• Introduction to Biology

    Introduction to Biology

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Recognizing Misconceptions about Evolution

Take this quiz to see if you can separate fact from fiction based on your knowledge about evolution.

 

Evolution

The word “evolution” frequently causes confusion because of varying definitions for the term. Even scientists use “evolution” to refer to theories about how evolution occurs or the effect evolution has on populations over time. The distinction between these three concepts (evolution, how evolution occurs, and the effect of evolution over time) is extremely important to recognizing how our knowledge and understanding of evolutionary theory affects all of biology.

Black and white color moth on mossy green treeAny change in the population frequency of a genetically inherited trait is evolution. Because changes in population frequency can occur over just a few generations, evolution can be directly observed, even in humans! The fact that evolution occurs is neither hypothesis nor theory. It is a directly observable fact. Stating that evolution does not occur is similar to stating that gravity does not exist. Both evolution and gravity are phenomena that we can observe in the world around us, and both have spawned theories about how they occur. Using the scientific definition for evolution, many examples of directly observed evolution are readily available in textbooks or on the Internet.

Individuals do not evolve, but populations do. Like all living organisms, humans are born with a set of genes that determine many physical characteristics, or traits. Other traits are determined by the environment or by an interaction between genes and environment. Although mutations occur to alter the genes in one individual, evolution only occurs if a heritable trait spreads to other individuals in a population. Spreading of a genetic trait is accomplished through transfer of genetic information, as occurs when genes are transmitted to offspring during reproduction.

Consider the following statements:

  1. Individuals inherit at least part of their appearance, personality, intelligence, and other traits directly through gene transfer from their parents.
  2. Individuals in the same population show variation in several genetically inherited traits.
  3. Some inherited genetic traits, such as physical characteristics that determine attractiveness to members of the opposite sex, can reduce a person’s reproductive success.

Charles DarwinThese major principles of Darwin’s theory of evolution by natural selection are considered standard knowledge today, yet non-scientists sometimes assume that Darwinian theory is weak or tragically flawed. Although some aspects of Darwin’s theory have been modified in response to new data since his original publication, Darwin’s hypothesis forms the basis for today’s theory of natural selection as a mechanism for evolutionary change.

Charles Darwin’s groundbreaking work was a hypothesis for how evolution occurs. Darwin proposed that some organisms in a population survive to reproduce more often than other organisms. When genetic traits that increase successful reproduction are present in an organism, the genetic material (including the more adapted traits) passes into more offspring than genetic material from a less adapted organism. Over multiple generations, a more adaptive trait may increase in frequency in the organisms living in a particular area, leading to evolution of the population.

Adaptation, is sometimes confused with improvement. Although a heritable trait may be adaptive, improving survival in a given environment, the same trait may decrease survival in a different environment. An adaptive trait is not an “improvement” towards perfection; it simply provides increased reproductive ability in the current environment. Species that exist today are not the top of an evolutionary ladder, they are adapted to their environment. Many current species would struggle to survive if they were transported to an environment that existed millions of years ago.

colony of bacteriaAlthough frequently described as a random process, natural selection is not random. The current environment determines which traits are adaptive, directing the progress of natural selection. Although directed towards adaptation, changes in evolutionary direction can arise through environmental change, the appearance of new traits through genetic mutation, and divergence of organisms to form isolated populations.

A common misconception in evolutionary theory is that natural selection causes adaptations to appear in a population. Adaptive traits must already exist in order for natural selection to increase their frequency in a population. For example, a colony of bacteria that are all sensitive to the antibiotic penicillin will all die if penicillin is added to their growth medium. But, if one bacterial cell already possesses a mutation that renders it insensitive to penicillin, that bacteria will survive and continue to grow while all other bacteria die. This is the basis for antibiotic resistance in a bacterial population, an evolutionary change. Antibiotics do not cause bacteria to become resistant, existing resistant bacteria spread throughout a population because antibiotics provide a selective advantage.

Evolution in Action

How does antibiotic resistance arise in bacteria?

The following animation illustrates the effect of natural selection on a bacterial population. We can observe evolution directly in many existing species, but bacteria provide an excellent opportunity to view evolution as it occurs due to bacteria’s short generation time. All populations exhibit genetic variation, which may be introduced by mutation or influx of organisms from neighboring populations. In bacteria, new gene forms are introduced frequently through a high mutation rate.

Although selective pressure affects the frequency of genetic traits, a genetic trait must already exist for it to spread through a population. In bacteria, if no individual organisms carry a gene from that provides antibiotic resistance and no random mutations convey resistance before the antibiotic kills the organism, then all bacteria would die, and no evolution would occur.

This animation simplifies the process of bacterial growth and antibiotic effects in order to demonstrate the principle. In reality, different antibiotics kill bacteria in different ways and with differing levels of effectiveness. Additionally, bacteria undergo many rounds of division each day, usually dividing about once every 20 minutes.

 

Common Descent

The most controversial aspect of Darwin’s theory of natural selection lies in the effect of evolution over time. Darwin’s On the Origin of Species proposed that the process of evolution by natural selection explains the appearance and existence of all currently existing species. Massive evidence collected over more than a century of research provides enormous support for Darwin’s hypothesis, leading to use of the word theory to describe evolution by common descent.

Some biologists prefer to separate the term evolution into two processes, microevolution and macroevolution. Microevolution is defined as changes in frequency of genetic traits within a single population over multiple generations. When microevolutionary changes are significant, a new species may arise. Changes that affect an entire species or have larger evolutionary implications involve macroevolution. While microevolution can be observed directly, macroevolution cannot. When discussing the controversy over evolutionary theory, macroevolution is usually the target. While no scientific hypothesis can ever be proven, the evidence supporting models for macroevolution and common descent is so great that biological scientists no longer test the overarching concepts of evolutionary theory.

Bee on yellow flowerEvidence of a common origin for all existing species is provided by documenting the appearance of heritable modifications and new species in the fossil record, DNA analysis and comparing living and extinct species, and comparative studies of anatomy, physiology, and embryonic development. In contrast to the massive evidence accumulated for the overall theory, specific processes involved in the evolutionary changes in some individual lines of descent are the subject of continued scrutiny and investigation. Evolutionary theory provides a global mechanism and explanation for the mountain of data available to scientists, and is as close to fact as science reaches. Scientific discussion over details of specific evolutionary processes does not question evolutionary theory as a whole, but provides evidence that scientists continue to examine and evaluate existing data to improve our understanding.

Moth that looks like a leafWhy study evolution? Understanding the factors that affect organism development and evolution over time allow us to understand more about the species that live today. Without evolution, very little of what we observe in nature makes sense. For example, the butterfly shown in the figure mimics the appearance of a dead leaf. Evolutionary theory leads to the conclusion that the dead leaf appearance increases the butterfly’s survival and reproductive capability by hiding it amongst dead leaves. The ability to hide in plain sight increases survival by reducing predation. If the environmental pressure of predation did not exist, what other reason might a butterfly have for resembling a leaf? Or conversely, why don’t all butterflies resemble dead leaves if it is advantageous? Evolutionary theory provides explanations for both questions.

 

Effect of Evolution

This activity will test your ability to identify the environmental effect—coevolution, mimicry, or camouflage—that leads to the spread of inherited physical traits.

Additional Resources

On the Origin of Species
Read the entire text of Charles Darwin's book on the theory of evolution.