Learning Objectives

Learning Objectives

In this section, you will explore the following questions:

  • How was the present-day theory of evolution developed?
  • What is adaptation, and how does adaptation relate to natural selection?
  • What are the differences between convergent and divergent evolution, and what are examples of each that support evolution by natural selection?
  • What are examples of homologous and vestigial structures, and what evidence do these structures provide to support patterns of evolution?
  • What are common misconceptions about the theory of evolution?

Connection for AP® Courses

Connection for AP® Courses

Millions of species, from bacteria to blueberries to baboons, currently call Earth their home, but these organisms evolved from different species. Furthermore, scientists estimate that several million more species will become extinct before they have been classified and studied. But why don’t polar bears naturally inhabit deserts or rain forests, except, perhaps, in movies? Why do humans possess traits, such as opposable thumbs, that are unique to primates but not other mammals? How did observations of finches by Charles Darwin, visiting the Galapagos Islands in the 1800s, provide the foundation for our modern understanding of evolution?

The theory of evolution as proposed by Darwin is the unifying theory of biology. The tenet that all life has evolved and diversified from a common ancestor is the foundation from which we approach all questions in biology. As we learned in our exploration of the structure and function of DNA, variations in individuals within a population occur through mutation, allowing more desirable traits to be passed to the next generation. Due to competition for resources and other environmental pressures, individuals possessing more favorable adaptive characteristics are more likely to survive and reproduce, passing those characteristics to the next generation with increased frequency. When environments change, what was once an unfavorable trait may become a favorable one. Organisms may evolve in response to their changing environment by the accumulation of favorable traits in succeeding generations. Thus, evolution by natural selection explains both the unity and diversity of life.

Convergent evolution occurs when similar traits with the same function evolve in multiple species exposed to similar selection pressure, such as the wings of bats and insects. In divergent evolution, two species evolve in different directions from a common point, such as the forelimbs of humans, dogs, birds, and whales. Although Darwin’s theory was revolutionary for its time because it contrasted with long-held ideas (for example, Lamarck proposed the inheritance of acquired characteristics), evidence drawn from many scientific disciplines, including the fossil record, the existence of homologous and vestigial structures, mathematics, and DNA analysis supports evolution through natural selection. It is also important to understand that evolution continues to occur; for example, bacteria that evolve resistance to antibiotics or plants that become resistant to pesticides provide evidence for continuing change.

Information presented and the examples highlighted in this section support concepts outlined in Big Idea 1 of the AP® Biology Curriculum Framework. The AP® Learning Objectives listed in the Curriculum Framework provide a transparent foundation for the AP® Biology course, an inquiry-based laboratory experience, instructional activities, and AP® exam questions. A Learning Objective merges required content with one or more of the seven Science Practices.

