2.1: Evolution

All organisms are products of evolution adapted to their environment. (a) Saguaro (Carnegiea gigantea) can soak up 750 liters of water in a single rain storm, enabling these cacti to survive the dry conditions of the Sonora desert in Mexico and the Southwestern United States. (b) The Andean semiaquatic lizard (Potamites montanicola) discovered in Peru in 2010 lives between 1,570 to 2,100 meters in elevation, and, unlike most lizards, is nocturnal and swims. Scientists still do not know how these cold-blood animals are able to move in the cold (10 to 15°C) temperatures of the Andean night. (credit a: modification of work by Gentry George, U.S. Fish and Wildlife Service; credit b: modification of work by Germán Chávez and Diego Vásquez, ZooKeys)

All living organisms, from bacteria to baboons to blueberries, evolved at some point from a different species. Although it may seem that living things today stay much the same, that is not the case—evolution is an ongoing process.

The theory of evolution is the unifying theory of biology, meaning it is the framework within which biologists ask questions about the living world. Its power is that it provides direction for predictions about living things that are borne out in ongoing experiments. The Ukrainian-born American geneticist Theodosius Dobzhansky famously wrote that “nothing makes sense in biology except in the light of evolution” (“Biology, Molecular and Organismic.” American Zoologist 4, no. 4, 1964: 449). He meant that the tenet that all life has evolved and diversified from a common ancestor is the foundation from which we approach all questions in biology.

In anthropology—and in the subfield of biological anthropology in particular—we are especially interested in evolution because it helps to explain the biological foundations of what it means to be 'human.' We learned that anthropology is an evolution-based discipline. This means it is one among many disciplines which recognizes the importance in understanding evolution in order to understand its subjects: humans. Further, by better understanding our place in the 'evolutionary tree' does two things for us. First, it helps us to recognize some of the problems with human exceptionalism—the belief that humans are unique and superior to other living beings, often based on characteristics like emotion, language, reason, culture, or morality. Second, by using comparative studies based on our relationship to other primate species, we can develop new research questions and investigations to better understand our past and present selves as a species. In some cases, our similarities can be even more revealing than our differences! This has led some to question traditional attempts to limit the label of 'human' to our current species Homo sapiens sapiens and continually push the title back in time to include many others including Homo neanderthalensis (Neanderthals), Homo erectus, and even Homo heidelbergensisand more.

Understanding Evolution

Evolution by natural selection describes a mechanism for how species change over time. Written record goes as far back as the ancient Greeks who expressed evolutionary ideas. The modern theory of evolution largely began in the eighteenth century. Naturalist Georges-Louis Leclerc Comte de Buffon reintroduced ideas about the evolution of animals and observed that various geographic regions have different plant and animal populations, even when the environments are similar. Some at this time also accepted that there were extinct species.

James Hutton, a Scottish geologist and naturalist, proposed a theory of slow geological change. It can be difficult for us to conceive of a time when this idea was new! Nineteenth century geologist Charles Lyell popularized Hutton's view. A friend to Charles Darwin. Lyell’s ideas were influential on Darwin’s thinking: the slowness a process of change rather than short, catastrophic bursts, provided an analogy for Darwin's observations about biological change. In the early nineteenth century, Jean-Baptiste Lamarck published a book that detailed a mechanism for evolutionary change. We now refer to this mechanism as inheritance of acquired characteristics: the environment causes modifications that offspring use or disuse during its lifetime and thus bring about change in a species.

Charles Darwin and Natural Selection

In the mid-nineteenth century, two naturalists, Charles Darwin and Alfred Russel Wallace, independently conceived and described the actual mechanism for evolution based on their observations having traveled around the world--through the Pacific and to South and Central America. On the Galápagos Islands west of Ecuador, Darwin observed species of organisms on different islands that were clearly similar, yet had distinct differences. Famously, the ground finches comprised several unique beak shapes (figure below). He observed that some finches closely resembled another finch species on the South American mainland. Darwin imagined that the island species might be species modified from one of the original mainland species. Upon further study, he realized that each finch's varied beaks helped the birds acquire a specific type of food. For example, seed-eating finches had stronger, thicker beaks for breaking seeds, and insect-eating finches had spear-like beaks for stabbing their prey.

