find variation not just at the morphologic (i.e., regarding physical shape) level, but at the chromosomal, biochemical, and DNA levels (Ridley 1993). The sources of this variaton are understood; they include chromosomal recombination, migration (influx of new material from different populations), and ultimately mutation (Ridley 1993). Thus condition #3 is fulfilled.
Empirical observations show that the fitness of individuals is differential: in any population, some individuals are more likely to survive to reproduce than others, based on their inherited characteristics. This condition is intuitive, given condition #3. Thus condition #4 is fulfilled.
When all four conditions are fulfilled, then natural selection is a logical deduction. An individual is most likely to survive to reproduce if its physical characteristics are such that it has high fitness. Those individuals who are less fit are less likely to reproduce. The offspring of that generation all have characteristics of their parents, including those which determine fitness. There are more offspring of the more fit individuals than of the less fit individuals, and since the characteristics of the parents are transfered to the offspring, the offspring on average are more fit than the parents on average (the less fit have been weeded out). Left to run indefinitely, the system will tend toward maximum fitness of all individuals.
The theory of natural selection itself is very simple. Its implications, however, are extraordinarily complex.
This isn't really a good question, because it assumes that a theory cannot be a fact. So to answer this question, a few definitions of terms used in science will be necessary first.
A law is a general statement about how nature behaves which has been shown to be valid over a wide variety of experimental conditions, while a theory is an explanation of why laws work (Giancoli 1995). Both can predict the results of future experiments; the difference is that theories explain laws.
The definition of a "fact" is fuzzier. The (Oxford Dictionary) lists five different definitions. For this argument I assume a definiton of "something known or shown to be true." It isn't a scientific term. Theories, laws, or single data could all be considered facts.
Evolution is a theory. It explains a wide range of observed phenomena in the fossil record, in the modern morphology of organisms, in the biochemical makeup of those organisms, etc. It is so widely verified that it is considered by biologists to be a fact, i.e. it has been shown to be true.
The second law of thermodynamics, generally stated, is this:
"The total entropy of any system plus that of its environment increases as a result of any
natural process" (Giancoli 1995).
The word total is important in this statement. According to Giancoli (1995), during the lives of organisms, waste products are eliminated continuously. The molecules that make up these wastes are simple and relatively unordered, and because so much waste is produced, the total entropy of the system of life on earth, from its origin to the present day, is greater.
In addition, the sun is part of the system of life on earth. Since the sun is constantly losing energy and thus becoming more disordered, the total entropy of the entire system is increasing.
Many people are quick to point out that nearly all mutations are harmful or at least neutral; beneficial mutations are rare indeed. Furthermore, all mutations are random. So how can mutations allow adaptations to come about?
The rate (or likelihood) of any given mutation can range from 0.1% to 0.0000001% (Ridley 1993); we can say the average could be 0.0001%. If just one in 100 mutations is beneficial (a generous estimate), that means that the chance of a beneficial mutation arising is 0.000001% -- only 1 in 100,000,000! That means that any given beneficial mutation will arise only once per 100,000,000 individuals -- while the chance of a given detrimental or neutral mutation will arise once per 1,000,000 individuals. How can this miniscule mutation rate produce adaptations, especially when it's so much more likely to be harmful or have no effect?
The answer comes in two parts. The first, of course, is that the whole system is not completely random; there is selection. The second part is that this selection is cumulative.
Imagine that you are picking up shells on a beach. Almost every one is broken or flawed. In fact, on the whole beach only about 1 in 100,000,000 shells is perfect. You walk from one end of the beach to the other, picking up every shell you see. You throw away the broken and flawed ones, and keep the rare perfect ones. At the end of a few weeks, you will have a handful of perfect shells, and no imperfect ones.
Evolution works the same way. The rare, random beneficial mutations are kept, because those individuals with such mutations survive better. The detrimental mutations go away in one generation because the individuals with those mutations die quickly. After many generations, the result is a collection of beneficial mutations, without the detrimental ones which arise more frequently but are not preserved.