Big Idea 1 The process of evolution drives the diversity and unity of life.
Enduring Understanding 1.A Change in the genetic makeup of a population over time is evolution.
Essential Knowledge 1.A.1 Natural selection is a major mechanism of evolution.
Science Practice 5.3 The student can evaluate the evidence provided by data sets in relation to a particular scientific question.
Learning Objective 1.9 The student is able to evaluate evidence provided by data from many scientific disciplines that support biological evolution.
Essential Knowledge 1.A.2 Natural selection acts on phenotypic variations in populations.
Science Practice 1.2 The student can describe representations and models of natural or man-made phenomena and systems in the domain.
Learning Objective 1.5 The student is able to connect evolutionary changes in a population over time to a change in the environment.
Essential Knowledge 1.A.4 Biological evolution is supported by scientific evidence from many disciplines, including mathematics.
Science Practice 2.2 The student can apply mathematical routines to quantities that describe natural phenomena.
Learning Objective 1.2 The student is able to evaluate evidence provided by data to qualitatively and quantitatively investigate the role of natural selection in evolution.
Essential Knowledge 1.A.4 Biological evolution is supported by scientific evidence from many disciplines, including mathematics.
Science Practice 5.3 The student can evaluate the evidence provided by data sets in relation to a particular scientific question.
Learning Objective 1.12 The student is able to connect scientific evidence from many scientific disciplines to support the modern concept of evolution.
Essential Knowledge 1.A.4 Biological evolution is supported by scientific evidence from many disciplines, including mathematics.
Science Practice 5.2 The student can refine observations and measurements based on data analysis.
Learning Objective 1.10 The student is able to refine evidence based on data from many scientific disciplines that support biological evolution.
Enduring Understanding 1.C Life continues to evolve within a changing environment.
Essential Knowledge 1.C.3 Populations of organisms continue to evolve.
Science Practice 7.1 The student can connect phenomena and models across spatial and temporal scales.
Learning Objective 1.26 The student is able to evaluate given data sets that illustrate evolution as an ongoing processes.
Essential Knowledge 1.C.3 Populations of organisms continue to evolve.
Science Practice 7.1 The student can connect phenomena and models across spatial and temporal scales.
Learning Objective 1.25 The student is able to describe a model that represents evolution within a population.
Essential Knowledge 1.C.3 Populations of organisms continue to evolve.
Science Practice 7.1 The student can connect phenomena and models across spatial and temporal scales.
Learning Objective 1.4 The student is able to evaluate data-based evidence that describes evolutionary changes in the genetic makeup of a population over time.

The Science Practices Assessment Ancillary contains additional test questions for this section that will help you prepare for the AP exam. These questions address the following standards:

  • [APLO 1.10]
  • [APLO 1.12]
  • [APLO 1.13]
  • [APLO 1.31]
  • [APLO 1.32]
  • [APLO 1.27]
  • [APLO 1.28]
  • [APLO 1.30]
  • [APLO 1.14]
  • [APLO 1.29]
  • [APLO 1.26]
  • [APLO 4.8]

The Origin of Life

The Origin of Life

For as long as humans have existed on Earth, we have wondered about our origins. Early on, many civilizations believed that life on earth originated from deities or other supernatural powers. Organisms on Earth were believed to be created to live in their environment and were therefore specifically designed to match our habitats. This meant that living things were immutable. Greek philosophers such as Aristotle believed that organisms on Earth occupied a ladder of increasing complexity and each species had a permanent spot on this ladder. Based on these beliefs, scientists such as Carolus Linnaeus designed classification schemes to organize the diversity of life based on this linear hierarchy.

As scientists observed the world around them, however, it became clear that life was not immutable. When Georges Cuvier studied fossils, for example, he found that life forms in older strata were more different than fossils in newer strata, suggesting that life changed over time. This led to some scientists such as Jean-Baptiste de Lamarck to hypothesize that organisms changed over time by adapting to their environment. As parts of the body were used more often, they were strengthened, and as parts of the body were used less, they were weakened. These traits would then be passed down from parent to successive generations. Although these theories were ultimately proven as unsupported, they formed a good basis for scientists such as Charles Darwin to form the theory of natural selection.

Charles Darwin and Natural Selection

Charles Darwin and Natural Selection

In the mid-nineteenth century, the actual 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 on H.M.S. Beagle, including stops in 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, like 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 species of organisms on different islands that were clearly similar, yet had distinct differences. For example, the ground finches inhabiting the Galápagos Islands comprised several species with a unique beak shape (Figure 18.2). 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 species 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, and insect-eating finches had spear-like beaks for stabbing their prey.

Illustration shows four different species of finch from the Galápagos Islands. Beak shape ranges from broad and thick to narrow and thin.
Figure 18.2 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.

Wallace and Darwin both observed similar patterns in other organisms and they 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.