Darwin observed that beak shape varies among finch species. He postulated that ancestral species' beaks had adapted over time to equip the finches to acquire different food sources.

Wallace and Darwin both observed similar patterns in other organisms and independently developed similar explanations for how and why such changes could take place. Darwin called this mechanism natural selection. Natural selection, or “survival of the fittest,” is that the more prolific reproduction of individuals with favorable traits that survive environmental change because of those traits. This leads to evolutionary change. Today, we might more accurately describe the process as “survival of the fit enough,” but the core idea remains. Natural selection, Darwin argued, was an inevitable outcome of three principles that operated in nature.

• First, most characteristics of organisms are inherited, or passed from parent to offspring (although no one, including Darwin and Wallace, knew how this happened at the time).
• Second, more offspring are produced than are able to survive. Resources for survival and reproduction are limited.
• Third, offspring vary among each other in regard to their characteristics and those variations are inherited.

Darwin and Wallace reasoned that offspring with inherited characteristics which allow them to best compete for limited resources will survive and have more offspring than those individuals with variations that are less able to compete. These three features working in concert are natural selection.

Given arguments today about evolution, you might not believe it really is that simple—but it is! If the three points outlined above are true (and more and more evidence of this is found every day)—then evolution exists as science understands it.

Remember, scientists will always hold short of claiming evolution has been 'proved'--but it is still the best theory given the preponderance of evidence so far and it has yet to be falsified!

Processes and Patterns of Evolution

Natural selection can only take place if there is variation (differences) among individuals in a population. We now know these differences have a genetic basis when it comes to evolution. This is critical because non-genetic reasons (such as diet) can also cause variation among individuals. Genetic diversity in a population comes from two main mechanisms: mutation and sexual reproduction.

Mutation, a change in DNA, is the ultimate source of new genetic variation in any population. The genetic changes that mutation causes can have one of three outcomes the observable characteristics created by genetic information. These observable characteristics are called phenotypes. A mutation affects the organism's phenotype in a way that:

• gives it reduced fitness—lower likelihood of survival or fewer offspring,
• gives it increased fitness—greater likelihood of survival or more offspring, or
• has no effect on fitness.

Sexual reproduction also leads to genetic diversity: when two parents reproduce, unique combinations of genes produce the unique features of offspring.

We call a heritable trait that helps an organism's survival and reproduction in its present environment an adaptation. Scientists describe groups of organisms adapting to their environment when a genetic variation occurs over time that increases or maintains the population's “fit” to its environment. A platypus's webbed feet are an adaptation for swimming. A snow leopard's thick fur is an adaptation for living in the cold. A cheetah's fast speed is an adaptation for catching prey.

Whether or not a trait is favorable depends on the current environmental conditions. The evolution of species has resulted in enormous variation in form and function. Sometimes, evolution gives rise to groups of organisms that become tremendously different from each other. We call two species that evolve in diverse directions from a common point divergent evolution. In other cases, similar traits evolve independently in distantly related species. For example, flight has evolved in bats, insects, and birds, and they all have structures we refer to as wings, which are adaptations to flight. However, these wings have evolved from very different original structures. We call this phenomenon convergent evolution, where similar traits evolve independently in species that do not share a recent common ancestry.

These physical changes occur over enormous time spans and help explain how evolution occurs.

Evidence of Evolution

The evidence for evolution is compelling and extensive. Looking at every level of organization in living systems, biologists see the signature of past and present evolution.

Fossils provide solid evidence that organisms from the past are not the same as those today. Similarities between them also show gradual evolutionary changes over time. Scientists determine the age of fossils and categorize them from all over the world to determine when the organisms lived relative to each other. The resulting fossil record tells the story of the past and shows the evolution of form over millions of years.

Another type of evidence for evolution is the presence of structures in organisms that share the same basic form. For example, the bones in human, dog, bird, and whale appendages all share the same overall construction (figure below) resulting from their origin in a common ancestor's appendages. Over time, evolution led to changes in the bones' shapes and sizes in different species, but they have maintained the same overall layout. Scientists call these homologous structures.

The similar construction of these appendages (human, dog, bird, and whale) indicates that these organisms share a common ancestor.

