Evolution and Classification

  1. Similarities among organisms allow us to group them and then study the groups. Characteristics are details of appearance or behavior; or a particular form or a particular function. Some basic characteristics will be shared by most organisms.
  2. The more characteristics two species will have in common, the more closely they are related. And the more closely they are related, the more recently they will have had a common ancestor.
  3. Two organisms belonging to the same genus are generally very similar and are likely to have a common ancestor in the recent past.
    Example: Dogs, jackals, wolves belong to the genus Canis.
  4. Classification of species is a reflection of their evolutionary relationship.

Example 1.
How are the areas of study evolution and classification – interlinked?
The more characteristics two species have in common, the more closely they are related. And they are likely to have a common ancestor more recently. Classification of organisms necessarily involves organizing them in different groups, based on the similarities and differences of characteristics. Classifying organisms helps us in recognizing the basic arrangement of a hierarchical structure among diverse species. It tells us about the similarities and evolutionary relationships between various organisms. Classification of species is a reflection of their evolutionary relationship. Thus, we can say, the areas of study – evolution and classification – interlinked

Tracing Evolutionary Relationships

Evolution and Classification Definitions, Equations and Examples

Homologous Organs

Those organs which are similar in structure, have the same developmental origin but perform different functions are known as homologous organs.

Example: forelimbs of mammals, birds, reptiles and amphibians.
The basic structure of the limbs is similar though it has been modified to perform different functions in various vertebrates. Such a homologous characteristic helps to identify an evolutionary relationship between apparently different species.

Analogous Organs

Those organs which perform similar functions but are structurally dissimilar are known as analogous organs.
Example: Wings of bats and wings of birds.

The 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 very different. They Look similar because they have a common use for flying, but their origins are not common.

Similarly thorns and spines seen in plants are also analogous structures. Both are pointed structures that are protective in function. However, thorn is modification of stem and spine is modified leaf.

Another example of analogous structures in plants is the tendrils of different types carrying out similar function.
Evolution and Classification 1


Fossils are the preserved traces of living organisms that lived millions of years ago.

Formation of fossils: Usually, when organisms die, their bodies will decompose and be lost. But every once in a while, the body or at least some parts may be in an environment that does not let it decompose completely. If a dead insect gets caught in hot mud, for example, it will not decompose quickly, and the mud will eventually harden and retain the impression of the body parts of the insect.

Analysis of the organ structure in fossils allows us to make estimates of how far back evolutionary relationships go.
Example: The fossil Archaeopteryx is considered to be the connecting link between birds and reptiles as it
has feathered wings and beak (like those of birds) but teeth and tail-like structures (like those of reptiles).

Evolution and Classification Definitions, Equations and Examples

Estimation of age of fossils:

  1. One method is relative. If we dig into the earth and start finding fossils, the fossils we find closer to the surface are more recent than the fossils we find in deeper layers.
  2. The second way of dating fossils is by detecting the ratios of different isotopes of the same element in the fossil material.

Example 2.
Explain the importance of fossils in deciding evolutionary relationships.
Fossils are the remains or preserved traces of dead plants or animals, which died millions of years ago and some of them are already extinct. The remains or impressions of body part, sometimes can be found between different layers of earth under its surface. The study of fossils have helped us to know about the evolution or the link between two species. Fossils tells us how new species are developed from the old. So fossils have an importance in deciding evolutionary relationship.

Comparing DNA

Changes in DNA during reproduction are the basic events in evolution. Comparing the DNA of different species give us a direct estimate of how much the DNA has changed during the formation of these species. This method is now extensively used to define evolutionary relationships.

Evolution by Stages

Evolution of Eyes: The complex body organs of animals such as eyes have been created in stages over many generations. Firstly, the rudimentary eye like that of a flatworm (Planaria) was formed, which are very simple and are just eye spots which can detect light. Starting from this basic design, more and more complex eyes were then evolved in various organisms. The evolution of eye is an example of evolution by stages.

Evolution by Artificial Selection

Farmers have been cultivating wild cabbage as a food plant for a very long time and have produced or evolved entirely different looking vegetables like cabbage, broccoli, cauliflower, kohlrabi and kale from it by artificial selection.

Evolution should not be Equated with Progress

Evolution is the production of a diversity of life forms and shaping of this diversity by environmental selection due to which more and more complex body designs have emerged. It is not as of when a new species is formed, the old species will disappear.
Note: It is not as if the body designs of older organisms were inefficient.

This is because many of the older and simpler forms of organisms still survive on earth. For example, one of the simplest and primitive life forms called ‘bacteria’ still inhabit some of the most inhospitable (or unfavourable) habitats such as hot springs, deep-sea thermal vents and the ice in Antarctica. Most other organisms cannot survive in such harsh environments.

Evolution and Classification Definitions, Equations and Examples

Example 3.
In evolutionary terms, can we say which among bacteria, spiders, fish and chimpanzees have a ‘better’ body design? Why or why not?
In evolutionary terms, we can not say which among bacteria, spiders, fish and chimpanzees have a ‘better’ body design. Because the evolutionary process takes into account the development of most efficient and suitable features in body designs of organisms for survival and adaptation favoring to a particular niche. For example, organisms with Complex and seemingly better body design, may not survive particular situations. Whereas, organisms like bacteria with its primitive body design and simple microbial life form may survive in the most inhospitable and extreme environmental conditions that are extraordinarily hot, cold or acidic.

Molecular phylogeny: Changes in the DNA during cell division would lead to changes in the proteins that are made from this new DNA and these changes would accumulate from one generation to the next. The idea behind molecular phylogeny is based on the idea that organisms that are more distantly related will accumulate a greater number of differences in their DNA. Such studies trace the evolution.

Human Evolution

  1. The same tools for tracing evolutionary relationships – excavating, time-dating, and studying fossils, as well as determining DNA sequences – have been used for studying human evolution.
  2. There is a great diversity of human forms and features across the planet. Over recent years, the evidence has become very clear. There is no biological basis to the notion of human races.
  3. All humans are a single species.
  4. The earliest members of the human species, Flomo sapiens, can be traced to Africa. Our genetic footprints can be traced back to our African roots. A couple of hundred thousand years ago, some of our ancestors Left Africa while others stayed on.

Class 10 Science Notes