For example, a population of giant tortoises found in the Galapagos Archipelago was observed by Darwin to 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 couldn’t reach the food source. Consequently, long-necked tortoises would be more likely to be reproductively successful 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 no one, including Darwin and Wallace, knew how this happened at the time, it was a common understanding. Second, more offspring are produced than are able to survive, so resources for survival and reproduction are limited. 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. Both Darwin and Wallace’s understanding of this principle came from reading an essay by the economist Thomas Malthus who discussed this principle in relation to human populations. 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. Because characteristics are inherited, these traits will be better represented in the next generation. This will lead to change in populations over 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.

Papers by Darwin and Wallace (Figure 18.3) 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. His book outlined in considerable detail his arguments for evolution by natural selection.

Paintings of Charles Darwin and Alfred Wallace are shown.
Figure 18.3 Both (a) Charles Darwin and (b) Alfred Wallace wrote scientific papers on natural selection that were presented together before the Linnean Society in 1858.

Demonstrations of evolution by natural selection are time consuming and difficult to obtain. One of the best examples has been demonstrated in the very birds that helped to inspire Darwin’s theory: the Galápagos finches. Peter and Rosemary Grant and their colleagues have studied Galápagos finch populations every year since 1976 and have provided important demonstrations of natural selection. The Grants found changes from one generation to the next in the distribution of beak shapes with the medium ground finch on the Galápagos island of Daphne Major. The birds have inherited variation in the bill shape with some birds having wide deep bills and others having thinner bills. During a period in which rainfall was higher than normal because of an El Niño, the large hard seeds that large-billed birds ate were reduced in number; however, there was an abundance of the small soft seeds which the small-billed birds ate. Therefore, survival and reproduction were much better in the following years for the small-billed birds. In the years following this El Niño, the Grants measured beak sizes in the population and found that the average bill size was smaller. Since bill size is an inherited trait, parents with smaller bills had more offspring and the size of bills had evolved to be smaller. As conditions improved in 1987 and larger seeds became more available, the trend toward smaller average bill size ceased.

Career Connection

Field Biologist

Many people hike, explore caves, scuba dive, or climb mountains for recreation. People often participate in these activities hoping to see wildlife. Experiencing the outdoors can be incredibly enjoyable and invigorating. What if your job was to be outside in the wilderness? Field biologists by definition work outdoors in the field. The term field in this case refers to any location outdoors, even under water. A field biologist typically focuses research on a certain species, group of organisms, or a single habitat (Figure 18.4).

Photo shows a scientist next to a tranquilized polar bear laying on the snow.
Figure 18.4 A field biologist tranquilizes a polar bear for study. (credit: Karen Rhode)

One objective of many field biologists includes discovering new species that have never been recorded. Not only do such findings expand our understanding of the natural world, but they also lead to important innovations in fields such as medicine and agriculture. Plant and microbial species, in particular, can reveal new medicinal and nutritive knowledge. Other organisms can play key roles in ecosystems or be considered rare and in need of protection. When discovered, these important species can be used as evidence for environmental regulations and laws.

Processes and Patterns of Evolution

Processes and Patterns of Evolution

Natural selection can only take place if there is variation, or differences, among individuals in a population. Importantly, these differences must have some genetic basis; otherwise, the 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 being taller because of better nutrition rather than different genes.

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 alleles, or new genetic variation in any population. The genetic changes caused by mutation can have one of three outcomes on the phenotype. A mutation can affect the phenotype of the organism in a way that gives it reduced fitness—lower likelihood of survival or fewer offspring. Alternatively, a mutation may produce a phenotype with a beneficial effect on fitness. And, many mutations will also have no effect on the fitness of the phenotype; these are called neutral mutations. Mutations may also have a whole range of effect sizes on the fitness of the organism that expresses them in their phenotype, from a small effect to a great effect. Sexual reproduction also leads to genetic diversity: When two parents reproduce, unique combinations of alleles assemble to produce the unique genotypes and thus phenotypes in each of the offspring.

A heritable trait that helps the survival and reproduction of an organism in its present environment is called an adaptation. Scientists describe groups of organisms becoming adapted to their environment when a change in the range of genetic variation occurs over time that increases or maintains the fit of the population to its environment. The webbed feet of platypuses are an adaptation for swimming. The snow leopards’ thick fur is an adaptation for living in the cold. The cheetahs’ fast speed is an adaptation for catching prey.