Some structures exist in organisms that have no apparent function at all (wings on flightless birds, leaves on some cacti, and hind leg bones in whales) and appear to be residual parts from a past common ancestor. We call these unused structures without function vestigial structures. In other cases, similar characteristics (such as wings in bats and birds) occur because of environmental constraints and not due to a close evolutionary relationship. We call these analogous structures.

Further, convergence of form in organisms that share similar environments provides evidence. For example, species of unrelated animals living in the arctic region have been selected for seasonal white phenotypes during winter blending in with the snow and ice. These similarities occur not because of common ancestry, but because of similar selection pressures—the benefits of predators not seeing them.

The white winter coat of the (a) arctic fox and the (b) ptarmigan’s plumage are adaptations to their environments. (credit a: modification of work by Keith Morehouse)

The study of the anatomy of an organism's development to its adult form (called embryology) also provides evidence of relatedness between now widely divergent groups of organisms. Structures that are absent in some groups often appear in their embryonic forms but then disappear. For example, all vertebrate embryos have 'pharyngeal arches' during embryonic development. These develop into gills and gill slits which fish keep these into adulthood and some amphibians keep as juveniles. In mammals (like humans) these become the mandible (jawbone) any hyoid (a bone in the throat). 'Great ape' embryos, including humans, have a tail structure during their development but which does not develop into a tail by birth.

The geographic distribution of organisms on the planet also follows patterns best explained best by evolution in conjunction with movement over geological time. Broad groups that evolved before the supercontinent Pangaea broke up (about 200 million years ago) are distributed worldwide. Groups that evolved since the breakup appear uniquely in regions of the planet which were connected when those species are thought to have developed. Perhaps the best example is marsupial diversification in Australia with the absence of other mammals. This reflects Australia’s long continental isolation.

Like anatomical structures, the molecular structures of life reflect descent with modification. The universality of DNA (Deoxyribonucleic Acid--the complex molecule serving as the 'genetic instruction manual' for all living organisms) reflects evidence of a common ancestor for all of life. We need not get into the complexities of this here, but in general, the relatedness of groups of organisms is reflected in the similarity of their DNA sequences—exactly the pattern that we would expect from descent and diversification from a common ancestor!

Misconceptions of Evolution

Although the theory of evolution generated some controversy when Darwin first proposed it, biologists almost universally accepted it, particularly younger biologists, within 20 years after publication of On the Origin of Species. Nevertheless, the theory of evolution is a difficult concept and misconceptions about how it works abound.

One of the most common misconceptions is that evolution is "just a theory." Critics of the theory of evolution dismiss its importance by purposefully confounding the everyday usage of the word “theory” with the way scientists use the word. Like other theories describing understood facts about the world, the theory of evolution describes facts about the living world. It has survived significant efforts to discredit it scientifically! The everyday version of the word "theory" meaning a guess is more akin to a "hypothesis." When someone says "just a theory," they are implying that there is little evidence supporting it and that it is still in the process of rigorous testing. This is a mischaracterization.

Another misconception is that species evolve 'on purpose.' Statements such as “species evolve in response to a change in an environment” are quite common, but statements like this run the risk of assigning intention to species. A changed environment results in benefits to some individuals in a population and produces a change in the population. Note that the variation that natural selection works on is already in a population and does not arise in response to an environmental change. Evolution is not 'goal directed.' Species do not become 'better' over time--just 'fit' for their environment. Evolution has no goal of making faster, bigger, more complex, or smarter species. What characteristics evolve in a species are a function of the variation present and the environment, both of which are constantly changing in a non-directional way. A trait that fits in one environment at one time may well be fatal at some point in the future. This holds equally well for every species from fungi to humans.

Also, evolution is the change in a population's genetic composition over time. Another misconception is that individuals evolve. Individuals do change over their lifetime, but this is development in coordination with the individual’s environment. When thinking about the evolution of a characteristic, it is probably best to think about the change of the 'average' of a characteristic in the population over time. Related to the point above "organisms" don't evolve but "species" do.

It is also a common misunderstanding that evolution includes an explanation of life’s origins. Some critics even use this as a means to critique the theory claiming that it fails to do so adequately. But the theory of evolution does not try to explain the origin of life--just how life has changed since it began. Put simply, only cnce a mechanism of inheritance was in place were living things subject to the principle of natural selection.