What makes natural selection a process powerful enough to bring about the evolution of adaptations?

This is a guest post by Carl Bajema, a retired evolutionary biologist, first posted on the Richard Dawkins website on Darwin's birthday.

Happy 198th Birthday Charlie Darwin from Carl Bajema...

Organisms with their intricate adaptations for surviving and reproducing could not have evolved by chance alone. Both creationists and evolutionary biologists agree with this conclusion. Charles Darwin understood that the designs we observe in nature could not have been produced by undirected random processes alone.

Selection was widely understood before Darwin's time to be strictly a negative editing process that selectively eliminated individuals with harmful traits. Darwin's great scientific achievement involved the identification of two additional natural processes that expanded our scientific understanding of selection and that make selection a powerful positive process capable of bringing about the evolution of adaptations. Science educators need to help their students and the general public gain a better understanding of how powerful selection can be.

Organisms with their intricate adaptations for surviving and reproducing could not have evolved by chance alone. Both creationists and evolutionary biologists agree with this conclusion. Charles Darwin understood that the designs we observe in nature could not have been produced by undirected random processes alone.

Selection was widely understood before Darwin's time to be strictly a negative editing process that selectively eliminated individuals with harmful traits. Darwin's great scientific achievement involved the identification of two additional natural processes that expanded our scientific understanding of selection and that make selection a powerful positive process capable of bringing about the evolution of adaptations. Science educators need to help their students and the general public gain a better understanding of how powerful selection can be.

Charles Darwin identified one of the two natural processes that make selection a powerful force capable of bringing about the evolution of adaptations in September 1838 when he read An Essay on Population by Thomas Robert Malthus. Darwin read Malthus' summary of Benjamin Franklin's views on the "constant tendency of all animated life to increase beyond nourishment" and on how rapidly a human population can double in numbers.

Ben Franklin's insights concerning competition and rapid human population growth in the American colonies provided Charles Darwin with the scientific answer to the problem he was trying to solve. Darwin now realized that the power of selection was generated by the "overproduction of offspring" that greatly intensified the "struggle for existence" that organisms have with the physical conditions of the environment, individuals of other species and individuals of the same species.

Young Charles Darwin went on to identify still another selective natural process that speeds up evolution—competition for mates. This powerful selective process also greatly affects the reproductive success of organisms and does so by involving competition for reproductive success among individuals who survived the rigors of natural selection. Darwin coined the metaphor "sexual selection" to describe this selective process.

Science educators need to help their students gain a better understanding of how powerful selection operating in nature can be. One of the pedagogical strategies that educators can use involves simple mathematics. The four genetic consequences of selection operating in nature can be represented in mathematical terms.

(1) The SELECTIVE ELIMINATION (subtraction) of harmful traits, that is, the removal of maladaptive genetic information; Darwin used the phrase "rejection of injurious variations" to describe this already well known outcome of selection in 1859 (Darwin, 1859, p. 81). (Biologists have used such terms as stabilizing selection, negative selection, purifying selection, editing, sieving, filtering, culling and selection against to describe this mode of selection that maintains the genetic status quo.)

(2) The SELECTIVE ACCUMULATION (addition) of new adaptive genetic information in the form of new mutations each generation. Darwin used the phrase "preservation of favourable variations" to describe this known outcome of selection (Darwin, 1859, p. 81).

Charles Darwin adopted Herbert Spencer's metaphor "Survival of the Fittest" to describe how natural selection operates in the fifth and sixth editions of the Origin of Species at the urging of Alfred Wallace. Unfortunately "Survival of the Fittest" is a very misleading metaphor. It does not help students gain an adequate understanding of what the natural processes are that really make selection a powerful process.

Darwin's unique scientific contribution involved the identification of the following two additional outcomes of selection that are very powerful and that give adaptive direction to evolution.

(3) The SELECTIVE MULTIPLICATION of adaptive genetic information each generation brings about an EXPONENTIAL increase in the frequency of adaptive genes in a population. (multiplication) Darwin (1859) used such phrases as "high geometric powers of increase" (pp. 4-5, 109), "overproduction of offspring," "success in leaving progeny" (pp. 62, 88) and "leaving a more numerous progeny" (Darwin, 1871, vol. 1, p. 279) when drawing attention to this exponential natural process.

