4.1.1: Understanding Evolution

Understanding Evolution

• Please read and watch the following Mandatory Resources
• Reading the material for understanding, and taking notes during videos, will take approximately 2 hours.
• Optional Activities are embedded.
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Learning Objectives

• Describe the historical influences on Darwin’s and Wallace's theory of evolution
• Explain the requirements of evolution.
• Identify the major misconceptions of evolution and why they are incorrect.

Introduction: Evolution

Video

This 12-minute video summarizes much of the material below. You may want to revisit this video at the end of the Unit. 
Question after watching: Darwin famously wrote that “endless forms most beautiful have been and are being evolved.” In what way do we know that evolution is still taking place today? 

The theory of evolution is the unifying theory of biology, meaning it is the framework within which biologists ask questions about the living world. The Ukrainian-born American geneticist Theodosius Dobzhansky famously wrote in 1964 that “nothing makes sense in biology except in the light of evolution.” ¹ The tenet that all species have evolved and diversified from a common ancestor is the foundation from which we approach all questions in biology. It provides a direction for predictions about living things and has been validated through extensive scientific experimentation. Evolution is simply a change in the gene pool of a species over time. More technically, evolution is the change in genetic composition of a population over time, specifically over generations, resulting from differential survival and reproduction of individuals. Evolution by natural selection describes one mechanism for this change. Natural selection is introduced here but is discussed in further detail in Section 2.2.3. Additional mechanisms are discussed in Section 2.2.4. 

Adaptations

A heritable trait that aids the survival and reproduction of an organism in its present environment is called an adaptation. A population is adapted when a change in the range of genetic variation occurs over time that increases or maintains the survivability of that population in its current environmental conditions. The webbed feet of platypuses are an adaptation for swimming. The snow leopard’s thick fur is an adaptation to living in the cold. The cheetah's fast speed is an adaptation for catching prey.

Figure \(\PageIndex{1}\): All organisms are products of evolution via adaptations 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 Sonoran 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.

Whether or not an individual trait is favorable depends on the environmental conditions at the time. The same traits are not always selected because environmental conditions can change. For example, consider a species of plant that grew in a moist climate and did not need to conserve water. Large leaves were selected because they allowed the plant to obtain more energy from the sun. Large leaves require more water to maintain than small leaves, and the moist environment provided favorable conditions to support large leaves. After thousands of years, the climate changed and the area no longer had excess water. The direction of natural selection shifted so that plants with small leaves were selected because those populations were able to conserve water to survive the new environmental conditions.

Video

This video identifies the three types of adaptations organisms may possess.
Questions after watching: What sorts of adaptations might you expect to arise in a plant species or a bacterial/unicellular species? Do you expect that the three types described in the video for animals will apply to these types of species?

  History of the Theory of Evolution  

Well before Darwin began to explore the concept of evolution, the idea that species change over time had already been suggested and debated. The view that species are static and unchanging was grounded in the writings of Plato more than 2000 years ago, yet there were also ancient Greeks who expressed ideas about evolution. During the eighteenth century, ideas about the evolution of animals were reintroduced by the naturalist Georges-Louis Leclerc Comte de Buffon who observed that various geographic regions have different plant and animal populations, even when the environments are similar. It was also accepted that there are extinct species.

During this time, a Scottish naturalist named James Hutton proposed that geological change occurs gradually by the accumulation of small changes over long periods of time. This theory contrasted with the predominant view of the time: that the geology of the planet is a consequence of catastrophic events that occurred during a relatively brief past. During the nineteenth century, Hutton’s views were popularized by the geologist Charles Lyell, who was a friend of Charles Darwin. Lyell’s ideas, in turn, influenced Darwin’s concept of evolution. The greater age of the earth proposed by Lyell supported the gradual evolution that Darwin proposed, and the slow process of geological change provided an analogy for the gradual change in species.

In the early nineteenth century, Jean-Baptiste Lamarck published a book that detailed a different mechanism for evolutionary change. This mechanism is now referred to as an inheritance of acquired characteristics. This idea states that modifications in an individual are caused by its environment, or the use or disuse of a structure during its lifetime. These changes can be inherited by its offspring, bringing about change in a species. While this mechanism for evolutionary change is now known to be incorrect, Lamarck’s ideas were an important influence on the initial concepts of evolution, especially in regards to epigenetics, or the influence of the environment on genetic expression.

