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.
aid
VIDEO
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,
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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.
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WEIRD SCIENCE
Weird Science: Species Flocks
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.
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PRACTICES OF SCIENCE
Practices of Science: Communication & Collaboration in
the Scientific Community
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.
Fig. 1.7. A male Indian peacock (Pavo cristatus) with white leucistic color mutation displays his full plumage. Genetic
mutations give rise to new genotypes. New genotypes may result in new phenotypes or traits such as body size or color
patterning.
Image courtesy of Félix Potuit, Wikimedia Commons
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.
Fig. 1.8. Diagram of genetic mutation and natural selection
Image courtesy of Elembis, Wikimedia Commons
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.
aid
VIDEO
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.