Download CHAPTER 7: INTRODUCING EVOLUTION Adaptions and Variations

CHAPTER 7: INTRODUCING EVOLUTION
Adaptions and Variations
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Organisms constantly face environmental challenges that limit their ability to survive. Severe weather, famine,
and competition for food, space, and mates are all challenges that organisms may or may not be able to survive.
Organisms that survive long enough to reproduce have the opportunity to pass along to their offspring the genetic
information that helped them survive.
The diversity within species and the interactions of organisms with their environment help explain how
populations can change over time and why some species of organisms survive while others become EXTINCT
(disappear completely from Earth).
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An ADAPTATION is a structure, behavior, or physiological process that helps an organism survive and
reproduce in a particular environment.
For example, camouflage is an example of adaptation. For instance, the colouring and structure of the stick insect
help camouflage the insect so that it blends in with its environment. The insect is almost invisible to predators
because the predators cannot distinguish the stick insect from the surrounding vegetation. As a result, more of the
camouflaged individuals survive and reproduce.
For example, hibernation is an example of a physiological adaptation. Hibernation allows an animal to survive in
climates with harsh winters by reducing their metabolism to save energy.
For example, many harmless species resemble a harmful species in colouration or structure. This phenomenon is
called MIMICRY. Predators that avoid the harmful species will also avoid the mimic. For example, the
viceroy butterfly (Limenitis archippus) is palatable to predators, but the monarch butterfly (Danaus plexippus) is
not. So, the viceroy is similar in structure and colouration to the monarch. Predators learn to avoid both
butterflies because the monarch tastes bad.
Development of Adaptations
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Adaptations are the result of gradual, accumulative changes that help an organism survive and reproduce.
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VARIATIONS are differences between individuals, which may be structural, functional, or physiological.
Not all variations become adaptations.
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Environmental conditions determine whether a variation in an individual has a positive or negative effect, or no
effect on the individual’s ability to survive and reproduce. Individuals possessing a helpful variation are more
likely to survive and have a higher chance of passing this variation on to their offspring. As a result, the variation
will become more frequent and will be considered to be a characteristic, or trait, of the population.
Interaction with environments is important to adaptation and variation because environments change: climates
change over time, and droughts, floods, and famines may occur. Human activitiess, such as deforestation, and
land cultivation for crops, also change environments. Therefore, a characteristic that may not give an individual
organism a particular advantage now, may become critical for survival later in the environment in which that
species lives.
For example, the English peppered moth (Biston betularia) is an example of how the proportions of some
inherited characteristics in a population change in response to changes in the environment. The English
peppered moth has three colour variations: greyish-white flecked with black dots, black, and an intermediate
colour. In the past, the black variety was extremely rare in Manchester, England. It only made up 2% of its
population. Yet, 50 years later (in 1898) 95% of the moths in Manchester were of the black variety. In rural
areas, black moths were still less frequent. Why?? Peppered moths are active at night. During the day, when
they rest in the trees, they are potential prey for birds. Until the mid-19th century, the flecked moths in
Manchester were camouglaged when they rested in the trees that were covered with light-coloured lichen. The
black moths, however, were easily seen and therfore easily preyed upon. The 50 years in which the black moths
gradually became much more common in Manchester coincided with the Industrial Revolution in England. The
air pollution from all the new factories killed the lichens, and soot began to cover Manchester’s trees. As a
result, the flecked moths were seen and eaten by birds, while more black moths survived long enough to
reproduce and pass on their genes to their offspring. The difference between the flecked and black forms of the
peppered moth is genetic. Before the Industrial Revolution, more flecked moths survived and therefore passed
on the genes for flecked colouring in the gene pool. However, when air pollution increased, more black moths
survived with each successive generation and the ration of flecked to black moths in the population essentially
reversed. In the 1950’s, England enacted clean-air legislation, and lichen began to grown on trees again. By
1989, the number of black moths were only 3 out of 10. It is estimated that by 2019, the black peppered moth
will again be rare in Manchester.
