4.4.3 Understanding Darwin’s Theory of Natural Selection

The theory of natural selection has five main components:

1. All organisms are capable of producing offspring faster than the food supply increases.
2. All organisms show variation.
3. There is a fierce struggle for existence, and those with the most suitable variations are most likely to survive and reproduce.
4. Variations, or traits, are passed on to offspring (inherited).
5. Small changes in every generation lead to major changes over long periods of time.

A popular but often-misunderstood concept related to natural selection is the term survival of the fittest. Survival of the fittest does not necessarily mean that the biggest and fastest survive; instead, it refers to those who are most evolutionarily fit. This means that an organism has traits that are sufficient for survival and will be passed on to future generations. The term survival of the fittest was not even introduced by Darwin; rather, it was first used by English philosopher, anthropologist, and sociologist Herbert Spencer, who promoted the now discredited ideology of social Darwinism. Social Darwinism applied the concept of Darwin’s biological evolution to human societies, proposing that human culture was progressing toward the “perfect human.” Spencer’s writings became integrally related to the 19th-century rise of scientific racism and European colonialism.

Examples of Darwin’s theory of natural selection can be found throughout the natural world. Perhaps one of the best known is the color change observed in peppered moths in England during the 19th century. Before the Industrial Revolution, peppered moths in England were a light grey color, well camouflaged on tree branches and less likely to be eaten by birds. Occasionally, through the process of mutation, black moths would appear in the population, but these were usually quickly eaten because they were more visible against light-colored bark. When soot from coal factories began to cover the bark of the trees, the black moths became better camouflaged and the white moths were now more visible. Consequently, the black moths were the ones to survive to reproduce, while the white ones were eaten. In a few decades, all the peppered moths in the cities were black. The process was termed industrial melanism. As coal usage decreased and the bark of the trees once again became lighter in color, white moths again dominated the urban areas.

Examples of natural selection in modern times are numerous. Pesticide resistance in insects is a classic example. Pesticide resistance refers to the decreasing susceptibility of a pest population to a pesticide that previously was effective at controlling it. Pest species evolve pesticide resistance via natural selection, with the most resistant individuals surviving to pass on their ability to resist the pesticide to their offspring. Another good example is the rise of “superbugs,” bacteria that have become increasingly resistant to antibiotics.

The Processes of Evolution

Mutation is the creative force of evolution and represents the first stage of the evolutionary process. Mutation is defined as an alteration in a genetic sequence that results in a variant form. For a mutation to have evolutionary significance, it must occur in the sex cells (sperm and ova). This is because only genetic information that is in the sex cells is passed on from generation to generation. Mutations in non-sex chromosomes will not be passed on from one generation to the next. Whereas other evolutionary forces can modify existing genetic material, only mutation can produce new genetic material. One of the most interesting things about mutations is the fact that they are random. There is no way of predicting when a specific mutation will occur; all scientists can do is estimate the probability of a mutation occurring. Mutations do not necessarily appear when they are needed.

The conventional view is that mutations are harmful, but this is not always true. Some mutations are harmful, some are advantageous, and some are neutral. Advantageous mutations lead to changes that improve an individual’s survival and/or chances of reproduction. The mutation that confers resistance to insecticide in mosquitos led to changes that improved their survival. Likewise, the mutation for black coloration in peppered moths led to increased survival during the Industrial Revolution. Neutral mutations have no effect on survival or reproduction. And some mutations are in fact quite harmful and do negatively affect certain individuals’ survival and reproduction.

Mutations generally occur spontaneously in response to conditions in the body or in the environment. The exact cause of a mutation cannot usually be determined, and the rate of mutation is very difficult to determine. This is because mutations that are neutral or do not lead to obvious changes often go unnoticed. The probability of a mutation at any given gene is between 1 in 10,000 and 1 in 100,000. While the probability that a specific point in an individual’s genetic material will have a mutation is clearly very low, the probability that the totality of an individual’s genetic material will have at least one mutation is much higher. The point is that while rare, mutation is also common. For example, although many mosquitoes have adapted to insecticides through a mutation that confers some resistance to the chemicals, if the mutation had not already been present in the population, the mosquitoes would have died out. The need for a specific mutation had no effect on whether the mutation appeared or not.