Charles Darwin and Alfred Russell Wallace 

In the mid-nineteenth century, the mechanism for evolution was independently conceived of and described by two naturalists: Charles Darwin and Alfred Russel Wallace. Importantly, each naturalist spent time exploring the natural world on expeditions to the tropics. From 1831 to 1836, Darwin traveled around the world to places like South America, Australia, and the southern tip of Africa. Wallace traveled to Brazil to collect insects in the Amazon rainforest from 1848 to 1852 and to the Malay Archipelago from 1854 to 1862. Darwin’s journey, as with Wallace’s later journeys to the Malay Archipelago, included stops at several island chains, the last being the Galápagos Islands west of Ecuador.

On these islands, Darwin observed that species of organisms on different islands were clearly similar, yet had distinct differences. For example, the ground finches inhabiting the Galápagos Islands comprised several species with unique beak shapes. The species on the islands had a graded series of beak sizes and shapes with very small differences between the most similar. He observed that these finches closely resembled another finch species on the mainland of South America.

Darwin imagined that the island species might be modified from one of the original mainland species. Upon further study, he realized that the varied beaks of each finch helped the birds acquire a specific type of food. For example, seed-eating finches had stronger, thicker beaks for breaking seeds, while insect-eating finches had spear-like beaks for stabbing their prey. In 1860, he wrote, “seeing this gradation and diversity of structure in one small, intimately related group of birds, one might really fancy that from an original paucity of birds in this archipelago, one species had been taken and modified for different ends.”

Figure \(\PageIndex{2}\): Darwin observed that beak shape varies among finch species. He postulated that the beak of an ancestral species had adapted over time to equip the finches to acquire different food sources.

Origin of the Idea of Natural Selection

Wallace and Darwin observed similar patterns in other organisms and independently developed the same explanation for how and why such changes could take place. Darwin called this mechanism natural selection. Natural selection, also known as “survival of the fittest,” is the more prolific reproduction of individuals with favorable traits that survive environmental change because of those traits. This leads to evolutionary change, the trait becoming predominant within a population. For example, Darwin observed that a population of giant tortoises found in the Galápagos Archipelago have longer necks than those that lived on other islands with dry lowlands. These tortoises were “selected” because they could reach more leaves and access more food than those with short necks. In times of drought, when fewer leaves would be available, those that could reach more leaves had a better chance to eat and survive than those that could not reach the food source. Consequently, long-necked tortoises would more successfully reproduce and pass the long-necked trait to their offspring. Over time, only long-necked tortoises would be present in the population.

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 how traits were inherited was unknown at the time. Second, more offspring are produced than are able to survive. The capacity for reproduction in all organisms outstrips the availability of resources to support their numbers. Thus, there is competition for those resources in each generation (note: when Darwin mentioned “survival of the fittest,” he was referring to the competition between members of the same species for resources, not between different species, a common misconception). Both Darwin and Wallace were influenced by an essay written by economist Thomas Malthus who discussed this principle in relation to human populations. Third, Darwin and Wallace reasoned that offspring with the inherited characteristics that 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. Because characteristics are inherited, these traits will be better represented in the next generation. This will lead to change in populations over successive generations in a process that Darwin called descent with modification. Ultimately, natural selection leads to greater adaptation of the population to its local environment; it is the only mechanism known for adaptive evolution. 

Video

This 9-minute video is an overview of Darwin’s concept of natural selection which is explained in a slightly different manner than the reading. Notably, it explains how evolution by natural selection has been harnessed in agriculture and farming.
Question after watching: Which aspects of natural selection are random and which are not random?

Papers by Darwin and Wallace presenting the idea of natural selection were read together in 1858 before the Linnean Society in London. The following year, Darwin’s book, On the Origin of Species, was published which outlined his arguments for evolution by natural selection.

Figure \(\PageIndex{3}\): Both (a) Charles Darwin and (b) Alfred Wallace wrote scientific papers on natural selection that were presented together before the Linnean Society in 1858.

Optional Activity \(\PageIndex{1}\)

Which scientific concept did Charles Darwin and Alfred Russell Wallace independently discover?

a. mutation
b. natural selection
c. overbreeding
d. sexual reproduction

Answer

b. natural selection

Requirements for Evolution

All of the following are required for evolution to occur.

Variation

One of the hallmarks of Darwin and Wallace's theory of evolution via natural selection is that it can only take place if there is variation, or differences, among individuals in a population. Importantly, these differences must have a genetic basis (their genotypes); otherwise, selection will not lead to change in the next generation. This is critical because variation among individuals can be caused by non-genetic reasons, such as an individual losing a limb due to an accident. Variations of this type would not be inherited by the next generation. 

