Classify the following examples based on what type of natural selection they describe.

Natural selection can take many forms. To make talking about this easier, we will consider the distribution of traits across a population in graphical form. In we see the normal bell curve of trait distribution. For example, if we were talking about height as a trait, we would see that without any selection pressure on this trait, the heights of individuals in a population would vary, with most individuals being of an average height and fewer being extremely short or extremely tall. However, when selection pressures act on a trait, this distribution can be altered.

Stabilizing selection

When selective pressures select against the two extremes of a trait, the population experiences stabilizing selection. For example, plant height might be acted on by stabilizing selection. A plant that is too short may not be able to compete with other plants for sunlight. However, extremely tall plants may be more susceptible to wind damage. Combined, these two selection pressures select to maintain plants of medium height. The number of plants of medium height will increase while the numbers of short and tall plants will decrease.

Directional selection

In directional selection, one extreme of the trait distribution experiences selection against it. The result is that the population's trait distribution shifts toward the other extreme. In the case of such selection, the mean of the population graph shifts. Using the familiar example of giraffe necks, there was a selection pressure against short necks, since individuals with short necks could not reach as many leaves on which to feed. As a result, the distribution of neck length shifted to favor individuals with long necks.

Disruptive Selection

In disruptive selection, selection pressures act against individuals in the middle of the trait distribution. The result is a bimodal, or two-peaked, curve in which the two extremes of the curve create their own smaller curves. For example, imagine a plant of extremely variable height that is pollinated by three different pollinators, one that was attracted to short plants, another that preferred plants of medium height and a third that visited only the tallest plants. If the pollinator that preferred plants of medium height disappeared from an area, medium height plants would be selected against and the population would tend toward both short and tall, but not medium height plants. Such a population, in which multiple distinct forms or morphs exist is said to be polymorphic.

Introduction to Evolution by Natural Selection

Without 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.

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Workshop: Natural Selection Introduction

Evolution 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,

• traits persist in a population because they contribute to the success of the organism, or
• traits are eliminated because they detract from the success of the organism.

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 Evolution

Model natural selection in a population of bacteria.

History of Evolutionary Biology

In 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 Selection

For 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.

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Workshop: Natural Selection Wrap-up

In summary:

1. Natural selection requires variation between individuals.
2. Mutations and sexual reproduction increase genetic variation in a population.
3. Natural selection occurs when environmental pressures favor certain traits that are passed on to offspring. The “big prize” in natural selection is passing on genetic information.
4. Natural selection acts on populations. Individuals do not evolve in genetic evolutionary terms. Individuals may mutate, but natural selection acts by shifting the characteristics of the population as a whole.

Activity

Activity: Simulate Natural Selection

Model how variation in prey color and predator foraging affects survival and reproduction of a prey population.

What are the 3 types of natural selection and examples?

What are different types of natural selection explain with suitable examples?

What are the 3 types of selection?

What are the 5 modes of natural selection?