Many of these adaptations are the result of thousands of generations of evolutionary changes and thus involve multiple genes. However, the mutation of just one gene can result in a phenotypic change that can increase or decrease an organism’s adaptation to its environment. For example, a single nucleotide change can change the color of a field mouse from black to white. In a single generation, this new mouse would be more adapted to survive in snowy terrains than its parents.

Whether or not a 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.

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. When two species evolve in diverse directions from a common point, it is called divergent evolution. Such divergent evolution can be seen in the forms of the reproductive organs of flowering plants which share the same basic anatomies; however, they can look very different as a result of selection in different physical environments and adaptation to different kinds of pollinators (Figure 18.5).

Photo showing a Dense Blazing Star (Liatrus spicata) and a Purple Coneflower (Echinacea purpurea).
Figure 18.5 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. (credit a: modification of work by Drew Avery; credit b: modification of work by Cory Zanker)

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, the wings of bats and insects have evolved from very different original structures. This phenomenon is called convergent evolution, where similar traits evolve independently in species that do not share a recent common ancestry. The two species came to the same function, flying, but did so separately from each other.

These physical changes occur over enormous spans of time and help explain how evolution occurs. Natural selection acts on individual organisms, which in turn can 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 the genotype of an entire species to evolve. It is over these large time spans that life on earth has changed and continues to change.

Evidence of Evolution

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. Darwin dedicated a large portion of his book, “On the Origin of Species”, to identifying patterns in nature that were consistent with evolution, and since Darwin, our understanding has become clearer and broader.


Fossils provide solid evidence that organisms from the past are not the same as those found today, and fossils show a progression of evolution. 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 (Figure 18.6). For example, scientists have recovered highly detailed records showing the evolution of humans and horses.

Photo A shows a museum display of hominid skulls that vary in size and shape. Illustration B shows five extinct species related and similar in appearance to the modern horse. The species vary in size from that of a modern horse to that of a medium-sized dog.
Figure 18.6 In this (a) display, fossil hominids are arranged from oldest (bottom) to newest (top). As hominids evolved, the shape of the skull changed. An artist’s rendition of (b) extinct species of the genus Equus reveals that these ancient species resembled the modern horse (Equus ferus) but varied in size.

Anatomy and Embryology

Another type of evidence for evolution is the presence of structures in organisms that share the same basic form. For example, the bones in the appendages of a human, dog, bird, and whale all share the same overall construction (Figure 18.7) resulting from their origin in the appendages of a common ancestor. Over time, evolution led to changes in the shapes and sizes of these bones in different species, but they have maintained the same overall layout. Scientists call these synonymous parts homologous structures.

Illustration compares a human arm, dog and bird legs, and a whale flipper. All appendages have the same bones, but the size and shape of these bones vary.
Figure 18.7 The similar construction of these appendages indicates that these organisms share a common ancestor.

Some structures exist in organisms that have no apparent function at all, and appear to be residual parts from a past common ancestor. These unused structures without function are called vestigial structures. Examples of vestigial structures include wings on flightless birds, leaves on some cacti, and hind leg bones in whales.

Link to Learning

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Visit this interactive site to guess which bones structures are homologous and which are analogous, and see examples of evolutionary adaptations to illustrate these concepts.

What is the basic difference between things that are homologous and things that are analogous?
  1. Things that are analogous look similar and things that are homologous do not.
  2. Things that are analogous have the same function and things that are homologous have different functions.
  3. Things that are analogous are not a result of evolution, whereas things that are homologous are.
  4. Things that are analogous result from convergence and things that are homologous result from common ancestry

Another evidence of evolution is the convergence of form in organisms that share similar environments. For example, species of unrelated animals, such as the arctic fox and ptarmigan, living in the arctic region have been selected for seasonal white phenotypes during winter to blend with the snow and ice (Figure 18.8ab). These similarities occur not because of common ancestry, but because of similar selection pressures—the benefits of not being seen by predators.