Individuals of a population often display different phenotypes, or physical, metabolic, or behavioral expressions of their genotypes. Populations with two or more variations of particular characteristics, or trait, are called polymorphic. The distribution of phenotypes among individuals, known as the population variation, is influenced by a number of factors, including the population’s genetic structure and the environment (Figure \(\PageIndex{4}\)). Understanding the sources of a phenotypic variation in a population is important for determining how a population will evolve in response to different evolutionary pressures.

Figure \(\PageIndex{4}\): The distribution of phenotypes in this litter of kittens illustrates population variation. (credit: Pieter Lanser)

The theory of natural selection stems from the observation that some individuals in a population are more likely to survive longer and have more offspring than others; thus, they will pass on more of their genes to the next generation. A big, powerful male gorilla, for example, is more likely than a smaller, weaker one to become the population’s silverback, the pack’s leader who mates far more than the other males of the group. The pack leader will father more offspring, who share half of his genes, and are likely to also grow bigger and stronger like their father. Over time, the genes for bigger size will incase in frequency in the population, leading to a greater percentage of larger gorillas in the population. As a consequence, the average size of a gorilla in this population will increase. That is, this would occur if this particular selection pressure, or driving selective force, were the only one acting on the population. In other examples, better camouflage or a stronger resistance to drought might pose a selection pressure. In addition to natural selection, below are the details of five other agents of evolutionary change that are all acting in concert with natural selection.

This variation, or genetic diversity, within a population comes from two main mechanisms: mutation and, in those species that reproduce sexually, sexual reproduction. Mutation, a change in the DNA sequence, is the ultimate source of new alleles, or new genetic variation in any population. The genetic changes caused by mutation can have one of three outcomes:

• Many mutations will have no effect on the genetic contribution of an individual to future generations; these are called neutral mutations.
• A mutation may affect the phenotype of the organism in a way that gives it a lower likelihood of survival or fewer offspring.
• A mutation may produce a phenotype with a beneficial effect on survival or its genetic contribution to future generations.

Different mutations will have a range of above effects on an organism that expresses them in their phenotype, from a small to great.

Sexual reproduction also leads to genetic diversity: when two individuals of a sexually-reproducing species reproduce, unique combinations of alleles assemble to produce the unique genotypes and thus phenotypes in each of the offspring. However, sexual reproduction does not lead to new genes, but rather provides a new combination of genes in a given individual.

Inheritance

The trait or adaptation being selected for or against must be heritable.

Differential Reproduction and Survival

Individuals with a certain version of the trait are more likely to survive and reproduce than individuals with a different version of the trait

Time

The version of the trait that helps individuals survive and reproduce becomes more common in the population over multiple generations. Individuals do not evolve.

Optional Activity \(\PageIndex{2}\)

Which of the following situations may lead to natural selection?

a. The seeds of two plants land near each other and one grows larger faster.
b. Two types of fish eat the same kind of food but one is better able to gather food than the other.
c. In a species of fungi, one colony spreads faster through the soil to reach nutrients than another colony of the same species.
d. All of the above may lead to natural selection.

Answer

d. All of the above may lead to natural selection.

Patterns of Evolution

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. We can see such divergent evolution in the forms of the reproductive organs of flowering plants which share the same basic anatomies (Figure \(\PageIndex{5}\)); however, they can look very different as a result of selection in different physical environments and adaptation to different kinds of pollinators.

Figure \(\PageIndex{5}\): Flowering plants evolved from a common ancestor. Notice that the (a) dense blazing star (Liatrus spicata) and the (b) purple coneflower (Echinacea purpurea) vary in appearance, yet both share a similar basic morphology.

In other cases, similar phenotypes evolve independently in distantly related species. For example, flight has evolved in both bats and insects, and they both have structures we refer to as wings, which are adaptations to flight. However, bat and insect 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 common ancestry. The two species came to the same function, flying, but did so separately from each other.

These physical changes occur over enormous time spans and help explain how evolution occurs. Natural selection acts on individual organisms, which can then shape an entire species. Although natural selection may work in a single generation on an individual, it can take thousands or even millions of years for an entire species' genotype to evolve. It is over these large time spans that life on earth has changed and continues to change.

Optional Activity \(\PageIndex{3}\)

Which situation is most likely an example of convergent evolution?

a. Squid and humans have eyes similar in structure.
b. Worms and snakes both move without legs.
c. Some bats and birds have wings that allow them to fly.
d. All of the above are examples of convergent evolution.

