NCERT Textbook - Heredity and Evolution Class 10 Notes | EduRev

Science Class 10

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Class 10 : NCERT Textbook - Heredity and Evolution Class 10 Notes | EduRev

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Science
142
Figure 9.1
Creation of diversity over succeeding
generations. The original organism at the top
will give rise to, say, two individuals, similar
in body design, but with subtle differences.
Each of them, in turn, will give rise to two
individuals in the next generation. Each of
the four individuals in the bottom row will be
different from each other . While some of these
differences will be unique, others will be
inherited from their respective parents, who
were different from each other.
Heredity and
Evolution
9 CHAPTER
W
e have seen that reproductive processes give rise to new individuals
that are similar, but subtly different. We have discussed how some
amount of variation is produced even during asexual reproduction. And
the number of successful variations are maximised by the process of
sexual reproduction. If we observe a field of sugarcane we find very little
variations among the individual plants. But in a number of animals
including human beings, which reproduce sexually, quite distinct
variations are visible among different individuals. In this chapter, we
shall be studying the mechanism by which variations are created and
inherited. The long-term consequences of the accumulation of variations
are also an interesting point to be considered. We shall be studying this
under evolution.
9.1 9.1 9.1 9.1 9.1 A A A A ACCUMUL CCUMUL CCUMUL CCUMUL CCUMULA A A A ATION OF V TION OF V TION OF V TION OF V TION OF VARIA ARIA ARIA ARIA ARIATION TION TION TION TION
DURING REPRODUCTION DURING REPRODUCTION DURING REPRODUCTION DURING REPRODUCTION DURING REPRODUCTION
Inheritance from the previous generation provides both
a common basic body design, and subtle changes in it,
for the next generation. Now think about what would
happen when this new generation, in its turn,
reproduces. The second generation will have differences
that they inherit from the first generation, as well as
newly created differences (Fig. 9.1).
Figure 9.1 would represent the situation if a single
individual reproduces, as happens in asexual
reproduction. If one bacterium divides, and then the
resultant two bacteria divide again, the four individual
bacteria generated would be very similar. There would
be only very minor differences between them, generated
due to small inaccuracies in DNA copying. However, if
sexual reproduction is involved, even greater diversity
will be generated, as we will see when we discuss the
rules of inheritance.
Do all these variations in a species have equal
chances of surviving in the environment in which they
find themselves? Obviously not. Depending on the
nature of variations, different individuals would have
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