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EVOLUTION BY NATURAL SELECTION
Evolution – Change over time. It the Process by which
modern organisms has descended from ancient species.
In 1859, On the Origin of Species, Darwin proposed a
mechanism for evolution called natural selection.
He describe Natural selection is the process by which
genetic mutation that lead to selective advantages and
increased fitness become, and remain, more common in
successive generation of a population. Natural selection
operates on the phenotypes of individuals produced by
their particular combination of allele.
Darwin’s process of natural selection has four components.

Variation.
Organisms (within populations) exhibit
individual variation in appearance and behavior. These
variations may involve body size, hair color, facial
markings, voice properties, or number of offspring. On
the other hand, some traits show little to no variation
among individuals—for example, number of eyes in
vertebrates.

Inheritance.
Some traits are consistently passed on
from parent to offspring.
whereas
other
traits
are
Such traits are heritable,
strongly
influenced
environmental conditions and show weak heritability.
by

High rate of population growth. Most
populations have more offspring each year than
local resources can support leading to a struggle
for resources. Each generation experiences
substantial mortality.

Differential survival and
reproduction. Individuals possessing traits well
suited for the struggle for local resources will
contribute more offspring to the next generation.
Natural selection describes how populations, over time,
adapt to their environments. Adaptation is based on
the generational selection of certain beneficial alleles
that will, over time, increase in frequency (and
conversely, less advantageous alleles will decrease in
frequency). For example, let's say in a certain species
of bird there is an allele that leads to curly wing
feathers. While these curly feathers may be nice to
look at, they may not lead to an increase in fitness.
On the contrary, these curly-feathered birds are
likely poor flyers and they do not live
sufficiently long enough in the wild to
reproduce. As a consequence, the curly-wing
allele is not very prominent in the population
because birds having this allele don't live long
enough to breed and contribute the allele to
the next generation.
There are main three types of natural selection:
Directional selection:
Directional selection is associated with gradually changing
conditions where the adaptive phenotype is shifted in one
direction and one aspect of a trait becomes emphasize. So, in
Directional selection Smaller individuals may have higher
fitness (i.e. produce more offspring) than larger individuals. It
favors smaller individuals and will, if the character is inherited,
produce a decrease in average body size. Directional selection
could, of course, also produce an evolutionary increase in body
size if larger individuals had higher fitness.
An example of directional selection: pink salmon
Pink salmon (Onchorhynchus gorbuscha) in the Pacific
Northwest have been decreasing in size in recent years. In
1945, fishermen started being paid by the pound, rather
than per individual, for the salmon they caught and they
increased the use of gill netting, which selectively takes
larger fish. After gill netting was introduced, smaller salmon
had a higher chance of survival; the selection favoring small
size in the salmon population was intense, because fishing
is thorough: about 75 - 80% of the adult salmon swimming
up the rivers under investigation were caught in these years.
The average weight of salmon duly decreased, by about
one-third, in the next 25 years.
Figure: the graph shows
the decrease in size of
pink salmon in two rivers
in British Columbia. Two
lines are drawn for each
river: one for the salmon
caught in odd-numbered
years, the other for even
years. Salmon caught in
odd
years
are
presumably
heavier
because of the two year
life cycle of the salmon.
From Ricker (1981).
Professor E. W. Bride wrote in "Nature":
Natural Selection' affords no explanation…of any…form of
evolution. It means nothing more than 'the survivors
survive.' Why do certain individuals survive? Because
they are the fittest. How do we know they are the fittest?
Because they survive.
In his book he describes neither natural selection nor
mutations introduce any new genetic data into the
organism's DNA. Natural selection only selects out the
disfigured, weak, or unfit individuals of a population. It
cannot produce new species, new genetic information,
or new organs and thus cannot make anything evolve.
One commonly cited evidence for evolution is the development