Variation within a Species
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The number of possible combinations of genes that offspring can inherit from their parents results in greater
genetic variation among individuals within a population.
Genetic variation in a population results from the variety of genetic information in all individuals of the
population. An important source of genetic variation in a population is gene flow, which is the transfer of alleles
from one population to another population through immigration of individuals. For example, if a bird from
population A immigrates to population B, which has fewer of the dominant alleles, and through mating
incorporates its alleles into the other population.
Mutations Lead to Genetic Variation
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MUTATIONS are a permanent change in the genetic material of an organism.
Mutations are the only source
of new genetic variation since they provide new alleles in a species.
Mutations happen continuously in the DNA of any living organism. They can occur spontaneously when DNA is
copied (during the S phase) before a cell divides. Mutagens, such as ultraviolet radiation, are environmental
agents that can also cause mutations in DNA.
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When there is a mutation in the DNA, a cell may exhibit new characteristics: the cell could die, malfunction, or
multiply more than it should, resulting in a cluster of cells that form a tumour.
Some mutations do not appear to have any effect on a cell. Whatever the result, if the mutation occurs in a
somatic cell, the mutation disappears form the population when the organism dies.
If the mutation alters the DNA in a gamete, the mutation may be passed on to succeeding generations as a new
allele.
Mutations Can Provide a Selective Advantage
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Mutations that were once no advantage, or perhaps were even a disadvantage, may become favourable in a
changing environment. In this situation, the mutation provides a selective advantage.
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A SELECTIVE ADVANTAGE is a genetic advantage that improves an organism’s chance of survival,
usually in a changing environment. A selective advantage causes the organism to be favoured in terms of survival
and reproduction.
For example, the water flea Daphnia, normally lives in water that is around 20°C, and it cannot survive in water
27°C or warmer. However, a mutation enables some populations of Daphnia to survive in temperatures between
25°C and 30°C. Daphnia without this mutation may not survive and reproduce when temperatures are high.
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Rapid Reproduction and Selective Advantage
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Some organisms reproduce very quickly, such as bacteria, viruses, and many insects. The reproduction time of
these organisms are very short compared with the reproduction times of most plants and animals.
In populations that reproduce this quickly, a new allele that resulted from a random mutation that was previously
insignificant in the population may provide a selective advantage to some individuals when the environment
changes. As a result, the organisms that have the new allele may survive long enough to reproduce and pass the
genetic information on to the next generation. In time, the gene that provided the selective advantage becomes
more prevalent in the population. One mutation can, in some cases, mean the survival of the whole population.
For example, the Staphylococcus aureus bacterium is a relatively common bacterium that can have minor or
major effects on human health, such as minor skin infections and more serious blood infections. The bacteria can
reproduce every 30 min. Such rapid reproduction helps adaptation to occur very quickly: the bacteria with an
advantageous mutation may survive a changing environment and reproduce. The surviving bacteria reproduce
rapidly, and this rapid reproduction often leads to problems when doctors treat patients with a Staphylococcus
aureus infection with an antibiotic.
Natural Selection and Artificial Selection
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NATURAL SELECTION is the process that results when the characteristics of a population of
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organisms change over many generations as organisms with heritable traits survive and reproduce, passing their
traits to offspring.
For natural selection to occur there must be diversity within a species.
An abiotic (non-living) environmental condition can be said to select for certain characteristics in some
individuals and select against different characteristics in other individuals. In this way, the environment exerts
selective pressure on a population.
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SELECTIVE PRESSURE is an environmental condition that selects for certain characteristics of
individuals and selects against other characteristics. Selective pressure may result from biotic factors, such as
predators, parasites, and competition for resources.