Skeptics of evolutionary theory often point to a lack of transitional forms in the fossil record as evidence against evolution without defining what they mean by "transitional form." If "transitional form" means a partially adapted species, then no transitional forms will ever be found. All creatures which survive long enough to be fossilized are fully adapted in the sense that they are adapted well enough to survive. A fully adapted species is not an ideal, "perfectly adapted" creature; those exist only in the minds of some people. A fully adapted species is a species which has all the adaptations it needs to survive in its environment.
Paleobiologists use a different definition for the term "transitional form": a species which is found midway between two other species on the phylogenetic tree. They show some characteristics of the descendant group, but not all. But no matter what combination of characteristics they have, they are fully adapted to their environments, because they were able to survive long enough to be fossilized.
These forms not only exist in the fossil record, they are relatively common. Ichthyostega is transitional between sarcopterygian fishes and amphibians; the therapsids are transitional between pelycosaur reptiles and mammals; Archaeopterix is transitional between archosaur reptiles and birds.
Ichthyostega is a good example of a transitional animal. It was able to survive on land but was also largely aquatic. It had a fish-like tail with a fin as well as a lateral line system (a sensory mechanism unique to fishes), all leftover from its fish ancestors. But it also had legs and toes as well as nostrils designed to allow air breathing (fish nostrils are adapted only for smelling), which are definite tetrapod (four-footed animal) features (Carroll 1998). It was fully adapted to its semi-aquatic environment, but it is also a transitional form, as paleontologists can see from its unique combination of characteristics.
The Transitional Forms FAQ includes much more information on known transitional forms, of which there are many.
Some people may note that while transitional forms are common in the fossil record, they aren't nearly as common as Darwin predicted they should be. Darwin supposed that the fossil record should show a fairly continuous, gradual range of transitions, and that the "spottiness" was due to the rather low odds of any species at all being preserved, found, and described. 140 years later, the known fossil record is far more complete, and yet it still shows a pattern of relative constancy, then a transitional form or two, then relative constancy again. This pattern was explained by Niles Eldredge and Stephen Jay Gould in their model of punctuated equilibrium.
A tautology is defined as "a statement that is necessarily true" (Oxford Dictionary).
It refers to a theory or statement which cannot be falsified, i.e. for which no condition can be stated which would prove the original statement false.
If evolutionary theory were a tautology, then it could not be disproven.
But evolution could be falsified if certain conditions were met.
If, for example, fossils from an organism were found in strata that were deposited long before said organism could have evolved, then evolution would be falsified.
Human fossils in Cretaceous strata along with dinosaur bones would falsify evolution.
Therefore evolution is not a tautology.
Of course, it has never actually been falsified.
Carroll, Robert L. 1988. Vertebrate Paleontology and Evolution. W.H. Freeman and Company. 698 p.
Concise Oxford Dictionary of Current English, The, ninth edition. 1995. Ed. Della Thompson. Clarendon Press, Oxford.
Darwin, Charles. 1964. On the Origin of Species. Facsimile of the First Edition, 1859. Harvard University Press. 513 p.
Giancoli, Douglas C. 1995. Physics: Principles With Applications, fourth edition. Prentice-Hall. 1020 p.
Ridley, Mark. 1993. Evolution. Blackwell Science, Inc. 670 p.
© 1997, 1998 by Robyn Conder Broyles. All rights reserved.
E-mail comments to Robyn Conder Broyles at ginkgo100@yahoo.com
2. How can natural selection be strong enough to create whole new species, instead of
variation within single species?
3. Isn't evolution just a theory, not a fact?
4. Doesn't the second law of thermodynamics say that evolution is impossible?
5. How can the order and complexity we see in nature arise by random chance mutations?
6. How does the theory of evolution explain the lack of transitional forms in the fossil record?
7. Is evolution an unfalsifiable tautology?
8. How can a theory which requires billions of years be reconciled with the Genesis account of creation?
Literature Cited
