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SAI International School
Subject- Biology
Chapter- Heredity And Evolution

Subtopic- Evolution and Classification, Tracing Evolutionary
Relationships

Module-27
Lesson Notes

EVOLUTION AND CLASSIFICATION
 According to Scientists estimations, there are about 8.7 million living species,
which is still increasing.
 Among which, some are known, while some are unknown. For the ease of study
and identification of these organisms, they have been classified into different
groups based on different criteria.
 This procedure of arranging the organisms in a hierarchical series of groups is
termed as the Biological classification.
 There were different criteria to be followed in the arrangement of organisms into
categories. Every classification started with basic design, characteristics,
similarities, dissimilarities, etc.

Characteristics:

 Characteristics are details of appearance or behaviour; in other words, a
particular form or a particular function.
 For e.g., i) we have four limbs is a characteristic. ii) plants can do photosynthesis
is also a characteristic.
 Some basic characteristics will be shared by most organisms.
 The cell is the basic unit of life in all organisms. The characteristics in the next
level of classification would be shared by most, but not all organisms.
 A basic characteristic of cell design that differs among different organisms is
whether the cell has a nucleus.
 Bacterial cells do not, while the cells of most other organisms do. Among
organisms with nucleated cells, which ones are unicellular and which ones multi-
cellular?
 That property marks a very basic difference in body design, because of
specialization of cell types and tissues.
 Among multi-cellular organisms, whether they can undertake photosynthesis or
not will provide the next level of classification.
 Among the multi-cellular organisms that cannot do photosynthesis, whether the
skeleton is inside the body or around the body will mark another fundamental
design difference.
 We can see that, even in these few questions that we have asked, a hierarchy is
developing that allows us to make classification groups (Refer to the picture
given below).
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