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For example, a population of young trees in a dense forest with low light. Those individual trees that are able to
survive in the shade will reproduce and pass on the alleles to survive those conditions. In the next generation, the
abundance of the alleles for these conditions will increase in the population because more of the individuals will
have survived and reproduced. Over time, the tree population will be able to survive the shady conditions.
Natural selection does not anticipate change in the environment. Instead, natural selection is situational. A trait
that at one time in one situation seems to have no apparent relevance to survival may be the trait that, at a
different time in a different situation, helps certain individuals in a population survive and reproduce. The alleles
for this trait will be inherited by the offspring of the survivors.
FITNESS is the relative contribution an individual makes to the next generation by producing offspring that
will survive long enough to reproduce. It is the contribution that an individual makes to the gene pool of the next
generation. A high degree of fitness means that an organism will survive and reproduce, thereby passing on its
advantageous genes to its offspring.
Fitness is often described as the number of reproductively viable offspring that an organism produces in the next
generation. An organism with many viable offspring has high fitness, whereas an organism with few or no viable
offspring has low fitness.
ARTIFICIAL SELECTION is selective pressure exerted by humans on populations in order to improve
or modify particular desirable traits. Selective breeding is an example of artificial selection in which humans have
purposely used technology to improve the genetic quality of farm animals.
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BIOTECHNOLOGY is the use of technology and organisms to produce useful products.
Artificial selection has had a huge impact on human survival. Most of the food we eat (grains, fruits, vegetables,
meat, and milk) comes from species that have been selectively bred. For instance, artificial selection has resulted
in cows that produce more milk; some varieties of chickens are bred to grow rapidly and have large muscles for
increased amounts of meat; other varieties of chicken have been bred to produce large numbers of eggs.
The food crops that we depend on for most of our diet (rice, corn, wheat, and vegetables) are the result of
selective breeding. We breed food crops to increase their nutritional value and harvest yield. We also breed them
to become drought-resistant or pest-resistant.
Artificial selection has its limits: if plants were bred to grow too quickly, they may not tolerate poor soil
conditions. So, there must be a balance to maintain genetic variation within the crops and the ability of the plants
to respond to conditions of environmental change.
Factors that affect artificially selected crops are: decreased variation, and a decreased ability to respond to
conditions of environmental change. For example, the 1896 Illinois Experiment Station. This study began with
163 ears of corn that had oil contents ranging from 4-6%. In each of 80 succeeding generations, corn plants were
divided into two separate groups. In one group, members with the highest oil content in the kernels were chosen
as parents of the next generation. In the other group, members with the lowest oil content were chosen. After
many generations, the oil content in the first group rose to over 18%. In the other group, it dropped to less than
1%. Corn oil is an agriculturally important product. Over the course of many generations, selective breeding has
had a major impact on the amount of corn oil made by the two groups of plants.
The following diagram shows the percentage of oil content (y axis) against the generations (x axis)
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Artificial selection has negative as well as positive consequences. For example, English bulldogs are selectively
bred for different traits, such as their flat faces. This trait results in severe respiratory problems. Another example
is hip dysplasia (a type of arthritis) is common in German shepherds and other large dog breeds.
Some artificial selection techniques, such as genetic engineering have been used to introduce new genetic
information into domesticated organisms. For instance, to make plants similar, it is necessary to reduce genetic
diversity. Plants are specialized through selective breeding to produce the qualities that growers want. Most
agriculture in the world is based on extensive plantings of the same varieties of a species over large expanses of
land. This is called MONOCULTURE. It is easier to manage fields in which there is only one kind of
plant growing. The entire crop will respond to the same type of pesticide as well as environmental conditions.
However, with monoculture, if a new disease infests the crop population, most of the individual plants will be
affected in the same way and the whole population could be killed or severely damaged.
The key difference between natural selection and artificial selection is than in natural selection, the environment
plays the role that humans play in artificial selection. In natural selection, the environmental conditions determine
which individuals in a population will survive and reproduce in the current conditions.
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