Download Natural Selection Evolution Evolution refers a change in the gene

Natural Selection
Evolution
Evolution refers a change in the gene frequency of a population. For example, suppose that
in a certain human population in 1990, 65% of the eye color genes were for blue eyes and
35% were for brown eyes. In 2000, the number of blue eye genes was 67%. This small
evolutionary change may not be noticeable, but over time, small differences accumulate to
produce larger differences. A number of natural phenomena can act to change gene
frequencies. Organisms moving into or out of a population (migration) can cause gene
frequencies to change. Random fluctuations can also cause changes, particularly in small
populations. Natural selection (described below) is particularly important in causing
changes in gene frequencies.
Adaptation
Adaptations are structures or behaviors that allow efficient use of the environment. For
example, the webbed foot of a duck enables it to swim better than a foot that is not webbed.
Adaptations are due to genes, that is, they are inherited.
Natural Selection
As was noted in the introductory paragraph above, natural selection is one of several
different mechanisms that cause evolutionary change in populations. Natural selection
operates to produce individuals that are better adapted to their environment. It is important
to keep in mind as you read below that natural selection does not act on individuals; it acts
on populations. Individual organisms cannot become better-adapted to their environment
because they cannot change their genes.
Natural selection produces changes in the genetic composition of a population from one
generation to the next. As a result, organisms become better adapted to their environment.
Natural selection occurs because
1.
2.
3.
4.
Individuals within a population vary; they are not all identical.
Some variants are “better” than others.
The traits that vary are heritable.
The “better” individuals will have more success reproducing; they will have
more offspring.
In successive generations, more offspring will have the better traits.
These items are discussed below.
Variation
For many traits that occur in a
population, individuals are often not all
identical. For example, if running speed
were measured, some individuals would
likely be able to run faster than others
but most individuals would probably be
intermediate.
If number of individuals is plotted
against the trait in question (running
speed for example), a graph like the one
shown is often produced.
We would get a similar bell-shaped curve if we plotted
height, weight, performance on exams, etc.
Some Variants are Better
Some individuals are bound to be better than others. Perhaps their body structure allows
them to escape predators better or to find food faster or to better provide for their young.
For example, suppose that the faster-running animals diagrammed below are better able to
escape predators than the slower ones. You would expect that more of the faster ones would
survive and reproduce than the slower ones.
The slower rabbits will not reproduce as much because predators kill them more than they
kill the faster rabbits.
Traits Are Heritable
Those individuals that survive better or reproduce more will pass their superior genes to the
next generation. Individuals that do not survive well or that reproduce less as a result of
"poorer genes" will not pass those genes to the next generation in high numbers. As a
result, the population will change from one generation to the next. The frequency of
individuals with better genes will increase. This process is called natural selection.
Fitness
We often hear natural selection described as "survival of the fittest." The word "fitness"
used in a biological context means "reproductive." It does not have anything to do with
physical fitness or strength. In the example above, it is the fastest rabbits that reproduce the
most, not the strongest.
Natural Selection Produces Evolutionary Change
If the conditions discussed above are met, the genetic composition of the population will
change from one generation to the next. This process is called natural selection.
The word "evolution" refers to a change in the genetic composition of a population. Natural
selection produces evolutionary change because it changes the genetic composition of
populations.
A variety of other mechanisms can also produce evolutionary change. For example,
suppose that 65% of the eye-color genes in a population were for individuals with blue eyes
and 35% of the genes were for brown eyes. If most of the immigrants entering the
population carried the blue gene, the overall composition might change from 65% blue to
70% blue.
Evolution occurs in populations, not in individuals. Although mechanisms exist for
individual bacteria to change their genetic composition, multicellular organisms do not
change their genetic characteristics and therefore cannot evolve. Natural selection does not
act on an individual to make it better adapted to its environment.
Example of Natural Selection: The Peppered Moth
There are two forms of the peppered moth (Biston betularia) in England- a dark-colored
form (carbonaria) and a light form (typica).
In the early 1800's, most moths were the light form. The first dark form was reported in
1848. The dark form increased in frequency during the last half of the 1800s. By 1895, 98%
of the Moths in Manchester were the dark form.
The increase in the dark (carbonaria) form of the moth occurred at a time of rapid
industrialization in England- the industrial revolution. During this time, an increase in the
amount of coal-burning factories caused widespread pollution. The pollution killed lightcolored lichens, causing the trees to be darker. The trees in polluted areas were also covered
with dark soot.
In 1896, J. W. Tutt proposed that bird predation was responsible for the increase in
abundance of the dark form of the moth. He reasoned that birds had difficulty seeing the
dark form on the dark trees; the moths were camouflaged and survived better. In clean
areas, the trees were covered with lichens, making the pale form more difficult for birds to
see. In the early 1800s, the trees were light and the light form were more difficult to see.
To test the bird predation hypothesis, H.B.D. Kettlewell released moths of each type and
then measured the number that were later recaptured. The experiment was performed in a
polluted area in Birmingham, England and in a clean area in Dorset. The moths were
marked with a dot of paint so that he could identify them after they were released and then
recaptured. In the unpolluted area, he recaptured 13.7% light, 4.7% dark indicating that the
light form survived better. In the polluted area, he recaptured 13% light and 27.5% dark
suggesting that the dark form survived better.
These results support the hypothesis that color change was due to bird predation. Birds ate
moths that were easiest to find.
Sexual Reproduction and Evolutionary Change
Variation
Later in this course, we will discuss how sexual reproduction acts to increase variation in
populations by shuffling genes. Offspring have some genes from each of two different
parents and therefore are not identical clones of their parents. The increased variation due
to sexual reproduction allows natural selection (and thus evolution) to produce changes in
populations as described above.
Fluctuating environments
Evolutionary change due to natural selection would not be necessary if the environment
never changed and the organisms within the environment were optimally adapted to the
environment. For example, imagine a plant that is adapted to an environment that has an
average annual rainfall of 100 cm. If the climate were to change so that the amount of
rainfall decreased, individuals that could tolerate less rain would survive and reproduce
better, thus establishing their drought-tolerant genes in subsequent generations. If there was
no variation in the plant population, there would not be any drought-tolerant individuals
and the species would likely go extinct in areas of decreased rainfall.
Sexual reproduction therefore, enables species to survive in fluctuating or changing
environments because it promotes variation, which in turn allows natural selection.