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Science, Evolution, and Creationism
National Academy of Sciences, 2008.
SUMMARY: According to how the modern synthesis is explained to the public, evolution is due to natural selection changing the preponderance of harmful mutations in each new generation to a preponderance of beneficial mutations entering the gene pool. But unless natural selection is 100% efficient this is not possible.
How is the mechanism of evolution most authoritatively presented to the public? Surely the gold standard is this book from the National Academy of Sciences, revised by a committee of fifteen eminent scientists chaired by Francisco Ayala.
Could it be flawed? Below, extracts, in italics, from pages 4-5, plus my comments.
"Evolution consists of changes in the heritable traits of a population of organisms as successive generations replace one another....The differential reproductive success of organisms with advantageous traits is known as natural selection... Natural selection can also reduce the prevalence of traits that diminish organisms' abilities to survive and reproduce."
Fine. Natural selection tends to both increase the prevalence of advantageous traits, and reduce the prevalence of deleterious traits. Clearly implied, this is the mechanism of evolution.
"Most organisms in any species, including humans, are genetically variable to some extent... the DNA of the two parents is combined in the offspring. In addition, DNA can undergo changes known as mutations from one generation to the next... "
Genetic variability has multiple causes, of which mutation is only one. That is, survivability and reproduction is determined by mutations less than 100% of the time. Let's say X%. Can we call that the "efficiency" of natural selection?
Now, some elaboration, with my summary below:
"The mutation may result in an altered trait that harms the organism, making it less likely to survive or produce offspring... Another possibility is that the mutation makes no difference to the well-being or reproductive success of an organism. Or the new mutation may result in a trait that enables an organism to take better advantage of the resources in its environment, thereby enhancing its ability to survive and reproduce...
"If a mutation increases the survivability of an organism, that organism is likely to have more offspring than other members of the population. If the offspring inherit the mutation, the number of organisms with that advantageous trait will increase from one generation to the next. In this way, the trait--and the genetic material (DNA) responsible for the trait--will tend to become more common in the population of organisms over time. In contrast, organisms possessing a harmful or deleterious mutations are less likely to contribute their DNA to future generations, and the trait resulting from the mutation will tend to become less frequent or will be eliminated in a population."
In other words, both creatures with beneficial mutations and those with harmful mutations can survive and reproduce, and those mutations can affect fitness and hence evolution by how their representation in the gene pool changes over time. Through the action of natural selection, individual harmful mutations end up with less representation in the gene pool.
NOTE: the NAS omits mentioning that the "great majority" of mutations are harmful (Huxley's phrase in "Evolution: The Modern Synthesis"). Here's contemporary confirmation from "Mutation" in Wikipedia: "Studies in the fly Drosophila melanogaster suggest that if a mutation changes a protein produced by a gene, this will probably be harmful, with about 70 percent of these mutations having damaging effects, and the remainder being either neutral or weakly beneficial." And among the remainder beneficial mutations are usually acknowledged to be rare. (The reference is to the Proceedings of the National Academy of Sciences.) I propose we assume harmful mutations start out 100 times as prevalent as beneficial mutations.
Now let's see how this works out in practice. I compare things before the action of natural selection in a single generation, with after:
In progeny, before the action of natural selection:
(prevalence of harmful mutations) = 100(prevalence of beneficial mutations)
Among genes entering the gene pool in a single generation, following the action of natural selection,
(prevalence of harmful mutations) = 100(prevalence of beneficial mutations)x(100-X)/100
ie, if natural selection identifies mutated genes with an "efficiency" ("X") of 5%, of 100 harmful mutations that would otherwise enter the gene pool natural selection will eliminate 5, leaving 95 harmful mutations to enter the gene pool, for every 1 (or 1.05) beneficial mutations. For that value of X, far more harmful mutations than beneficial mutations enter the gene pool in each generation, they will accumulate rapidly with each succeeding generation, and lead swiftly to extinction.
So why aren't all species extinct? Possibly because the cell's repair apparatus is so accurate there may be no incidence of mutations for natural selection to work on, and evolution would then have to be due to some other mechanism.
I am of course looking at the action of natural selection through a less usual end of the telescope. Usually one considers how natural selection, acting over millions of generations, is able to amplify the frequency of a single beneficial mutation to dominance of its allele, no matter how slight the efficiency of natural selection is. Seen this way round, individual harmful mutations quickly fade out of existence. But when you take relative prevalence of harmful and beneficial mutations into account, for all practical values of its efficiency natural selection can't in each generation change a prevalence of harmful mutations into a prevalence of beneficial mutations. Can it?
I plan to ask members of the committee responsible for the NAS book for a way out of what appears to me to be a dilemma. But until I hear from them I'd welcome your opinions.