Evidence for Natural Selection

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Evidence for Natural Selection

• How Selection Works...

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• NATURAL SELECTION IS A MAJOR CAUSE OF CHANGES IN
  GENE FREQUENCIES OCCURING IN POPULATIONS.
• Populations produce more offspring than can
  survive.
• There are variations among the individuals in a
  population and many of these variable
  characteristics are heritable.
• Those individuals with the most adapted
  (desirable within the specific environment)
  traits are more likely to survive and reproduce.
• Those individuals with less adapted traits are
  less likely to survive and reproduce.
• Over time highly adapted traits (and the genes
  that cause them) become more common within a
  population.

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• Some Factors that affect Natural Selection. . .
• Nonrandom mating - there is sexual selection.
  Color patterns, mating behaviors, etc.
• Nonrandom fecundity - there are differences in
  the number of offspring produced by different
  individuals.
• Nonrandom survival to reproductive age - some
  characteristics better enable individuals to
  survive to reproductive age.
• Adaptability to environmental changes -
  individuals that are better able to cope with
  environmental changes are more likely to survive
  (heterozygote advantage).

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FITNESS. . . Fitness is an important aspect of
natural selection. Fitness measures the
reproductive success of one phenotype to another.
Various phenotypes may have different degrees of
fitness in different environments. The more fit
an individual is, the greater its genetic
contribution will be to future generations. Adapti
ve Value (Fitness) W Selection Coefficient
S 0 minimal elimination 1 maximal
elimination W 1 - S
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If 23 of a phenotype are eliminated, then S
.23 W 1.0 - .23 .77 W .77 The Fitness for
individuals with this phenotype would be 77.
Selection is described as negative or positive
(not good or bad). Negative selection eliminates
unfit individuals, while positive selection
favors the more fit individuals. Survival of the
Fittest - does not mean the best fighters (the
strongest) will always live. There is a general
misconception that there must be direct
competition for evolution to occur. What
actually happens is that, in the presence of two
or more phenotypes, one will commonly be better
adapted to its environment and also produce more
viable offspring.
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The Fit individual is the one with the most and
best babies. Hence, the quiet little rabbit
that can hide the best and has a lot of healthy
babies is the most fit.
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• Genetic Variation makes up the raw material for
  evolutionary change. As long as there is a
  constant influx of new genetic material coming
  into a population, evolution will occur.
• Sources of new genetic information
• Sexual reproduction increases genetic variability
  through recombination that occurs during crossing
  over, independent assortment, and fertilization.
• Mutation increases variation.
• Genetic variation is natural selections
  playground - beneficial gene combinations remain
  in the gene pool and harmful ones are weeded out.

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HETEROZYGOSITY is more common that what
Hardy-Weinberg would suggest. It works like this.
. . In many cases there is an advantage to being
heterozygous for a trait. This is called the . .
.Heterozygote Advantage - duh. Heterozygosity
helps to maintain variation in a population. The
Heterozygote Advantage can be used to explain
resistance to malaria, tuberculosis, and
cholera. Hybrids (corn, wheat, dogs, etc.)
usually show more adaptive traits than
purebreds. Example - Thoroughbred horses are more
inbred. They are bred for specific traits such
as speed, but their health is usually more
fragile, more susceptible to illness,etc.
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Two different alleles (Aa) may code for two
slightly different proteins which may compliment
each other. Two forms of an enzyme are called
allozymes. There is a selective advantage for the
individuals that contain both - they tend to be
able to operate in a wider range of
conditions. Selecting for the heterozygote keeps
both homozygotes in the population. One or the
other may be negative - but they are still kept
in the population (sickle cell anemia, cystic
fibrosis, and tay sachs disease).
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Adaptive Evolution - examples of this would
include Camouflage, and Mimicry. A combination
of traits are selected for by the environment
which dramatically increase the fitness of the
individuals that contain them. Coadapted Gene
Complex - this would be a collections of genes
that affect the same trait or function. When all
of these genes are present together, then the
organism is well adapted to its environment.
Examples would include coloration and
shape. These gene complexes may occur close
together on the same chromosome, so they tend to
be inherited together.
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Evolution is a tinkerer not an engineer. It
does not create perfect phenotypes - it merely
chooses the best phenotypes present at any
point in time. Polygenes will cause a phenotype
to show as a bell curve. In the case of
polygenes, several genes control one trait - such
as human height. Shifts in population phenotypes
can be seen with three types of
selection. Stabilizing Selection - narrows a bell
curve, and is associated with a population well
adapted to its environment. Human birth weight
is affected by polygenes and environmental
factors. Those babies with birth weights near
the medium are more likely to survive. Too
little or too large are less likely to survive.
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Stabilizing selection tends to decrease variation
- the bell curve narrows. This usually doesnt
happen in nature, other forces act against it.
Stabilizing selection is more common in an
environment that has been stable for a very long
time. Directional Selection - If the environment
changes over time, directional selection may
favor one of the phenotypes at one of the
extremes of the normal distribution. One
phenotype can gradually replace another. This
only works if the appropriate gene is already
present in the population. Example - Peppered
moths. Disruptive Selection - Extreme changes in
the environment may favor two or more variant
phenotypes at the expense of the mean. The
average phenotype is selected against more than
one other phenotype is selected for.
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• Batesian mimicry give an example of disruptive
  selection. Some places in Africa have three
  species of bad tasting butterflies. Different
  females of edible swallowtail butterflies mimic
  each of the distasteful species.
• As was mentioned before, natural selection is a
  tinkerer - not an engineer. There are all kinds
  of examples of less than perfect results of
  natural selection. . .
• The too-large tail of peacock males attracts
  females but make the male easy to spot by
  predators.
• Transition from quadrupedal locomotion to bipedal
  locomotion, has give humans a propensity to lower
  back pain and difficult childbirth.
• When organisms are unable to adapt adequately to
  a changing environment, the result is extinction.