It was in nature that Darwin sought God. He spent much of his free time searching for insects in the forests and fields around the university and reading accounts of archaeological expeditions in South America. He was fascinated by the way other cultures lived and the strange places they called home. An adventurous itch began to build in Charles, and when he was given the opportunity to join Captain Robert FitzRoy and the crew of the
Beagle
on a five-year expedition to map out new trade routes of Argentina and neighboring countries that had just been delivered from Spain’s control, he jumped right in.The twenty-two-year-old boarded the
Beagle
on December 7, 1831, a beautiful autumn day in Plymouth, prophetically carrying with him for the voyage a copy of his favorite verse,
Paradise Lost
.
For five years the crew of the
Beagle
charted out the southern waters, past the Canary and Cape Verde islands, on toward Montevideo, Tierra del Fuego, and around Cape Horn. They veered north through the Strait of Magellan, carefully avoiding the iceberg fields that emanate from the Antarctic Circle to within a few hundred miles of the South American coast in spring. Onward, they skirted the shoreline of Chile, stopping for approximately one month at a tiny group of islands known as the Galapagos, just south of the equator.
The time Darwin spent on these islands gathering specimens and the details that he recorded of how functionally well adapted each species seemed to be to its environment had a profound impact on his thinking about evolution, but not until some two years after the visit. While on the islands he recorded a rich variety of birds, particularly the finches, wrens, and warblers that, surprisingly, looked almost identical except for their differing beaks. Some had large, blunt beaks that were used for crushing large seeds, while others had rather narrow and elongated beaks (think needle-nose pliers) that were used to extract small seeds and insects from difficult-to-reach places. Darwin dutifully noted these distinctions and sailed on, returning to England in the fall of 1836 a changed man.
Once home Charles began to receive reports from the leading British zoologists about the specimens he brought back from his travels. These caused confusion at first, but a theory soon emerged that would change the world, and it stemmed from the differences in the birds he saw in the Galapagos.What Darwin failed to realize at the time, but soon learned after returning to England, was that these were
all
finches. The primary factor distinguishing them was beak morphology. It was puzzling how so many different species of finch came to inhabit such a small area, and he reasoned that perhaps they all descended from a common ancestor and gradually, after many generations, began to diverge in appearance. He thought of Lamarck and his “transmutation of acquired characteristics,” but he did not accept the idea that changes accumulating within a lifetime could be passed down to offspring. Alternatively, he speculated that the different beak shapes must give each finch a special advantage to living in its local environment. For example, finches with long, narrow beaks would have an advantage in securing food in places that might not be reached by a finch equipped with a larger beak. With a more reliable food supply, the long-beaked finches would have an edge over their natural competitors and be more likely to reproduce. Likewise, in other parts of the islands where elderberry brush was plentiful, finches with more powerful and compact beaks dominate, since they alone can manage to grind the hard casing of their seeds to an edible pulp.
Darwin also knew what farmers understood for years—offspring tend to resemble their parents. Farmers select crops for their next harvest by determining which strains produced the best product this year and replant them hoping to build on their success in future seasons. Eventually this process of farmer selection results in enough accumulated differences that entirely new varieties emerge. In an instant of recognition, Darwin was terrified by the implications of his theory. If he was right, finches (and other animals) are selected by natural competition for food, sex, water, and all means of subsistence, and those that just happen to have an adaptive edge (perhaps because of a longer beak, or keener eyesight, or faster flight) are more likely to reproduce and bear similar offspring, thus ensuring a continuation of that lineage. Other, less successful finches that do not possess the advantages are less likely to survive to reproductive age in that environment, thus minimizing the spread of their characteristics to offspring. The different varieties of finches he saw on the islands were functional success stories—the good seeds. Each had some adaptations that secured reproductive success in their given environment at the expense of competing birds.
This was a deeply sad story for Charles, a religious man who had great difficulty accepting the idea that finches and other birds—indeed, all animals including humans—must evolve over time. Animal forms are not fixed through eternity, but rather molded by a process of natural selection—imagine, selection of a species without a Selector. It took Darwin fourteen years of confidence-building before he made his theory public in the magnum opus
On the Origin of Species by Means of Natural Selection
. The public outcry of blasphemy was, as expected, enormous, and included many of his scientific colleagues, friends, and even family (his wife wasn’t thrilled with the theory). Of course, an unwillingness to accept species selection without divine intervention continues today.
Darwin concentrated on evolution by natural selection in
Origin of Species
, which made virtually no reference to human behavior. This is not an oversight. Darwin was troubled by the lack of an obvious way to account for the development of so many uniquely human activities, such as making music and art, by the theory of natural selection. What is the survival function of singing a pretty melody, making someone laugh, being able to tell a good story, or creating a work of art? None of these human qualities seemed to fit into the same theory as that of the finches growing different-size beaks to adapt to distinct environmental challenges.
In his follow-up work
The Descent of Man and Selection in Relation to Sex
, Darwin reconciles the appearance of these behaviors by developing the theory of evolution by sexual selection—a special form of natural selection. The bottom line of evolution is the survival of genes down through the generations.An organism may survive to a ripe old age, but if it fails to reproduce, its genes die right along with it. For selection to drive evolution, an organism must survive and reproduce.
The adaptations that result from sexual selection are referred to as ornaments by evolutionary biologists. The example given earlier was of the peacock’s enormous and well-decorated plumage, which plays a direct role in attracting the attention of peahens. Darwin argued that the evolution of this display was driven by female choice.
