Advocates of Intelligent Design contend that complex biological features cannot arise by chance, the implication being that chance equates to sentient forces. From a scientific vantage, however, the driving force of adaptive evolution–natural selection– is itself the antithesis of chance. Hereditary factors that promote organismal survival and reproduction in a particular environment tend to be precisely those that proliferate across the generations and thereby come to characterize natural populations. Whenever genetic variation and differential reproduction exist in nature (as they do in all known species), natural selection is inevitable, both logically and empirically. Biological traits that emerge from this inexorable operation may have the superficial aura of intelligent artistry, but that appearance is illusory (under a scientific interpretation). Natural selection can be a highly creative process (given a suitable supply of genetic variation to work from), but it is merely a mechanistic phenomenon– as inescapable and insentient as gravity.
This is not to say that evolution is devoid of important stochastic (i.e. chance) elements. Natural selection can sift only among the genetic variants available for its scrutiny, and two of the three primary sources of genetic variability– de novo mutation and recombination– occur essentially at random with respect to forging adaptations. The new mutations and recombinant genotypes that arise in each generation have no biased tendency to enhance either an individual’s genetic fitness (its reproductive success relative to other individuals) or the adaptive needs of a species. In other words, favorable alleles and more fit genotypes have no known mutational tendency to arise disproportionately when needed. In this important sense, the genetic fodder upon which natural selection acts can indeed be characterized as stochastic or chancy in origin.
The third source of population genetic variation entails a mixture of “chance and necessity.” Apart from de novo mutations and recombinant genotypes, the genetic variety available for natural selection in any generation is also a function of historical circumstance, that is, of idiosyncratic genealogical outcomes that have been affected by both stochastic and directive evolutionary processes across all prior generations. Evolution going forward can work only with the biological substrates–”ghosts of evolution past”– are not supernatural legacies, but instead they are real genetic lineages and real species that have been subjected for eons to the full panoply of evolutionary processes including natural selection (the directive agent of adaptive evolution) as well as idiosyncratic mutation, recombination, and genetic drift (stochastic forces in the sense described above).
The temporal nature of heredity also means that evolution is inherently a phylogenetic process, involving descent with modification. So, for example, when two or more species share exquisite details in some complex biological feature (such as a long nucleotide sequence for a protein-coding gene), the usual evolutionary interpretation is that these species inherited copies of that trait from a shared ancestor. The creationist explanation, by contrast, posits that God created such traits independently in each species, starting in each case from scratch. At least at a superficial explanatory level, evolutionary and creationist scenarios both seem plausible, in principle, for complex traits that perform their functions well. A more acid test comes from complex traits that are more harmful to their bearers. …Many complex genetic traits (such as pseudogenes and mobile elements) that often are functionless or even detrimental to the organisms that house them are rampant in the genomes of vertebrate animals, humans included. Did a Creator God repeat these apparent errors of genomic construction time and time again? Or are such genomic flaws merely the footprints of phylogenetic history?
Evolutionary processes do not contrive complexity directly from nothing. Natural evolutionary processes operate on genetic lineages much like homeowners work on their houses– by taking advantage of available construction materials to make individually small but sometimes cumulatively substantial alterations to previously existing structures and functions. On occasion, fairly extensive renovations may occur rather quickly, but the norm–both for houses and biological lineages–is evolutionary gradualism in which renovated forms closely resemble their immediate predecessors and increasingly diverge from the preexisting entities after longer periods of time and impetus. In the case of biology, fossils and molecular evidence indicate that most grand evolutionary transitions (such as from ancestral fishes to various derived groups of terrestrial vertebrates) require millions of years and involve many intermediate steps.
The analogy of natural selection to a homeowner can be carried to far, however. A homeowner presumably has a longer-term plan or intent for her renovations, whereas natural selection renovates biological features without foresight. The force of natural selection acts as if myopic, unable to perceive the longer-term consequences of its immediate decisions, which are based solely on the fit of available genotypes to current adaptive needs.
At any horizon in time, history-laden genetic lineages both facilitate and constrain what evolution might accomplish going forward. Natural selection is facilitated in the sense that it need not re-contrive complex biological features ex nihilo in each generation; but it is also constrained because it must operate within the framework of the available (phylo)genetic materials that underlie the existing biological structures and functions in each evolved lineage. This raises a key point germane to this book. Evolutionary causation via natural processes leads to a biological expectation not shared by most versions of ID: a routine appearance of suboptimal organic design.