(4) The SELECTIVE RECOMBINATION of adaptive genetic information each generation. (division and selective addition)

Darwin drew attention to this important selective process when he discussed how "sexual selection" which involves both competition between members of the same sex (usually males) and mate choice (usually "female choice") resulted in successful individuals having a more numerous progeny (Darwin, 1859, pp. 87-90 and Darwin, 1871, vol. 1, p. 271, etc.). Scientists have learned that the process of sexual reproduction involves both division of genetic information via meiosis and selective addition of genetic information via selective mating and blind fertilization of gametes.

Selective mating produces new coalitions of genes that results in some individual offspring possessing numerous adaptive genes from many different genetic ancestors.

Sexual selection is a natural process with a very long evolutionary history. Insects such as fruit flies engage in it and it is very obvious that fish, amphibians, reptiles, birds and mammals do it, that is, engage in evaluating potential mates and attempting to mate selectively.

The important role that selective mating plays in evolution has been discussed by the evolutionary psychologist Geoffrey Miller in his book, The Mating Mind: How Sexual Choice Shaped the Evolution of Human Nature (2000, see especially p. 174). Miller described natural selection as manufacturing—producing the product, and courtship which plays a major role in selective recombination as marketing. Sexual selection theory provides a scientific explanation for why human beings and individuals of other species spend so much time acquiring resources for displays, displaying, and evaluating displays.

The selective recombination of the more adaptive genes each generation is an important natural process that plays a major role in the evolution of adaptations for survival and reproductive success. Selective recombination by incorporating the genetic effects of evolution-in-parallel, that is, by incorporating coalitions of adaptive genes from more than one individual ancestor each generation, makes selection an even more exponential multiplicative process with respect to the evolution of adaptations.

The two natural processes involved in selection that Darwin identified make selection a very powerful process producing exponential genetic change in populations by multiplying genes (genetic outcome 3), and producing individual organisms having new and more adaptive coalitions of genes by combining adaptive genes from many different ancestors(evolution-in parallel) into a descendant (genetic outcome 4). Modern day biologists have used such terms as directional selection and positive selection to describe the adaptive evolution producing mode of natural selection.

This more scientifically accurate "Speedy strong man" version of Darwinian adaptive evolution by ecological selection theory stands in stark contrast to the "Slow straw man" version which restricts selection to merely performing the negative task of editing, that is, "weeding out" harmful mutations. Science educators and especially biology textbook authors need to be more explicit when they discuss how selection brings about the evolution of adaptations. One of the strategies science educators can use involves describing the four genetic outcomes of selection in simple mathematical terms (selective subtraction, selective addition, selective multiplication, and division followed by selective addition.).

Natural selection is an adaptive evolutionary process that has ecological causes and genetic consequences. Charles Darwin's classic definition of the ecological causes ("struggle for existence") and genetic consequences of selection (page 62 in the Origin of Species) can be restated in modern scientific language as follows:

Natural/sexual selection is the ecological interactions an organism has with (1) the physical conditions of the environment, (2) individuals of other species, and (3) individuals of the same species that affects the number of times the organism successfully reproduces the genes the organism is carrying.

The modern version of Darwinian selection theory states that the adaptive evolution is a two step process that occurs every generation—(1) the production of blind genetic variation and (2) the selective multiplication and recombination of genes by the ecologically directed processes of natural and sexual selection. That is, Natural/sexual selection is the set of environmentally directed adaptive processes that are superimposed on the undirected processes that cause genetic mutations. Natural/Sexual selection selectively multiplies adaptive genes and selectively recombines them each generation.

Science educators need to stand on Darwin's shoulders and work to bring about a more scientifically accurate understanding of the Darwinian theory of adaptive evolution by selection. Presentations that include discussions of all three categories of the ecological causes and all four genetic consequences of selection that Darwin identified would be a very nice start toward this important goal.

Again—Happy 198th Birthday Charlie Darwin from Carl Bajema!!! (February 12, 2007), Department of Biology, Grand Valley State University, Allendale MI 49504 USA [bajemacj@gvsu.edu]