CLASS WORK -6

Class X-D&XH                                   BIOLOGY

31/01/2014

Evolution and Classification – Classification refers to the arrangement of organisms into groups on the basis of similarities/dissimilarities and placing them in a hierarchy that reveals their relationship similarities or dissimilarities are determined on the basis of presence or absence of characteristics in the organisms

Characteristics of organisms refer to details of external and internal appearance or behavior that distinguish them from one e.g. Plants can performs photosynthesis. Some basic characteristics are shared by large number of organisms. More characteristics two species have in common more closely they are related. If they are more closely related they will have had a common ancestor (explain the example of brother sister and cousins)

Conclusion- Classification of species is a reflection of their evolutionary relationship

Evolutionary relationships can be traced with the help of following –

1- Morphological and Anatomical Evidences – There evidences are based on the similarities in the external and internal features of the different kinds of organisms

a) Homologous organs – The organs which have similar basic structure and mode of origin but perform different functions in different animals are called homologous organs, e.g. forelimbs of amphibians, reptiles, birds and mammals. The basic structure limbs is similar though it has been modified to perform different functions in various vertebrates. Homologous organs give an evidence of common ancestory and evolutionary relationship between apparently different species.

Analogous organs – Analogous organs are those structures, which are different in their basic structure development and origin but appear similar and perform similar function Example of analogous organs wings of bird and bat. Wings of bats are skin folds stretched mainly between elongated fingers. But the wings of birds are a feathery covering all along the arm. The designs of the two wings, their structure and components, are thus very different. They look similar because they have a common use for flying but their origins are not common.

2. Fossils – They are impression of the body / body parts or the remains of organisms living in the past, which got preserved in sediments of earth. When living organisms die their bodies decompose but some parts of their bodies may be in such an environment that they do not decompose e.g. if a dead insect gets caught in hot mud it will not decompose quickly but the mud will harden and retain impressions of the body parts of the insects.

The age of fossil can be estimated in two ways.

-    The fossils that occur closer to earth surface are more recent to those found in deeper layers.

-    The second method is isotope dating i.e. detecting the ratio of different isotopes of the same element in the fossil material.

Importance of fossils – Fossils provide one of the most acceptable evidences in support of evolution, because we can study the evolutionary past or individuals in the form of their fossils. By studying fossils occurring in different strata of rocks, geologists are able to reconstruct the time course of evolutionary events. Complete fossil record of animals like horse, camel, man has helped us to study the stages of evolution.

3. Comparison of genetic material – DNA is the genetic material. It is a long molecules made up of smaller unit called nucleotides. Evolutionary relationships can also be worked out by comparing DNA of different species. During formation of new species from old one, DNA changes make the base of reproductive barrier. This barrier is created as the geographically isolated populations accumulate adaptations or inheritable traits with survival advantages in the form of differences in DNA sequences. Species which are closely related will have more common nucleotide sequences than those which are distantly related.  

Recapitulation Exercise

Q.1 Give an example of characteristics being used to determine how close two species are in evolutionary terms.

Q.2 Can the wing of a butterfly and the wing of a bat be considered homologous organs? Why or why not?

Q.3 What are fossils? What do they tell us about the process of evolution?

Q.4 Will two species considered close enough to be classified under on group if they have analogous organs? Give reasons.

Q.5 What are the evidences of organic evolution? List and explain any two.

Q.6 Explain the different ways to know the age of fossils.