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Does evolution exhibit progress or purpose?
David H. Bailey
Updated 31 March 2019 (c) 2019
One question that is often raised in discussions of evolution is whether the process of biological evolution exhibits any fundamentally trend for progress or purpose. Creationists and intelligent design writers tend to favor the affirmative, although as we will see below it not completely clear that this view is supportive of their overall objective.
The late Stephen J. Gould was a leading proponent of the view that evolution against the notion of evolutionary progress. In his 1990 book Wonderful Life, he argued at length, based in part on observations of organisms in the Burgess shale, that evolution does not exhibit anything resembling progress or direction. He further suggested that this conclusion also applies to the emergence of human evolution: if we "[w]ind back the tape of life to the early days of the Burgess Shale [and] let it play again from an identical starting point," Gould asserts, "the chance becomes vanishingly small that anything like human intelligence would grace the replay." [Gould1990, pg. 14].
In a later book, Full House, Gould subsequently modified this view to acknowledge that complexity is a one-way street in the sense that it cannot go below zero. Consider a drunk randomly staggering along a sidewalk bounded by a wall on one side and a street on the other. It is a straightforward exercise in the theory of random walks to conclude that eventually the drunk will venture far enough from the wall that he falls into the gutter. In a similar way, Gould argues, evolution can be expected eventually to branch out to unexplored reaches in many directions (i.e., the "branching bush" metaphor of evolution), but this should not be seen in any way as evidence of progress: "The vaunted progress of life is really random motion away from simple beginnings, not directed impetus toward inherently advantageous complexity." [Gould1996, pg. 149-151]. Elsewhere in Full House, Gould writes [Gould1996, pg. 133-134]:
If a sequence of local environments could elicit progressive advance through time, then some expectation of progress might be drawn from natural selection. But no such argument seems possible. The sequence of local environments in any one place should be effectively random through geological time -- the seas come in and the seas go out, the weather gets colder, then hotter, etc. If organisms are tracking local environments by natural selection, then their evolutionary history should be effectively random as well.
But other scientists and scholars find difficulties with Gould's line of reasoning. Robert Wright, for one, notes that Gould fails to acknowledge the effect of "positive feedback," namely complex interactions between competing organisms and competing species within a local environment (and not just large-scale geological effects). When these effects are considered, a significantly different picture emerges, since these positive feedback effects, often called "arms races," can definitely give evolution a direction. Wright notes the following examples
[Wright1999, pg. 265-297; Wright2001]:
- Bombadier beetles. These beetles have evolved to carry two compartments, where one contains a harmless mix of chemicals and the second contains a catalyst, and a nozzle where the two chemicals are combined and ejected. The beetle can aim the poisonous mixture in any direction. This by itself is an astonishing level of complexity. But it is even more remarkable that other animals, including skunks and one species of mouse, have evolved instinctive and highly specialized behavior patterns that result in the spray being discharged harmlessly, permitting them to capture and eat the beetles.
- Western hemisphere mammals. Researchers found that relative brain size (i.e., cranium capacity in proportion to body size) of the fossils of various carnivorous mammals in North America show a definite trend to increase over a time period spanning several tens of millions of years. These researchers found that relative brain size also increased among the herbivorous mammals that were the principal prey of these carnivores. However, similar analyzes of comparable South American herbivores, which faced no predators, showed almost no growth in relative brain size. Evidently species-vs-species competition is conducive to increases in brain size.
- Homo sapiens. Even human evolution shows signs of progress, most likely from positive feedback effects. From Australopithecus africanus to Homo habilis to early Homo sapiens to modern Homo sapiens, spanning roughly three million years of evolution, one sees a steady and rather brisk increase in relative cranium size in fossil specimens, with no signs of backtracking and hardly any pauses.
Another line of evidence that argues against Gould's "tape" scenario is seen in the many examples of "convergent" evolution, namely the phenomenon wherein two or more lines of species independently give rise to very similar features. For example, there is evidence that flight independently arose at least three different times. Numerous biologists haved pointed out that convergences such as these are the equivalent of "replaying the tape," and yet nature again produces similar (albeit not necessarily identical) features.
Simon Conway Morris, a evolutionary paleobiologist at Cambridge University, has written extensively on the subject of convergence. In his recent books on the topic, he highlights the following examples, among others
Conway Morris summarizes his survey of evolutionary convergence with the observation that while "the routes are many, the destinations are few" [ConwayMorris2003, pg. 297].
- Eyes. Eyes have independently developed dozens of times. Even the human-like camera eye, which has an adjustable lens to focus the image on the retina and thus is far superior in acuity to the compound eyes of insects, has independently evolved at least three different times. It is interesting, though, to note that while vertebrate eyes have the nerve endings on the surface of the retina, mollusks (e.g., squid and octopus) have nerve endings behind the retina, which is arguably a superior design because no blind spot is required. In any event, it is remarkable that these camera-eye designs have evolved from lines of evolution that diverged at least 100 million years ago.
