The Surprising Science of Classification and Evolutionary Relationships | Cambridge IGCSE Biology

[Please watch the video attached at the end of this blog for a visual explanation of this topic]

In the previous article on the same topic, we discussed what classification is, the Linnaean system of classification, and the Binomial naming system, and in this article, we will be focusing on how classification systems reflect evolutionary relationships between organisms.

The aim of a successful classification system is to group organisms together in terms of how closely related they are to one another. In other words, a classification system will try to reflect the evolutionary relationships between the organisms.

This might lead you, as a biology student, to ask yourself, “how is it that certain organisms share similar features?” For example, if we consider a group of mammals, we know that all mammals have bodies covered in hair and they feed their young from mammary glands.

The answer happens to be really simple. The reason that they share common features is  because all these mammals have descended from a common ancestor that lived about 200 million years ago. So all mammals are related because they shared this common ancestor.

This leads us to the conclusion that organisms share features because they originally descend from a common ancestor.

Classification then and now

Originally, organisms were grouped or classified using morphology and anatomy, i.e. they would be put into different groups based on their form, their shape and their internal body structures.

But as technology advanced, microscopes, knowledge of biochemistry, and eventually DNA sequencing allowed us to take classification and this grouping system down a more scientific path. 

In the modern day, the DNA sequences of different species are considered and the more similar the base sequences are in the DNA of these species, the more closely related these two species are. 

The more similar DNA base sequences are = more closely are the organisms related

How do you determine whether organisms are closely related looking at DNA base sequences?

If you look at the diagram given above, you will find the DNA base sequences of a few vertebrates such as a mouse, rat, human, pig, dog, a chicken and a frog. 

Among these animals, if we consider the mouse, rat, human, pig and the dog, we know that they are mammals, the chicken is a bird, and the frog is an amphibian.

We will then look at the DNA base sequences of the organisms given in the diagram and come to a few conclusions based on these DNA base sequences.

The DNA base sequences of the mouse, rat, human, pig and dog, are very similar to one another. Except in very few points, the base sequences are quite similar.

This means that these five organisms are very closely related to one another, and we know that because we know that they are mammals. This shows us how DNA base sequences reflect evolutionary relationships:

All these organisms who have very similar base sequences have evolved from a common ancestor and they are all grouped in the group mammals.

If you were to compare the DNA sequence of a mammal such as the dog and a bird (chicken in this example), it can be seen that there are a few points where the base sequences are similar to each other, but there are more variations among these base sequences. 

More variations in their DNA base sequence means that they are more distantly related to one another.

This is exactly what we mean when we say that DNA base sequences can show how closely related organisms are. 

For example, base sequences in a mammal’s DNA are more closely related to all other mammals than any other vertebrates because they are similar to one another, when compared with other vertebrate animals.

Revising Classification:

DNA sequence questions get tied in with the lesson variation sometimes, but it is also essential you know your basics.

Since Biology questions tend to get repeated, practise as many as you can find. Some questions can be found here as well, and you can time your answers to see if you can stick to the time limit given.

If you are struggling with IGCSE revision or Biology in particular, you can reach out to us at Tutopiya to join revision sessions or find yourself the right tutor for you.

Watch the video below for a visual explanation of the lesson and make sure to attempt the quiz once you’re done!

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