There is currently a controversial pilot program in Florida aimed at dealing with mosquitoes against which insecticide sprays have increasingly become ineffective. The first genetically modified mosquitoes were released in the Florida Keys in May of 2021. The genetically altered mosquitoes produce female offspring that die in the larval stage, preventing them from growing to adulthood, in which they can then bite and spread disease. Genetic science currently has the power to use mutations to control or even wipe out an entire species. Genetic engineering has the potential to benefit humanity, but it will undoubtedly also raise ethical questions and controversy.

Genetic Drift

Genetic drift is defined as the effect of random chance on a population, notably the way in which it determines whether an individual survives and reproduces or dies. Imagine that you stick your hand into a bucket filled with Halloween candy. What is the probability you will withdraw a Snickers bar? The composition of Halloween candy in your bucket will be affected by the proportion of people handing out Snickers bars compared to other candy. If each bucket of Halloween candy were a population, then one could say that genetic drift—random chance—was affecting the composition of the candy in your Halloween bucket. An important point about genetic drift is that it is directly and inversely related to population size. The smaller the population, the larger the influence of genetic drift; the larger the population, the smaller the influence of genetic drift. In a large population, say 100,000, removing a couple of individuals will have a truly miniscule effect on the population. Note that in early human evolution, however, population sizes were small, so the effect of genetic drift may have been substantial.

Gene Flow

Gene flow is another important evolutionary force, involving the exchange of genetic material between populations and geographic regions. Without gene flow, there would be no diversity—and without diversity, a species is at higher risk of extinction. Gene flow can be seen in the process of pollination, in which bees or butterflies carry and transfer pollen from one area to another. Anytime a gene is introduced to a new population where it did not exist before, that is gene flow.

Speciation

Speciation is the rise of a new species in response to an environmental change or pressure. Allopatric speciation, mentioned previously, is the most common form of speciation event. During allopatric speciation, a species diverges when two populations become isolated from one another and continue to evolve. This isolation is created by geographic barriers such as mountains, rivers, or oceans. A good example of allopatric speciation is the different species of squirrel found on the two sides of the Grand Canyon. Descended from a common ancestor, the squirrels became reproductively isolated from one another by the Grand Canyon, eventually resulting in different species.

Sympatric speciation involves species that are descended from a common ancestor and remain in one location without a geographic barrier. A good example is the East African cichlid fish, which experience reproduction isolation due not to a physical barrier but to females’ selection of mates with certain coloration. The amount of light that reaches different levels and depths of the lake impacts how colors in the males appear to the females. The East African cichlid fish are also a good example of adaptive radiation. Adaptive radiation is seen when one or more species give rise to many new species in a relatively short time. Research shows that an explosion of about 250 very diverse species of cichlids in Lake Tanganyika occurred in less than 10 million years (Takahashi and Koblmüller 2011). Other research suggests that the common ancestor was the result of a hybrid swarm from two different locations, as seen in Figure 4.16. (Meier et al. 2017).

In peripatric speciation, members of the same population are separated and over time evolve as separate species. Ring speciation is considered by some to be a type of peripatric speciation. Ring speciation occurs when several species coexist for a time in a region near one end of a geographic barrier. When part of the population migrates away from the original population (or gene pool) to the other side of the barrier, reproductive isolation results. Reproductive isolation is strongest for that part of the population that is farthest away from the original population. When too much variation has occurred between two groups, they will no longer interbreed, and as a result, speciation—the development of two separate species—can occur. While fairly rare, ring speciation is believed to explain the different species of the California salamander genus Ensatina.

Gradualism vs. Punctuated Evolution

Biological anthropologists are interested not only in how a species is best defined but also in how often and by what means new species are developed. The traditional view of evolution assumes that morphological, behavioral, and genetic changes occur gradually and accumulate in a single unbroken and unbranching line; this view of evolution is known as gradualism. If this perspective is correct, scientists would expect to find numerous fossils exhibiting evidence that they are slowly and gradually transitioning into new and distinct species. However, while fossils are rare, fossils showing evidence of transitional forms are even rarer. While the dearth of transitional fossils is often attributed to the incompleteness of the fossil record, it has caused some biological anthropologists to question if evolution is truly gradual.