- Rhodopsin. Rhodopsin, a special light-sensitive biomolecule that is responsible for color vision, has evolved at least twice. This by itself is remarkable enough, but it is also interesting to see variations of rhodopsin that yield three-way and even four-way color sensitivity, or which permit some species to see violet and ultraviolet light much better than humans.
- Electric fish. Michael Faraday was the first to comment on the electric properties of what we now know as the electric eel from South America. Interestingly, similar properties have evolved in a species of mormyrid fish in lakes and rivers of Africa.
- Myoglobin. Myoglobin is a biomolecule related to hemoglobin, the agent that carries oxygen in many animals and plants. But there is evidence that myoglobin that occurs in cyanobacteria evolved independently of the myoglobin that is found in mammals. What's more, the myoglobin of diving mammals (such as whales) and burrowing mammals have independently evolved a specific strain of myoglobin that stores additional oxygen while in longer dives (or burrows).
- Brains. Humans are known to have the largest brains and the most advanced cognition, by far, among animals species. But numerous other species have developed rather large brains and advanced cognition. Chimps are known to use tools and some have developed a modest facility with language. Whales have been known to thank humans who have helped them. Dolphins have exhibited evidence of self-recognition and have advanced language and social skills. By some measures, the brain size of dolphins approaches that of humans, and this increase in brain size has occurred many millions of years after the divergence between primates and cetaceans. Along this line, a 2011 study indicate that mollusks (octopuses, squid, etc.), which are the most intelligent of the invertebrates, evolved brains at least four times, independently and concurrently [Jabr2011].
- Signal processing. One item not mentioned by Conway Morris (because it was discovered subsequent to the most recent of his books) is the convergent evolution of the on/off signaling mechanism, which is used to reliably process incoming signals in plants an animals, using what is known as tyrosine phosphorylation. In a 2011 study, researchers at the Salt Institute discovered that although plants and animals took distinct evolutionary paths, both hit upon essentially the same mechanism [SD2011c].
- Genetic analyses. A 2012 genetic study (also not mentioned by Conway Morris because it was published more recently than his books) by researchers at Princeton University analyzed the DNA sequences of 29 distantly related insects. They found, remarkably, that 14 of these species have evolved nearly identical changes to a key protein, which grants them resistance to certain cardenolides that plants such as milkweed and dogbane produce to ward off insects. Jianzhi Zhang of the University of Michigan, who was not involved in the study, noted that: "The finding of parallel evolution in not two, but numerous herbivorous insects increases the significance of the study because such frequent parallelism is extremely unlikely to have happened simply by chance." [SD2012d].
Evolution and purpose
The question of whether evolution exhibits purpose is a more difficult question, mainly because it is hard to frame an objectively definable concept of purpose that can be tested through the empirical methods of scientific inquiry. For this reason, it is not at all clear that scientific research can say anything one way or the other on this question. Indeed, one can argue that humans bring purpose to nature rather than the other way around.
While Stephen J. Gould's observations regarding the contingency of nature in general and the contingency of evolution in particular are valuable reminders of the singular nature of our own existence, nonetheless many scientists and scholars believe that he went too far in this line of thought. Indeed, there are many examples in nature of where competition between species has led to acceleration of progress, and where evolution "discovered" remarkably similar solutions to a common problem in distinct evolutionary pathways. The latter examples of "convergent evolution" provide convincing counters to Gould's assertion that if the "tape" of life were replayed, nothing like the modern intelligent human species would arise. To the contrary, scientists such as Conway Morris believe that the emergence of something closely resembling the human species, not only in brainpower but perhaps even in general physical form, was inevitable [ConwayMorris2003, pg. 331-332].
In any event, it is not clear that any of these considerations lend much support to the worldview of either traditional young-earth creationists or their more sophisticated intelligent design colleagues. To begin with, science can say nothing about "purpose" in nature one way or the other, because it is not at all clear that "purpose" can be defined in a testable manner. And while many scientists believe that evolution does exhibit progress in some real sense, note that none of the above-mentioned research suggests that progress in nature requires supernatural intervention. Indeed, to even say "nature" and "supernatural" in the same sentence is an oxymoron, underscoring the difficulty in trying to comment one way or the other on the supernatural from a discipline, namely scientific research, that is fundamentally grounded in the study of natural laws and processes. What's more, the history of those who have sought "impossible" features of nature as "proof" of God's hand is not encouraging -- this is very definition of the "God of the gaps" theology, which has mainly been a source of disillusionment over the ages as science has continued its relentless advance.
For additional discussion, see