The left photo depicts an arctic fox with white fur sleeping on white snow, and the right photo shows a ptarmigan with white plumage standing on white snow.
Figure 18.8 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)

Embryology, the study of the development of the anatomy of an organism to its adult form, also provides evidence of relatedness between now widely divergent groups of organisms. Mutational tweaking in the embryo can have such magnified consequences in the adult that embryo formation tends to be conserved. As a result, structures that are absent in some groups often appear in their embryonic forms and disappear by the time the adult or juvenile form is reached. For example, all vertebrate embryos, including humans, exhibit gill slits and tails at some point in their early development. These disappear in the adults of terrestrial groups but are maintained in adult forms of aquatic groups such as fish and some amphibians. Great ape embryos, including humans, have a tail structure during their development that is lost by the time of birth.


The geographic distribution of organisms on the planet follows patterns that are best explained by evolution in conjunction with the movement of tectonic plates over geological time. Broad groups that evolved before the breakup of the supercontinent Pangaea, about 200 million years ago, are distributed worldwide. Groups that evolved since the breakup appear uniquely in regions of the planet, such as the unique flora and fauna of northern continents that formed from the supercontinent Laurasia and of the southern continents that formed from the supercontinent Gondwana. The presence of members of the plant family Proteaceae in Australia, southern Africa, and South America, for example, is best explained by their presence prior to the southern supercontinent Gondwana breaking up.

The great diversification of marsupials in Australia and the absence of other mammals reflect Australia’s long isolation. Australia has an abundance of endemic species—species found nowhere else—which is typical of islands whose isolation by expanses of water prevents species from migrating. Over time, these species diverge evolutionarily into new species that look very different from their ancestors that may exist on the mainland. The marsupials of Australia, the finches on the Galápagos, and many species on the Hawaiian Islands are all unique to their one point of origin, yet they display distant relationships to ancestral species on mainlands.

Molecular Biology

Like anatomical structures, the structures of the molecules of life reflect descent with modification. Evidence of a common ancestor for all of life is reflected in the universality of DNA as the genetic material and in the near universality of the genetic code and the machinery of DNA replication and expression. Fundamental divisions in life between the three domains are reflected in major structural differences in otherwise conservative structures such as the components of ribosomes and the structures of membranes. In general, the relatedness of groups of organisms is reflected in the similarity of their DNA sequences—exactly the pattern that would be expected from descent and diversification from a common ancestor.

DNA sequences have also shed light on some of the mechanisms of evolution. For example, it is clear that the evolution of new functions for proteins commonly occurs after gene duplication events that allow the free modification of one copy by mutation, selection, or drift (changes in a population’s gene pool resulting from chance), while the second copy continues to produce a functional protein.

Direct Observations

Evolution has also been well documented in direct observations made by scientists. For mammalian species such as humans with long reproductive cycles, it takes thousands if not millions of years for significant changes in allelic frequency to be observed in the gene pool. However, in species such as bacteria with faster reproductive cycles, it is possible to observe natural selection at work. When bacteria are exposed to antibiotics, for example, the alleles that help bacteria resist the antibiotic increase rapidly Figure 18.9. This is because non-resistant individuals quickly die off, leaving only resistant individuals in the population.

