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Signed in but can't access contentOxford Academic is home to a wide variety of products. The institutional subscription may not cover the content that you are trying to access. If you believe you should have access to that content, please contact your librarian. Institutional account managementFor librarians and administrators, your personal account also provides access to institutional account management. Here you will find options to view and activate subscriptions, manage institutional settings and access options, access usage statistics, and more. Introduction to Evolution by Natural SelectionWithout physically counting bacteria or viruses, scientists estimate that there are more than 8.7 million species of organisms living on Earth today. The origin and extinction of so many species has fascinated scientists for thousands of years, since the days of ancient Greece. The great diversity of living organisms on Earth is best explained by the evidence-based scientific concept of evolution by natural selection. Paid Video Workshop: Natural Selection IntroductionEvolution is the change in inherited characteristics or traits in a population of organisms over many generations. The mechanism that best explains evolution is a phenomenon known as natural selection. Natural selection is the process by which certain inherited traits—such as the color of a fish, height of a person, or shape of a leaf—are favored within a population. A population is a group of organisms that mate and reproduce with one another. In general,
Thousands of years of selective pressures have determined the shapes, colors, sizes, and behaviors that optimize the survival and reproductive success of organisms in the environment in which they evolved. In fact, it is often possible to tell a lot about where something lives by how it looks and behaves. Evolutionary fitness and success refers to surviving long enough to pass genetic material on to offspring. Traits that are passed on to offspring because they contribute to success are “selected for continuation.” Traits that are eliminated from the population because they detract from success are “selected against continuation.” Any force in the environment that favors or disfavors traits is a selective agent. A force can be biological, like a predator, or physical, like temperature. Over time, populations subjected to different selective agents may become so different that they are no longer able to breed with one another. The biological definition of species is a group of organisms that can successfully breed with each other. Under this definition, when populations can no longer breed with each other, they are considered to be different species. Activity Activity: Modeling EvolutionModel natural selection in a population of bacteria. History of Evolutionary BiologyIn the 1800s, many people were trying determine why there were so many different kinds of plants and animals in the world. Charles Darwin wondered about the diversity of animals he saw while in the Galapagos Islands. This led him to develop the theory of natural selection, which is the best explanation we have for the diversity of life. Alfred Russell Wallace also hypothesized that the environment could help to shape the diversity of life by favoring certain traits over others. Wallace noticed that insects in the jungles of Africa and South America were very well adapted to unique environments. These two men, working independently of one another, developed the same basic explanation for the diversity of life: natural selection. These principles are supported by current scientific research. Genetic Variation is Essential for Evolution by Natural SelectionFor natural selection to occur, a population must have a wide variety of individuals with different traits. For example, natural selection would not influence fish body color if all individuals in a population were exactly the same color. The term phenotype is used to describe these physical traits. All phenotypes are the expression of genetic information in an individual’s DNA molecules. The term genotype describes the specific genetic code in the DNA molecular structure that produces a certain phenotype. Variations in genotypes can also produce variations in phenotypes. New genotypes can be produced through the natural process of genetic mutation. A mutation is an error that is made during the DNA copying process. The mutation results in a change in the genetic code or genotype. Sometimes mutations errors occur within the sections of the DNA strand that do not code for any phenotype or trait. Similarly, some mutations are minor and do not cause any significant change in the appearance, physiology, or behavior of the organism. Other times, mutations can cause changes in the phenotype (Fig. 1.7). Over millions of generations of copying, even little errors in that influence phenotype can add up to big changes. Sometimes a single change in a regulating gene that controls other parts of an organism’s DNA can also have a big effect. To picture how this could work, think of the game “telephone.” In this game, one person whispers a phrase to their neighbor, who whispers it to their neighbor, and so on. The last person who receives the message compares the phase they think they heard to the original phrase. Often mistakes in repeating or understanding the sample phrase lead to changes in the meaning. The same thing happens when DNA is replicated again and again. The changes to the message in the game are similar to what happens when there are mutations. Changes in the meaning of the “message” of the DNA can cause changes in the appearance, physiology, or behavior of an organism. A mutation can be an especially powerful force for change if it has a significant impact on the survival of an organism. Some mutations, for example, have provided resistance to antibiotics in bacteria. Because bacteria reproduce very rapidly, these mutations can quickly take effect in changing the population of bacteria. It is also important to remember that mutations are random. An organism cannot pick or choose its mutation. For example, some species of animals that live their whole lives in caves without light have no pigment, or coloration. Because the caves are dark, there is no benefit to being camouflaged to avoid a predator or having coloration to attract a mate. However, not all animals that live in the dark lack color. In order for a species to lose its coloration, mutations must occur that allow the elimination of pigment. If the mutation never arises, the animals will stay pigmented. Sexual reproduction can also increase genetic variation in a population. Many single-celled microorganisms reproduce simply by duplicating their genetic material (i.e., DNA) and dividing themselves in half. This process is called asexual reproduction. The new cells produced by asexual reproduction are genetically identical to the original parent cell unless some mutation occurs. Sexual reproduction is the production of offspring through the combination of specialized sex cells. These sex cells (also called gametes) contain only a half-copy of an individual organism’s genetic material. When the two halves—one from the male and one from the female—are combined, the offspring ends up with a brand new combination of genes. Mutations and sexual reproduction increase genetic variation in a population. Individual organisms with unfavorable traits (e.g. malformed wings in fruit flies or bright white color patterning in male peacocks) are less likely to survive and reproduce. These individuals and the genes they carry are “selected against” or disfavored by natural selection (Fig. 1.8). In contrast, individuals with beneficial genes are more likely to survive and reproduce. Paid Video Workshop: Natural Selection Wrap-upIn summary:
Activity Activity: Simulate Natural SelectionModel how variation in prey color and predator foraging affects survival and reproduction of a prey population. How does genetic variation affect a species ability to adapt to its environment?Genetic variations that alter gene activity or protein function can introduce different traits in an organism. If a trait is advantageous and helps the individual survive and reproduce, the genetic variation is more likely to be passed to the next generation (a process known as natural selection).
How does variation affect adaptation?Populations adapt to novel environments in two distinct ways: selection on pre-existing genetic variation and selection on new mutations. These alternative sources of beneficial alleles can result in different evolutionary dynamics and distinct genetic outcomes.
How does genetic diversity affect adaptation?The greater the variation in genes, the more likely is that individuals in a population will possess the differentiated genes which are needed to adapt to an environment. The theory of natural selection suggests that it is this variety of genes that allows species to evolve, adapt and propagate successfully.
Does genetic variation help species adapt?Genetic diversity is important because it gives species a better chance of survival. However, genetic diversity can be lost when populations get smaller and isolated, which decreases a species' ability to adapt and survive.
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How does genetic variation affect an organisms ability to adapt to its environment?
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