What can be observed in the fossil record are static populations that are interrupted by sudden bursts of change. This phenomenon of long periods of stasis, or no change, followed by quick periods of change is known as punctuated equilibrium. Instead of a gradual accumulation of small changes, punctuated equilibrium suggests that rapid changes due to a variety of environmental factors, including climate change, are characteristic of the formation of new species. The fossil data for a large number of organisms show just this—long periods of stasis followed by rapid and massive change. The scarcity of intermediary forms in the fossil record has led some to conclude that punctuated equilibrium is the dominant theory. However, the fact that intermediary forms do exist suggests that gradualism is also an important factor in the evolution process. One research study found that 30 to 35 percent of speciation events occurred as the result of a sudden event or change, while the remainder showed evidence of gradualism (Phillips 2006). In both the gradual and punctuated models, speciation takes the form of branches through time rather than a linear progression. Evolution is neither linear nor progressive, but rather a branching process—a tree of life containing both areas of divergence and points of a shared common ancestry.

The Tree of Life: Showing Evolutionary Relationships

During Darwin’s time, evolutionary relationships had to be determined largely by structural morphologies and physical characteristics. Molecular science had not yet been developed. The binomial nomenclature discussed earlier not only allowed distinction between species but also provided clues to evolutionary relationships. For example, which of the below species of butterfly would be the most distantly related?

• Danaus gilippus
• Danaus genutia
• Limenitis archippus
• Danaus plexippus
• Danaus petilia

The answer, of course, would be Limenitis archippus, the viceroy butterfly, which is a mimic of the monarch butterfly (Danaus plexippus). The first part of the viceroy’s name, Limenitis, is the genus. The fact that it is different from the others shows that it is more distantly related.

It is important to note that the Linnaean classification system has limits. Sometimes, species can be difficult to identify by physical characteristics alone. Species that exhibit mimicry and larval forms in different stages of development can take on the appearance of other organisms, resulting in errors in classification. Can you tell which of the butterflies in Figure 4.19 is the monarch? Close examination reveals that the markings on the wings are a bit different. The monarch is on the left, and the monarch mimic, the viceroy, is on the right. Likewise, in Figure 4.20, you can see how it might be difficult to correctly classify barnacles, crabs, and limpets based on physical appearances. One may be tempted to classify the barnacle and the limpet as being closely related due to the conical shells that they share, when in actuality, the barnacle is more closely related to the crab, as they are both crustaceans. The conical shells of the barnacle and the limpet are similar adaptations in response to similar environmental pressures, not evidence that they are closely related or share a common ancestor.

Structural Morphologies as Evidence of Relationship

Structural similarities may be derived traits (homologous structures), inherited from a common ancestor, or they may have developed independently (analogous structures). An example of a homologous structure is the grasping hand found in both humans and chimpanzees, which suggests that humans and chimpanzees share a common ancestor that also had a grasping hand. Analogous structures are seen in the wing of a butterfly and the wing of a bat. While both wings serve a similar function, these two organisms likely developed their wings independently and do not necessarily share a common ancestor. Identifying homologies is essential for creating hierarchies of phylogenetic relationships because homology indicates that shared features are due to common descent. However, homologies can be difficult to identify in nature, and they are easy to confuse with analogous traits.

Cladistics, or the use of cladograms, is a method of visually distinguishing between homologous ancestral and derived characteristics. Ancestral characteristics are found in the common ancestor of the species being classified, whereas derived characteristics are only found in the groups in question. An ancestral characteristic that humans share with common ancestors is opposable thumbs. In contrast, a derived trait that is only found in modern humans is the chin. By exclusively looking at derived characteristics, biological anthropologists can develop a clearer understanding of the relationships between the groups being studied.

The Molecular Tree of Life and Phylogenetics

The emergence of genetic and molecular science has provided additional tools and lines of evidence to verify evolutionary relationships. The phylogenetic tree is a model used by modern taxonomists to reveal the complexity and diversity of life and its many branches. Phylogenetic trees show how species and other taxon groups evolved from a series of common ancestors. They are based on both physical and genetic evidence.