On Evolution

17. Evolution’s Unanswered Questions

We have come far in recent centuries and decades toward understanding how life evolves. Yet many outstanding questions remain, and undoubtedly countless more are not yet even imagined. Every evolutionary biologist must have a list of favorite scientific questions, and here are four of mine. . What are the evolutionary bases of animal behaviors and instincts? Especially in recent years, scientists have come a long way toward deciphering the genetic and developmental underpinnings of many morphological, physiological, and biochemical traits in a wide range of species. Far more refractory, it seems, are attempts to understand the precise mechanistic bases of animal behaviors and instincts. How in the world do genes program into a honeybee the capacity to communicate to a hive, through details of its dance, the distance and direction of a nectar source? How do segments of DNA dispose and enable a female sea turtle to navigate thousands of kilometers of ocean in a return migration, after an absence of more than a decade, to nest at her natal site? How in the world do physical pieces of genetic material instill an orb-weaving spider or a hummingbird with the drive and ability to craft intricate webs of silken strands and lovely nests of lichens, respectively? An amazing but indisputable empirical fact is that such instincts and behaviors gradually evolve in various lineages, just as other categories of organismal phenotype do. Thus, heritable genetic variation for such traits must be present. A major challenge for evolutionary biology is to illuminate, in particular instances, precisely how information stored and transmitted in physical molecules (of DNA) can ultiEvolution ’s Unanswered Questions 17 mately translate into such seemingly ethereal outcomes as “instinctual knowledge.” . Why did sleep evolve? At face value, sleep might seem to be highly maladaptive, wasting precious time that otherwise could be spent in fitness-enhancing activities such as foraging, finding mates, and rearing offspring. By regularly disabling a creature’s senses and awareness, sleep also increases risks to predation, and in general diminishes an animal’s capacity to respond properly to immediate threats or opportunities in its environment . Of course, the pat medical response is that sleep is necessary for energetic rejuvenation, but the deeper question remains— why? In theory, the behavioral tactic “sleep” should be evolutionarily fragile, because any genes that predispose animals to get by on less sleep should enjoy a large selective advantage over their sleepinducing counterparts. If natural selection for sleeplessness is indeed strong, why the phenotype has seldom or never evolved presents an enigma. The paradox of sleep is emblematic of similar queries that likewise apply to many suboptimal phenotypes. Scientists understand that evolutionary traits can be slipshod, history-laden, and generally unlike what a conscious omnipotent agent might engineer. But exactly why, in each particular instance, do various organismal traits fall far short of designer perfection? . What is the evolutionary significance of “junk DNA”? The genomes of complex organisms hold vast amounts of DNA that at face value appear superfluous if not detrimental to an organism’s well-being. In most multicellular species, including humans, more than  of the nuclear genome is composed of highly repetitive elements that have proliferated (quite like viruses or miniature parasites ) to oft-astounding numbers within all cells of an organismal lineage . Such elements act as if selfish, that is, vested in self-replication and evolutionary survival even if these actions negatively impact an  On Evolution organism’s genetic fitness. Other categories of DNA such as pseudogenes and intergenic spacer regions add to the total pool of DNA without obvious utility to the organism. A wonderful challenge for evolutionary biologists is to gain a much better understanding of what formerly was considered “junk DNA.” What fraction of this genomic trash (either fresh or recycled) will actually prove to be of near-term or longer-term evolutionary benefit to its hosts, and how so? . What made us human? A draft sequence of the full three-billion-base-pair human genome was published in , and the sequence of the similar-size chimpanzee genome followed just a few years later. Everything that differentiates humans from chimpanzees has evolved within the past five million years (following separation of these species from a common ancestor), and the genetic alterations responsible must reside somewhere within the relatively small fraction of nucleotides (ca. ) that proved to distinguish the aligned human and chimp genomes. Sequencing these two genomes was...