Answer

D. All of the above are examples of convergent evolution.

The Modern Synthesis: Natural Selection and Genetics

The mechanisms of inheritance, genetics, were not understood at the time Darwin and Wallace were developing their idea of natural selection. This lack of understanding was a stumbling block to comprehending many aspects of evolution. In fact, blending inheritance was the incorrect, but predominant, genetic theory of the time. This made it difficult to understand how natural selection might operate. Darwin and Wallace were unaware of the genetics work by Austrian monk Gregor Mendel, which was published in 1866, not long after the publication of On the Origin of Species. Mendel’s work was rediscovered in the early twentieth century at which time geneticists were rapidly coming to an understanding of the basics of inheritance. Initially, the newly discovered particulate nature of genes made it difficult for biologists to understand how gradual evolution could occur. But over the next few decades, genetics and evolution were integrated into what became known as the modern synthesis—the coherent understanding of the relationship between natural selection and genetics that took shape by the 1940s and is generally accepted today. In sum, the modern synthesis describes how evolutionary pressures, such as natural selection, can affect a population’s genetic makeup, and, in turn, how this can result in the gradual evolution of populations and species. The theory also connects this gradual change of a population over time, called microevolution, with the processes that gave rise to new species and higher taxonomic groups with widely divergent characters, called macroevolution.

Misconceptions about Evolution

The theory of evolution has been amply tested by researchers over the past 200 years. From this work, confidence in the theory’s basic tenants has been bolstered. Today, the vast majority of scientists accept the theory of evolution by natural selection as the most plausible, supported mechanism to explain how organisms change over time. 

However, certain groups, led by values other than the scientific search for evidence, have objected to the theory of evolution including as an explanation for the diversity of life. This has led to the spread of some misinformation and misconception about the theory. Below, these misconceptions are explicitly addressed.

Concept in Action

This website addresses some of the main misconceptions associated with the theory of evolution.

Misconception #1: 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. In science, a theory is understood to be a concept that has been extensively tested and supported over time. We have a theory of the atom, a theory of gravity, and the theory of relativity, each of which describes the strong evidence that scientists have to support our understanding of nature and the universe. In the same way, the theory of evolution describes our scientific understanding about life. As such, a theory in science has survived significant efforts to discredit it by scientists, who are naturally skeptical. While theories can sometimes be overturned or revised, this does not lessen their weight but simply reflects the constantly growing state of scientific understanding. In contrast, a “theory” in common vernacular means a guess or suggested explanation for something. This meaning is more along the lines of the concept of a scientific hypothesis, which is a tentative explanation for something that is proposed to either be supported or disproved. When critics of evolution say evolution is “just a theory,” they are mischaracterizing the science and inferring that there is little evidence supporting it. This is far from accurate. Evolution is widely accepted, extensively tested, heavily supported, and can be monitored in real-time, such as with Darwin's finches. 

Misconception #2: Individuals Evolve

An individual is born with the genes it has—these do not change as the individual ages. Therefore, an individual cannot evolve or adapt through natural selection. Individuals do change over their lifetime, but this is called development; it involves changes programmed by the set of genes the individual acquired at birth in coordination with the individual’s environment. When thinking about the evolution of a characteristic, it is probably best to think about the change in the average value of the characteristic in the population over time. For example, when natural selection leads to a bill-size change in medium-ground finches in the Galápagos, this does not mean that individual bills on the finches are changing. If one measures the average bill size among all individuals in the population at one time, and then measures the average bill size in the population several years later after there has been a strong selective pressure, this average value may be different as a result of evolution. Although some individuals may survive from the first time to the second, those individuals will still have the same bill size. However, there may be enough new individuals with different bill sizes to change the average bill size.

Misconception #3: Evolution Explains the Origin of Life

It is a common misunderstanding that evolution includes an explanation of life’s origins. Conversely, some of the theory’s critics complain that it cannot explain the origin of life. The theory of evolution explains how populations change over time and how life diversifies—the origin of species. It does not shed light on the beginnings of life including the origins of the first cells, which is how life is defined. The mechanisms of the origin of life on Earth are a particularly difficult problem because it occurred a very long time ago, over a very long time, and presumably just occurred once. Importantly, biologists believe that the presence of life on Earth precludes the possibility that the events that led to life on Earth can be repeated because the intermediate stages would immediately become food for existing living things. The early stages of life included the use or formation of organic molecules such as carbohydrates, amino acids, or nucleotides. If these were formed from inorganic precursors today, they would simply be broken down by living things. The early stages of life also probably included more complex aggregations of molecules into enclosed structures with an internal environment, a boundary layer of some form, and an external environment. Such structures, if they were formed now, would be quickly consumed or broken down by living organisms.

