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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.
There are three main 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
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).
Disruptive selection
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.
An example of disruptive selection: bristles on fruitflies.
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. (Reference: Evolution; Mark Ridely,
Third edition.)
Stabilizing selection:
Stabilizing selection is a type of natural selection that favors the average individuals in a
population. This process selects against the extreme phenotypes and instead favors the
majority of the population that is well adapted to the environment. Stabilizing selection is
often shown on a graph as a modified bell curve that is narrower and taller than the norm.
Diversity in a population is decreased due to stabilizing selection. However, this does not
mean that all individuals are exactly the same. Often, mutation rates in DNA within a
stabilized population are actually a bit statistically higher than those in other types of
populations. This and other kinds of microevolution keep the population from becoming too
homogeneous.
Stabilizing selection works mostly on traits that are polygenic. This means that more than
one gene controls the phenotype and there is a wide range of possible outcomes. Over time,
some of the genes that control the characteristic can be turned off or masked by other genes,
depending on where the favorable adaptations are coded. Since stabilizing selection favors
the middle of the road, a blend of the genes is often what is seen.
Examples:
Many human characteristics are a result of stabilizing selection. Human birth weight is not
only a polygenic trait, but it is also controlled by environmental factors. Infants with average
birth weight are more likely to survive than a baby that is too small or too large. The bell
curve peaks at a birth weight that has the minimum death rate.
Sexual Selection:
Evolution by sexual selection involves any physical or behavioural changes that enhance
winning a mate. Although the physical or behavioural changes contribute to reproductive
success they have no survival value. There are two forms of sexual selection (i) Intrasexual
where male fight one another for access to female or for resources required by females. (ii)
Intersexual where females decide on which mate to choose based on personal attributes such
as song, morphology or colouring.
Conclusion:
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.
Reference:
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‘Evolution’ by Mark Ridely, Third edition
Is Bacterial Resistance to Antibiotics an Appropriate Example of Evolutionary Change? Kevin L.
Anderson; Creation Research Society Quarterly; volume 41, No. - 4; March 2005.
‘Nature’ by Professor E. W. Bride.
Prepared by Mohammod Towhid, Fahmida Jahan, Vanesssa Hamilton, Dorji lama