Download Chapter 5 • Lesson 28

Chapter 5 • Lesson 28
Natural Selection
Objectives: 3,4,2, 3.4.3
Key Terms
• evolution • species • natural selection • phenotype • adaptation • gene pool • fitness • speciation • resistance •
immunity • antibiotic
Getting the Idea
The concept of evolution is a cornerstone of modern biology. Biological evolution is the
process of change in species and populations of organisms over time. In the mid-1800s,
Charles Darwin proposed a theory to explain how adaptations that developed in a
population could lead to the evolution of new species.
Natural Selection
The theory of evolution accepted by most scientists today was first proposed by Charles
Darwin, a British naturalist. Darwin traveled widely, observing and recording data on diverse
life forms. He used his observations to explain how species develop. He published his ideas
in 1859, in his book On the Origin of Species by Means of Natural Selection. Recall that a
species is a group of similar organisms that can interbreed and produce fertile offspring.
Natural selection is the process by which organisms best suited to their environment
survive and reproduce. The four key principles of natural selection are overproduction of
offspring, variation, adaptation, and descent with modification.
Overproduction of Offspring
Animals and plants have tremendous reproductive potential. However, most offspring are
lost to predators, disease, or other factors. Relatively few survive to reproduce. Organisms
that produce more offspring increase the likelihood that some of those offspring will survive.
However, increasing the number of offspring reduces the resources available to each.
Variation
Within any population, the phenotypes, or physical traits, of the organisms in the population
will vary somewhat. Recall that differences in traits are known as variations. Some
variations are genetic, while others are acquired in the organism's lifetime. All these
variations can affect an organism's ability to survive or reproduce. As you learned in
Chapter 3, the amount of space, food, water, shelter, and other resources in nature is
limited, and organisms compete for these resources. Organisms with variations that make
them better suited to their environments are more likely to survive and produce more
offspring. If a phenotype that makes an organism successful results from its genes, the
variation can be passed on to the organism's offspring. Variations that do not result from
genetics are not passed to offspring. A variety of phenotypes in a population makes it more
likely that some members of the population will survive and reproduce in a variety of
different conditions.
Adaptation
Structures and behaviors that increase an organism's chance of survival tend to become
more common in a population. Recall from Chapter 2 that any feature of an organism that
improves its chance of survival and reproduction in its environment is called an adaptation.
Adaptations can be structural (related to an organism's form), functional (related to the way
its body works), or behavioral.
Over several generations, a population's gene pool, the total genetic information of all
members of the population, will change. A population's gene pool includes all the alleles
carried by members of the population, whether or not the alleles are expressed. The term
adaptation is also used to describe the process that makes helpful traits more common.
Descent with Modification
All organisms inherit traits from previous generations. Because different traits are useful in
different environments, the phenotypes of a population may change through natural
selection when conditions change. The new traits are better adaptations to the new
conditions than the traits found in previous generations would be. As long as the new traits
are useful, the genes for them will spread through the population. Genes for less useful
phenotypes will decrease in frequency. This process is referred to as descent with
modification.
Natural Selection at Work
Natural selection requires and operates on the genetic variations within a population. The
individuals with the most useful traits are selected. Organisms inherit many of their
variations from their parents. As you have learned, some variations arise from mutations.
Others arise from the rearrangement of chromosomes during independent assortment and
crossing-over when gametes form.
Genetic variations that result in helpful traits can increase an organism's fitness. Fitness is
an organism's ability to survive and reproduce. Traits that help an organism survive or
reproduce increase its fitness. Natural selection ensures that only organisms that are fittest,
or best adapted to their environment, survive and reproduce. (Natural selection is also
described as survival of the fittest.) In many cases, natural selection occurs when
organisms must adapt to changing conditions. Because a population shares a gene pool, a
successful adaptation will increase in frequency within the population, and variations for
adaptations that do not help organisms survive and reproduce will disappear. Over time,
changes in a population as a result of natural selection can have either of two major
outcomes: speciation or extinction.
Speciation is the evolution of a new species from an existing species. Speciation can be a
result of environmental change if natural selection favors certain traits in the changed
environment. For example, if an environment becomes colder, animals that inherit thicker
fur will be better able to survive, reproduce, and pass this trait on than animals that inherit
thinner fur. If the populations of plants also change, the animals with teeth best suited to
eating the new plants will be more likely to survive and reproduce. They will tend to pass
those teeth on to their offspring. Similarly, if a new predator moves into an area, prey
animals that can run faster to escape are more likely to survive and reproduce.
Geographic isolation may also lead to speciation. Geographic isolation occurs when a physical
barrier such as water divides a population into two separate populations that can no longer reach
each other to interbreed. Over time, natural selection occurs within each of the two populations,
causing them to become genetically different. When the populations come into contact again, their
genetic differences are great enough that they cannot interbreed.
The illustrations below show adaptations involving beak shape in three of 14 species of finches that
Darwin observed in the Galapagos Islands. Darwin inferred that all 14 species developed from a
single mainland species. Each of the islands has a different food source. Isolated on the islands, the
mainland finches evolved into new species adapted to their new environments.
Resistance, Immunity, and Natural Selection
Some organisms have evolved adaptations that protect them from injury. Some plants, for
example, have thorns or spines that protect them from predators. Many animals have hard
shells or bony skeletons that protect their soft tissues or internal organs. In other cases, natural
selection produces populations with traits that enable them to survive disease or harmful
chemicals. Resistance and immunity are examples of such adaptations.