In species where there is competition for mate selection, the elaboration of secondary sexual characteristics (that is, those not serving a direct function in reproduction) usually occur in males to vie for the attention of females, who have a greater metabolic investment in reproduction.
The English geneticist Angus Bateman observed that in many species, females bear a much larger burden for producing an offspring than males. This inequity begins with the production of sex cells. Women produce approximately four hundred nutrient-rich ova during an entire lifetime, while men produce billions of sperm that are replenished at a rate of about eleven million to twelve million per hour. A female has a relatively small number of eggs at any given time, and a single male can fertilize all of them; hence the female will not produce more offspring by mating with more than one male. Contrasting this, males are capable of fathering many more offspring than any one female can bear if he mates with several at a time. Of course, fertilization and gestation occur internally within females, consuming great amounts of time and metabolic resources. If a successful birth occurs, this is followed by potentially several years of lactation to feed the child and still many additional years of investment in raising the toddler to an autonomous age. Thus, in many species where these conditions exist, females must be far more choosy in selecting mates than males.
Female choice has been shown to drive a broad range of adaptive traits or ornaments in males. A key question that has troubled biologists for decades is how particular traits are selected in the first place. For a trait to form through sexual selection there must be some initial preference for it by the female. Early theorists such as Sir Ronald Fisher, a geneticist who also made pioneering contributions to modern statistics, argued that the initial preference might be completely arbitrary. Say, for example, that a small number of female peahens developed a preference for mating with peacocks with brighter-than-usual plumage. Peacocks having brighter plumage would be more likely to mate with these peahens and produce peacock offspring with brighter plumes and peahens with a preference for bright plumes. Assuming the traits (producing the bright plume and preferring the bright plume) become genetically correlated, this would produce a positive feedback system where both traits become increasingly magnified over many generations until plumes become so large that they begin to have survival costs associated with their production that temper the process. At this point, greater elaboration of the plume should be curtailed, since it will result in survival deficits. An equilibrium point would emerge where the positive sexual selection effects of large, bright plumes are perfectly balanced by their negative survival costs.
But why would peahens develop a preference for larger, brighter plumes in the first place? We can find some help here in the work of biologists such as William D. Hamilton and Marlena Zuk, who pointed out that many ornaments are excellent indicators of genetic fitness. The fitness indicator theory suggests that any population where there is an imbalance of investment in producing offspring should theoretically result in the sex with the greater investment also having more at stake in mate selection. This would create pressure for that sex to be able to identify and pair with the most genetically fit mate available. In this example, female choice involves finding the best male genes with which to join with her own so that her offspring have the best chance of survival and reproducing. Of course, finding the best genes is tricky.
One argument might be that the best male genes are those that are fairly different from the female’s own, since this will reduce the possibility of recessive combinations being expressed (genetic diversity prevailing over homogeneity).Another argument might be to simply find genes that are healthy in general. Either way, it has been shown that in many sexually reproducing animals (including humans), key traits that are reliably associated with genetic fitness can be detected and used for mate selection.
The fitness indicator theory of sexual selection is fairly convincing, since it explains a great deal of empirical observations across many species where the development of a particular trait seems to have extended well beyond the limits imposed by survival costs. The case of the peacock’s plume is a perfect example in this regard, since its exaggerated growth makes it a target for predators. Indeed, the handicap principle championed by biologist Amotz Zahavi (see chapter 8) says that it is exactly this cost that makes it a relevant fitness indicator.
Fitness indicator theory goes a long way in explaining why some traits seem to be taken to extremes beyond which survival costs would be accrued. It also helps explain how initial female preferences for a trait, if reliably associated with genetic fitness, can emerge. Peahens that have preferences for traits that have poor or no correlation with genetic fitness are in trouble. If they use these traits for mate selection, they are essentially gambling with their reproductive success. If they have chosen poorly, their genes, along with the trait preferences they support, will be less likely to thrive and be propagated.
In this context it is easy to imagine how pleasure might play a pivotal role in this process. As we have seen in the chapters to this point, the pleasure instinct drives the emergence of distinct receiver biases—preferences for certain forms of sensory stimulation that are critical for normal brain development and maturation. If pleasure-associated preferences for particular forms of stimulation guide an organism toward traits that are also good fitness indicators, this combination may prove to be very useful during mate selection.
Imagine Sally is being pursued by Harry. If Sally makes her selection of a mate purely on the basis of fitness indicators without regard to whether they bring her pleasure, she might do so by simply summing up the tally and attributing an equal value to each indicator. A completely different approach would be to use the pleasure associated with the appearance of a particular fitness indicator to gauge its current importance relative to others. In this context, pleasure is the common currency for organizing and prioritizing competing goals and interests. Using this approach, Sally would be able to assess and rank-order which fitness indicators are more important than others so she could make a more informed choice that matches her current needs. This process would be far more flexible and adaptive to changing environmental circumstances than one based on a simple summation of overall values across all indicators.
If, at this particular time, Sally has not eaten for two days and is faced with a choice between Harry, who has taken her to dinner, and Tom, who has taken her to a movie sans dinner, the choice will be very different depending on the approach. If she chooses solely on the basis of fitness indicators, either Harry or Tom will do, since they have both done things to display their fitness. Harry has shown that he can provide food, a critical resource for survival. Tom has shown that he has sufficient wealth that he can waste money on things not directly related to survival. Both have provided evidence of their fitness. If forced to choose simply on the basis of fitness indicators, Sally might have to toss a coin to decide. A different outcome would occur, however, if pleasure is used to rank-order the relative importance of fitness indicators to match her current needs. In this scenario, Harry is the clear winner, since he has delivered what she needs most at present—nutrition.