The figure shows a line graph. The X-axis is labeled Year and the Y-axis is labeled percent resistant. The X-axis has tick marks for 1983, 1985, 1987, 1989, 1991, 1993, 1995, 1997, 1999, and 2001.  The Y-axis has tick marks for 0, 5, 10, 15, 20, 25, and 30. The data is presented annually and there are red circles showing the percent for each year. From 1983 to 1988, the value is 0 percent. In 1989, the value is just above the 0 percent line. The line follows a clear upward path, reaching approximately 30
Figure 18.9

Adaptations for homeostasis

Adaptations for homeostasis

Many organisms have similar anatomical, morphological, and molecular features because they have all descended from the same common ancestor. As organisms have evolved and adapted to their environment, they have developed new mechanisms to maintain homeostasis in their new niche. For example, organisms need oxygen to perform cellular respiration. When they were unicellular, oxygen simply diffused across the cellular membrane. As the organisms grew larger and more complex, they evolved a vasculature to bring dissolved gases to cells too far away from the external environment to allow the passive diffusion of gases. Later, when organisms adapted to land, they developed lungs so that gas exchange could occur with blood to bring fresh oxygen to the tissues.

Misconceptions of Evolution

Misconceptions of Evolution

Although the theory of evolution generated some controversy when it was first proposed, it was almost universally accepted by biologists, 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.

Link to Learning

QR Code representing a URL

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

>Select one misconception about evolution and explain what you might say to someone to dispel it.

  1. Misconception—Evolution is not a well-founded theory. Correction—Although evolution cannot be observed occurring today, there is strong evidence in the fossil record and in shared DNA sequences to support the theory
  2. Misconception—Humans are not currently evolving. Correction—The environmental pressures humans face are different than the ones they faced several thousands of years ago, but they are still there, and they are still producing evolutionary change.
  3. Misconception—Evolution produces individuals that are perfectly fit to their environment. Correction—Evolution produces random changes in the genetic code that sometimes lead to adaptations
  4. Misconception—Evolution is a random process. Correction—Evolution is a force that makes animals adapt to perfectly fit the environment they are living in

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 body of thoroughly tested and verified explanations for a set of observations of the natural world. Scientists have a theory of the atom, a theory of gravity, and the theory of relativity, each of which describes understood facts about the world. In the same way, the theory of evolution describes facts about the living world. As such, a theory in science has survived significant efforts to discredit it by scientists. In contrast, a theory in common vernacular is a word meaning a guess or suggested explanation; this meaning is more akin to the scientific concept of hypothesis. When critics of evolution say evolution is just a theory, they are implying that there is little evidence supporting it and that it is still in the process of being rigorously tested. This is a mischaracterization.

Individuals Evolve

Evolution is the change in genetic composition of a population over time, specifically over generations, resulting from differential reproduction of individuals with certain alleles. Individuals do change over their lifetime, obviously, but this is called development and 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 of the average value of the characteristic in the population over time. For example, when natural selection leads to 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, this average value will be different as a result of evolution. Although some individuals may survive from the first time to the second, they will still have the same bill size; however, there will be many new individuals that contribute to the shift in average bill size.

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 believe that it cannot explain the origin of life. The theory does not try to 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, and presumably it 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.

However, once a mechanism of inheritance was in place in the form of a molecule like DNA either within a cell or 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.

Organisms Evolve on Purpose

Statements such as “organisms evolve in response to a change in an environment” are quite common, but such statements can lead to two types of misunderstandings. First, the statement must not be understood to mean that individual organisms evolve. 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 change in the population if the characteristics 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 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 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 antibiotic.

In a larger sense, evolution is 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, despite the commonness of this kind of language in popular discourse. 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 the human species.

Science Practice Connection for AP® Courses


Using information from a book or online resource such as Jonathan Weiner’s “The Beak of the Finch”, explain how contemporary evidence drawn from multiple scientific disciplines supports the observations of Charles Darwin regarding evolution by natural selection. Then, in small groups or as a whole class discussion or debate, present an argument to dispel misconceptions about evolution and how it works.

Lab Investigation

AP® Biology Investigative Labs: Inquiry-Based, Investigation 8: Biotechnology: Bacterial Transformation. You will explore how genetic engineering techniques can be used to manipulate heritable information by inserting plasmids into bacterial cells.

Think About It

What selection pressures may affect the survival and reproduction of a group of pea seeds scattered by a person along the ground?


This section may include links to websites that contain links to articles on unrelated topics.  See the preface for more information.