Pesticide Resistance
Resistance is an organism's ability to withstand the effects of a harmful agent. Recall that
people have developed chemicals called pesticides to kill pest organisms, such as insects.
When a pesticide is first used on an insect population, it may kill all or most members of the
population. Occasionally, however, some insects in the population have slight variations that
enable them to survive exposure to that chemical. As these organisms survive and reproduce,
they may pass the trait that makes them pesticide-resistant to the next generation of insects.
Over many generations, this trait can spread to all members of the population. As the trait
spreads, the pesticide will become obsolete because it can no longer kill the pest population.
Immunity
Many diseases that affect organisms can spread rapidly through a population. Infectious
diseases are caused by pathogens. Pathogens include some bacteria, some fungi, some
protists, and many viruses. An organism's ability to fight disease is called immunity. Like other
mammals, you have a body system called the immune system that helps you maintain
homeostasis by protecting you from disease.
One kind of immunity present in humans and other mammals is active immunity. Active
immunity against a disease is produced by exposure to a pathogen. Exposure stimulates
the immune system to produce proteins called antibodies, which help destroy specific
pathogens. Antibodies attach to the pathogens and cause them to clump together. The
clumps of pathogens and antibodies are then engulfed and destroyed by white blood cells.
If the body has made antibodies to a pathogen once, the immune system can respond
quickly if the person is exposed to that pathogen again. One way to develop active immunity
to a disease is to get the disease. Another way is to be vaccinated against it. A vaccine is a
small dose of a weakened or deactivated pathogen that is introduced into the body to help
the immune system fight the disease caused by the pathogen. A vaccine stimulates your
immune system to make antibodies against the pathogen. Because the pathogen has been
weakened or deactivated, it usually cannot cause disease. (In some cases, weakened
viruses can cause a mild form of the disease.) If you are later exposed to a dangerous form
of the pathogen, your body will quickly respond by sending out antibodies against the
pathogen.
Vaccines are used to prevent diseases caused by many viruses and bacteria. It is important
to remember that vaccination does not alter the DMA of the organism that receives the
vaccine. As a result, the protection offered by a vaccine is not a heritable variation, and
natural selection does not apply to organisms with active immunity.
Passive immunity is immunity produced by the transfer of antibodies made by one organism
to another organism. For example, a person bitten by a dog or bat may be given antibodies
taken from people who have been vaccinated against rabies. This is done because rabies
may progress too quickly for the person to produce his or her own antibodies. The injected
antibodies slow the spread of the virus, giving the individual's immune system time to make
sufficient antibodies.
Passive immunity is often acquired by the developing fetuses of mammals, including
humans. As the fetus develops, it receives antibodies from its mother. Newborn mammals
can also receive antibodies in their mothers' milk. The antibodies can help protect the
offspring from disease. However, passive immunity lasts for only a short time after birth.
After that, an organism must develop its own antibodies.
Resistance to Antibiotics, Antiviral Drugs, and Vaccines
Bacteria usually reproduce asexually by binary fission. This method of reproduction allows
bacteria to reproduce very quickly. Because they reproduce quickly, diseases caused by
bacteria can spread rapidly throughout a population.
Many bacterial infections in humans and other animals are treated with antibiotics. An
antibiotic is a drug used to kill bacteria or slow their growth. An antibiotic may initially be
very effective at killing the bacteria that cause a disease. However, as the bacteria
reproduce, mutations may give some of them a trait that makes them resistant to the
antibiotic. As the resistant bacteria survive and reproduce, they will pass the trait on to their
offspring. Over time, the successful adaptation will spread through the gene pool of the
population until the antibiotic that once killed the bacteria becomes ineffective against them.
This problem is called antibiotic resistance.
Antibiotics are not effective against viruses. Scientists have developed medications called
antivirals that can be used to treat some infections caused by viruses. Many viral infections
can also be prevented with vaccines.
Unlike bacteria, viruses are not considered living organisms because they are not made up
of cells and do not have most of the characteristics used to identify organisms. However,
viruses do contain genetic material (either DMA or RNA), and they can make more viruses,
in a process called viral replication, when they are inside the cells of an organism. Many
viruses can replicate quickly, enabling them to spread rapidly throughout a population.
When they replicate, viruses make copies of their genetic material, which is passed on to
new viruses. Like that of organisms, the genetic material of viruses changes through
mutation. These changes can produce viruses that are resistant to the effects of antiviral
drugs. Like antibiotic resistance, resistance to an antiviral drug can spread through a
population of viruses.
The ability of viruses to mutate quickly can make developing vaccines to protect against
viruses difficult. For example, several different viruses can cause influenza (flu). Each of
these viruses mutates and forms new viral strains. The mutations make it difficult to develop
a single vaccine that can be used to prevent flu throughout an individual's lifetime. However,
scientists continue to develop vaccines that can help protect people from certain strains of
flu viruses. Each year's flu vaccine is designed to protect against the strains that are
expected to be common that year.
Discussion Question
Fitness depends on an organism's environment. Give some examples of traits that
would be favorable in one environment and unfavorable in a different environment. How
would natural selection affect the frequency of these traits if the environment changed?