of resistance in bacteria against antibiotics. Fifty years ago,
penicillin killed many types of disease-causing bacteria.
However, it is not that effective today. The development of
resistance against antibiotics is hypothetically direct
evidence for evolution.
The magazine Scientific American has to say the following in
its March 1998 issue: "Many bacteria possessed resistance
genes even before commercial antibiotics came into use."
Before the development of penicillin, some bacteria
species were already resistant. Many were not. After
penicillin was used against the bacteria, the nonresistant bacteria were killed. The resistant bacteria
survived and reproduced to produce more resistant
bacteria. The population increase of resistant bacteria
is not evolution. A new species of bacteria did not
evolve. The bacteria are still the same species as they
were before. What happened was only a weeding out
of non-resistant bacteria.
Evolutionists frequently point to the development of antibiotic
resistance by bacteria as a demonstration of evolutionary
change. However, molecular analysis of the genetic events
that lead to antibiotic resistance do not support this common
assumption. Many bacteria become resistant by acquiring
genes from plasmids or transposons via horizontal gene
transfer. (Is Bacterial Resistance to Antibiotics an Appropriate Example
of Evolutionary Change? Kevin L. Anderson; Creation Research Society
Quarterly; volume 41, No. - 4; March 2005)
Disruptive selection
Diversifying selection (also referred to as disruptive
selection) favors individuals at both extremes of the
phenotypic
range,
usually
during
periods
of
environmental change. Disruptive selection could favor
both extremes over the intermediate types. Disruptive
selection favours individuals with variation at opposite
extremes of trait over individual with intermediate
variations. Sometimes environmental conditions may
favour more than one phenotypes. It is associated with a
fluctuating environment and gives rise to balance
polymorphism in the population.
Imagine a population in which the main food supply
has decreased. Because less preferred food
exists, the variants of both extremes may be able
to
utilize
different
food
supplies,
whereas
intermediates may not be able to adjust. Selection
against intermediates would result, due to the
decrease in available preferred food.
AN EXAMPLE OF DISRUPTIVE SELECTION: BRISTLES
ON FRUITFLIES.
In nature, sexual dimorphism is probably a common example
of disruptive selection; but here we use an experiment by
Thoday and Gibson on the Drosophila melangaster fruitfly
as an example. Thoday & Gibson (1962).
Thoday and Gibson bred from fruitflies with high, or low,
numbers of bristles on a certain region of the body;
individuals with intermediate numbers of bristles were
prevented from breeding. As the graph shows, after 12
generations of this disruptive selection, the population had
noticeably diverged.
Figure: experimental
disruptive selection on
sternopleural bristle
number in fruitflies.
Individuals with many
or few bristles were
allowed to breed, those
with intermediate
numbers were not; and
the population rapidly
diverged.
www.execulink.com/~ekimmel/mixed_flash.h
tm
 Tutorial 23.1 Natural Selection
bcs.whfreeman.com/thelifewire/content/chp
23/2302001.html
 Three Modes of Natural Selection
wps.pearsoncustom.com/wps/media/objects
/3014/.../17_A02.swf

SEXUAL SELECTION
Definition:
Sexual selection involves any features physical or
behaviour that affect reproductive advantage over
members of the same sex. It has led to sexual
dimorphism which is the differences between male and
females in the same species.
SEXUAL SELECTION
Two types of sexual selection
(i) Intrasexual : where male compete with one another for
access to females or for resources required by females
(ii) Intersexual: when females decide on which mate to
choose base on personal attributes such as song
morphology or coloration.
SEXUAL SELECTION
Intrasexual
Where males compete with one another. Main forms of
competition are:
- Contests
- Scrambles
- Endurance rivalries
SEXUAL SELECTION
Intersexual
-
Where female choose.
In 95% of species females spent a lot more time raising offspring
than males so they have a lot more vested in who they choose as a
mate.
Tungara Frog
SEXUAL SELECTION
SEXUAL DIMORPHISM
Why are males and females so different in many species? Darwin
believed that sexual selection would account for differences in
colouring in both sexes.
THE PEACOCK
SEXUAL SELECTION
NATURAL SELECTION
VS
SEXUAL SELECTION
- Natural selection deals with forces between species while sexual
selection deals with forces within a species
- Sexual selection focuses on reproduction rather than on survival
while natural selection relies on both.