Charles Darwin returned from his seminal voyage to the Galapagos Islands in 1836 armed with two very important scientific hypotheses in the field of evolutionary biology, natural selection and sexual selection.  Darwin had observed that plants and animals in this isolated archipelago had undergone changes in size, shape, and features in response to various environmental, social, and geographical changes.  He called this “the struggle for existence” and hypothesized that while populations of plants and animals may increase indefinitely, the environment can only support so many individuals, that the variation brought about by these changes affect the survivability of individuals, and that these variations are then subsequently transferred to offspring of successful individuals.  This is known as Darwin’s theory of natural selection and his hypothesis has been confirmed after repeated observation and testing.  In fact, advanced studies in evolutionary biology, underway in the primate world today, have brought Darwin’s theories of natural selection into the forefront of human endeavors to understand its own beginnings, evolution, and, eventually, the dominant species on planet Earth.

Sexual selection is a type of natural selection specific to competition for mates, in contrast to the resource-centric activities involved in survivability and the transfer of genes to offspring that are hallmarks of natural selection proper.  Specifically, sexual selection is expressed in traits that define animal morphology, creating attractive, showy, or emblematic adornments in the animal kingdom, for instance, that are designed to make one of the sexes very appealing for choosy mates.  Scientists have identified two types of sexual selection, including intrasexual and intersexual selection.  Intrasexual selection occurs in species in which females cannot choose their mates, such as primates.  This intrasexual selection is characterized in groups of animals in which male members compete with one another for access to females.  In species that employ intrasexual selection, sexual dimorphism is a defining characteristic of individuals – males are large and strong, and/or have sharp teeth, for example – to effectively outcompete other male members of the species for access to females for reproduction.  Intersexual selection is a strategy employed by species where female members have evolved choosing males based on phenotypical aspects of the organism.  The classic example of intersexual selection comes from the bird world, in which the male peacock, at great expense both physically and with regards to maintenance, safety, and energy, grows enormously decorative tail feathers, where the boldness of color and the spots of the feathers are determining factors when female members choose a mate.  Primates, such as the chimpanzee, human beings’ earliest, living relatives primarily exhibit intrasexual selection, and are thus most relevant in the story of human evolution.

Darwin observed that sexual selection involves several costs – the pressures associated with, in primates, for example, living in large social groups in the competition for mates and resources.  These costs include the almost-constant need to find food, extra calories needed to support babies and milk production, and, for males, mate guarding and the energy required to deploy various strategies in that process.  Additionally, sexual selection pressures impact the parental investment made by both female and male members of all species.  Generally speaking, selection factors vary substantially between species based on whether the male or female individual invests more in parenting efforts.  Males, for example, tend to not invest too much time in parenting when resources are readily available, allowing them to easily copulate with many female members, or when caring for a child will not help its chances of survival to any significant degree.  And, of course, primate and even human females invest enormously in parenting, as carriers and mothers of infants, ultimately responsible for feeding, socializing, and supporting offspring.  In the end, sexual selection, then, compels individual members to select for traits based on a variety of factors, including fitness, costs, parental investment, and territory-maintenance strategies.

About 5-7 mya human beings’ earliest ancestors diverged from modern day chimpanzees.  The chimpanzee and later, intermediate species that evolved into humankind experienced natural and sexual selection pressures that work to explain the human experience today.  Living in highly complex social groups and a hierarchal intrasexual selection environment, alpha male chimpanzees, along with beta males to a lesser-degree, have access to many females for reproduction.  Arboreal nests and great strength allow the chimpanzee to both protect territory and vulnerable group members from predation.  And mothers work as a group to raise, nurse, and socialize infants, requiring enormous amounts of vegetation each day.  These patterns of behavior, brought about by selection pressures, are the expression of genes whose replication was made possible by the success of these myriad behaviors.

To wit, Lucy, an early hominid, and likewise a distant relative of the chimpanzee, went on display in Portland at the Oregon Museum of Science and Industry in September of 2010.  Museum-goers lined up around the massive building to catch a glimpse of her, and were awestruck as they filed through the dimly lit corridor that wound around the fossilized ancestor, sealed carefully in a protective case.  People today feel a close kinship to Lucy precisely because those natural and sexual selection pressures that were so important in shaping the human animal resonate with them today.  Beautifully adorned modern human women watch fit, competitive potential mates jockey for attention, and subsequently invest enormous resources into carrying, feeding, and nurturing an infant child.  The roots of the human story can be found in the natural and sexual selection pressures that Darwin identified while observing isolated species in the Galapagos – adaptation by natural selection saw the rise of the great apes, our early hominid ancestors like Lucy, and modern humans.  Both forms of selection pressure favored bipedalism, large brains, muscular bodies, large testes, and high energy-requirement ovum that are key features of human beings today.

References

Boyd, Robert, and Joan B. Silk. How Humans Evolved. 7th ed. New York. W.W. Norton & Company. 2015.