However, once a mechanism of inheritance was in place in the form of a molecule like DNA or RNA, either within a cell or within a pre-cell, these entities would be subject to the principle of natural selection. More effective reproducers would increase in frequency at the expense of inefficient reproducers. So while evolution does not explain the origin of life, it may have something to say about some of the processes operating once pre-living entities acquired certain properties.

Misconception #4: Organisms Evolve on Purpose

Statements such as “organisms evolve in response to a change in an environment,” are quite common. There are two easy misunderstandings possible with such a statement. First of all, the statement must not be understood to mean that individual organisms evolve, as was discussed above. The statement is shorthand for “a population evolves in response to a changing environment.” However, a second misunderstanding may arise by interpreting the statement to mean that the evolution is somehow intentional. A changed environment results in some individuals in the population, those with particular phenotypes, benefiting and, therefore, producing proportionately more offspring than other phenotypes. This results in a change in the population if the characters are genetically determined.

It is also important to understand that the variation that natural selection works on is already in a population and does not arise in response to an environmental change. For example, applying antibiotics to a population of bacteria will, over time, select for a population of bacteria that are resistant to antibiotics. The resistance, which is caused by a gene, did not arise by mutation because of the application of the antibiotic. The gene for resistance was already present in the gene pool of the bacteria, likely at a low frequency. The antibiotic, which kills the bacterial cells without the resistance gene, strongly selects for individuals that are resistant, since these would be the only ones that survived and divided. Experiments have demonstrated that mutations for antibiotic resistance do not arise as a result of the antibiotic application.

In a larger sense, evolution is also not goal-directed. Species do not become “better” over time; they simply track their changing environment with adaptations that maximize their reproduction in a particular environment at a particular time. Evolution has no goal of making faster, bigger, more complex, or even smarter species. This kind of language is common in popular literature. Certain organisms, ourselves included, are described as the “pinnacle” of evolution, or “perfected” by evolution. 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. What trait is fit in one environment at one time may well be fatal at some point in the future. This holds equally well for a species of insect as it does for the human species.

Misconception #5: Evolution Is Controversial among Scientists

The theory of evolution was controversial when it was first proposed in 1859, yet within 20 years virtually every working biologist had accepted evolution as the explanation for the diversity of life. The rate of acceptance was extraordinarily rapid, partly because Darwin had amassed an impressive body of evidence. The early controversies involved both scientific arguments against the theory and the arguments of religious leaders. It was the arguments of the biologists that were resolved after a short time, while the arguments of religious leaders have persisted to this day.

The number of working scientists who reject the theory of evolution, or question its validity and say so, is small. A Pew Research poll in 2009 found that 97 percent of the 2500 scientists polled believe species evolve.² The support for the theory is reflected in signed statements from many scientific societies such as the American Association for the Advancement of Science, which includes working scientists as members. Many of the scientists who reject or question the theory of evolution are non-biologists, such as engineers, physicians, and chemists. There are no experimental results or research programs that contradict the theory. There are no papers published in peer-reviewed scientific journals that appear to refute the theory. The latter observation might be considered a consequence of the suppression of dissent, but it must be remembered that scientists are skeptics and that there is a long history of published reports that challenged scientific orthodoxy in unpopular ways. Examples include the endosymbiotic theory of eukaryotic origins, the theory of group selection, the microbial cause of stomach ulcers, the asteroid-impact theory of the Cretaceous extinction, and the theory of plate tectonics. Research with evidence and ideas with scientific merit are considered by the scientific community. Research that does not meet these standards is rejected.

Video

This 4-minute video provides an overview of the main misconceptions about evolution, how they came to be, and what the accurate interpretation should be.
Question after watching: How can the expression “survival of the fittest” be misunderstood? Explain how it can be misunderstood and the accurate concept. 

Footnotes

1. 1 Theodosius Dobzhansky. “Biology, Molecular and Organismic.” American Zoologist 4, no. 4 (1964): 449.
2. 2 Pew Research Center for the People & the Press, Public Praises Science; Scientists Fault Public, Media (Washington, DC, 2009), 37.Contributors