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PinkMonkey Online Study Guide-Biology
CHAPTER 11 : EVOLUTION OF LIFE
11.0 Introduction
Scientists have long tried to discover the origin of
living organisms. Various observations about the varieties of plants and
animals, the diversities in their structures and the reproductive patterns,
lead to the concept of evolution. The process of evolution involves
a gradual change of organisms generation after generation. Essentially
it means that the present-day organisms have arisen from ancestors that
were more simple in organization. Evolution involves changes in genetic
composition of a population, generation after generation. Modification
and development of a species takes place through hereditary transmission
of slight changes (variation) from one generation to another. It results
in new characteristics in a species, and the eventual formation of new
species. Three main observations support the idea of evolution.
a) The development of present organisms can be
traced back to the organisms living in the past. The study of fossils
reveals that fossils in successive strata of rocks show a gradual change
from simple forms to complex forms of life. The geological time scale
explains that the oldest rocks formed show the fossils of simple animals
only. The rocks formed later show more complex forms of living organisms,
and thus a gradual evolution of more complex living organisms from simple
forms is suggested. The Proterozoic era shows fossils of invertebrate
animals. Paleozoic era shows fossils of fishes and amphibians in the
lower strata and reptiles in the upper strata. The Mesozoic era shows
dominance of reptiles. Coenozoic era shows appearance of birds and mammals.
The recent epochs show evolution of man.
b) Another observation about the diversity in the
structure of living organisms explains that there is unity in diversity
of life. The animals show variations in their structures but the
basic pattern of working is the same. There may be variations in the
food and feeding habits, but the basic components of food are same.
The proteins, carbohydrates and lipids are converted into aminoacids,
monosaccharides and fatty acids, respectively. The biochemical changes
during digestion are similar. The pattern of getting energy is basically
the same. Similarly the nucleic acids which are involved in the inheritance
of characters are similar in most of the organisms. The protein synthesis
mechanism is also similar. Structures of many organs also suggest a
common basic pattern, (e.g. the forelimbs of the frog, bird, horse,
whale, bat and man have the same basic skeleton pattern). But necessities
like running, flying, swimming, perching or grasping, variations result
from various changes in shape.
c) The unit of evolution is population : An
individual may show adaptation and survive. But development of one fit
individual will not result in evolution. The individuals may pass on
their advantageous characters to their offspring. When the individuals
with new characters interbreed in a population, the characters
are spread amongst the population. The varieties in the population thus
have common ancestors and thus in this way a gradual evolution of more
fit individuals must have taken place.
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Table of Contents
11.0 -
Introduction
11.1 Darwin's Theory of Natural Selection
Chapter 12
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