|Monkeys, Apes and Humans
Department of Anthropology, University of Missouri-Columbia
In Lecture 2, I wrote that chimps are more closely related to humans than they are to gorillas. This is correct. However, the three evolutionary branches of chimps, gorillas, and humans, may have split at about the same time. More on this later.
Evolutionary principles (cont.)
Adaptive for whom?
An adaptation is a physical trait or behavior produced by natural selection. For an adaptation to persist and spread in a population it must be beneficial. But beneficial for whom? The individual? The group? The species?
Consider the honey bee. When a male is able to find a female and copulate with her, he explodes. Why? His genitals are lodged into the female. By this adaptation, the male is able to fertilize the female AND prevent other males from copulating with the female. He "locks the door." The male, however, dies.
The gene is a replicator; it is able to make an identical versions of itself. Individuals (bees, humans) are vehicles used by replicators to make more replicators.
From this perspective, the exploding male honey bees make sense. The survival of the vehicle, the male honey bee, does not matter as long as replicators, the bee's genes, are successfully promoted. (Exploding males become adaptive only in mating systems where the chance of a second copulation is extremely small.)
Recall "suicidal" lemmings. Early biologists believed that lemmings (a) practice mass suicide and (b) that this trait is an adaptation benefiting the group. They reasoned that if the lemming population exceeds the carrying capacity of the local environment (if they exhaust food supplies) that the group will become extinct. To prevent group extinction, lemmings kill themselves. The gene for mass suicide is an adaptation benefiting the group to the disadvantage of the individual.
This explanation has several problems. First, lemmings don't kill themselves. They migrate to new areas. They are excellent swimmers. By swimming across fjords in groups, an individual lemming is less likely to be swallowed by predators (safety in numbers).
Second, a gene for suicide will not persist. Vehicles (lemmings) with the suicide gene do not reproduce—they kill themselves. In a population of lemmings with suicide genes, consider that a non-suicidal mutation would be very successful. If some lemmings refrain from killing themselves, they would be reproduce more than suicidal individuals and nonsuicidal genes would quickly predominate.
How can altruism be adaptive?
Altruism is cooperative behavior. Individuals who behave altruistically help others to their own disadvantage.
Species with sterile castes are extreme examples of altruism. Many species of insects and one species of mammals—the naked mole rat—live in large communities. Only the queen and few males reproduce in these communities. Worker castes are sterile.
From the gene's perspective, individuals are vehicles that perpetuate genes (replicators). But how do sterile workers perpetuate their genes? Workers do not reproduce but, rather, help the queen to reproduce.
One idea, part of kin selection theory, is that workers are promoting copies of their genes by helping relatives. Relatives have many genes in common; your brother is likely to have many of your genes. By helping relatives to reproduce, you are helping to promote the genes you share. Workers in social insects and naked mole rats are closely related to the reproducing queens. By helping the queen to reproduce, workers perpetuate copies of their genes.
A primate example of altruism: Vervet monkeys emit alarm calls when they see a predator. This allows their fellow monkeys to avoid the predator. However, alarm-calling is costly: the predator will spot and try to kill the monkey giving the alarm. If this cooperative gesture is risky, and if monkeys are vehicles perpetuating genes, why do vervets call?
Recall that in most primate species, females remain together in their birth group. Males move from their birth group into a new group (exception: humans and chimps). That means that in most primate groups, including vervets, females will be closely related, adult males distantly related. Females are more likely to emit alarm calls than males. Why? Because by being altruistic—warning others of predators—females help relatives. By doing so, they promote copies of their genes.
You could view the same behavior as a group-level adaptation. Groups of alarm-calling vervets may be more successful (reproduce more) than groups of vervets without alarm-calling. However, for several reasons, this explanation is weaker than a individual-level explanation. First, individuals reproduce quickly, groups and species reproduce very slowly. Suppose a group consist of individuals each producing two offspring. What happens if a mutation causes an individual to produce three offspring? In several generations, that mutation will spread and predominate in the population. Evolution by individual-level adaptation is much faster than evolution by group-level adaptation.
Second, which gene will be more successful: a self-sacrificing gene benefiting the group or a gene maximizing an individual's reproduction? The second. A self-sacrificing gene, such a suicide gene in lemmings, is not going to persist and spread in a population.
Generally, when we ask whether and how a primate feature or behavior is adaptive, we consider how it benefits the individual and we avoid "good for the species" explanations.
Kinds of Explanations
Why do male howler monkeys howl? If you ask this question to faculty members in different departments you may get four kinds of explanations.
Proximate Mechanisms: An endocrinologist would say that howlers howl because of a hormonal surge. Song birds sing in spring when lengthening day light stimulates testosterone production. This kind of explanation answers the question, why do howlers howl, by referring to environmental cues and physiological mechanisms.
Ontogeny: A developmental psychologist might tell you that male howlers learn to howl as they mature. An ontogenic explanation describes how a trait or behavior develops during an individual's life-span.
Phylogeny: A paleontologist might tell you that howlers howl because their ancestors had the trait. This kind of explanations describes how a trait is constrained by a species' evolutionary history. Pigs don't fly because they did not experience the same evolutionary conditions that birds and insects did. And pigs are unlikely to evolve wings because they are constrained by their evolutionary history.
Evolutionary Function: A primatologist would tell you that male howlers howl to attract a mate and defend their territory. Presumably, males that howl reproduced more than quiet males. Howling, by this kind of explanation, is viewed as an adaptation.
Another example: why do humans grow old and die? Ask this question to a faculty member in the medical school and she will tell you that our bodies (heart, bones, muscles, brain) wear out and break down. She is providing a proximate mechanism and ontogenic explanation. An evolutionist, however, would consider the evolutionary basis of aging (e.g., why humans, compared to other primates, age at the rate they do).
Genes and environment
Usually, genes do not directly determine a trait or behavior. Almost always, genes and environment interact to produce the phenotype. From a gene's point of view, the vehicle (phenotype) carrying genes should be sufficiently flexible. Vehicles that adjust to environmental conditions are more successful than less flexible vehicles.
The genotype-environment relation varies by norm of reaction. In some species, the environment has little influence on how phenotypes develop. In other species, behavior and physical appearance is dependent on environmental context. Humans are at the extremes; because of language, culture, and our imagination, we are highly flexible to changing environmental conditions.
An example of phenotypic flexibility: the Arctic hare. Genes and environment determine fur color. In winter, the fur turns white, in summer, brown. Both colors function to camouflage the hare. Genes predispose fur to change color, changing environment provides the cues. This flexibility is adaptive. A Missouri rabbit, unable to change hair color, would probably not last long during the Arctic winter.
Department of Anthropology
| College of Arts and Science |
| University of Missouri-Columbia |
revised: fall 2004
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