Theory Of Evolution By Natural And Sexual Selection

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Term Paper Title	Theory Of Evolution By Natural And Sexual Selection
# of Words	3789
# of Pages (250 words per page double spaced)	15.16

Theory Of Evolution By Natural And Sexual Selection

Introduction

     It is commonly thought today that the theory of evolution originated from Darwin in the

nineteenth century.  However, the idea that species mutate over time has been around for a long

time in one form or another.  Therefore, by Darwin’s time the idea that species change from one

type into another was by no means new, but was rejected by most because the proponents of

evolution could not come up with a satisfactory mechanism that would explain this change.

     The most influential evolutionary theories prior to Darwin were those of Lamarck and

Geoffroy St. Hilaire, developed between 1794 and 1830.  Lamarck suggested that species evolve

through the use or disuse of particular organs.  In the classic example a giraffe that stretches its

neck slightly to reach higher leaves will gain in neck length, and this small gain would be passed

on to its offspring.  Geoffroy, on the other hand suggested that the change was discontinuous,

large in magnitude, and occurred at the production of offspring.  However, these theories of

evolution were based on a priori explanations that offered no demonstrated mechanism.

     Darwin’s theory of evolution differs in that it is based on three easily verified observations.

“First, individuals within a species vary from one another in morphology, physiology, and

behavior.  Second, variation is in some part heritable so that variant forms have offspring that

resemble them.  Third, different variants leave different number of offspring”.  Darwin than

proceeded to elaborate on the mechanism of evolution by suggesting that in the universal struggle

for life, nature “selects” those individuals who are best suited (fittest) for the struggle, and these

individuals in turn reproduce more than those who are less fit, thus changing the composition of

the population.  In addition to natural selection, Darwin also suggested that species also evolve

through the complementary process of sexual selection.  According to Darwin, in sexual selection,
one gender of a species develops a preference for individuals of the other gender who possess

certain features.  The individuals who possess these features will than have a reproductive

advantage over others, resulting in a greater number of offspring, and thus, again, a change in the

composition of the population.  Therefore, it was Darwin who made the theory of evolution

feasible...

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