Notes on Evidence for Relationships Between Organisms

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Comparing protein structures

Because the primary structure (sequence of amino acids) of a protein is determined by DNA base sequence, you can compare amino acid sequences of proteins which have the same function in different species to see how close they are: the more similar the amino acid sequence, the closer the evolutionary relationship.

For example, you could compare the amino acid sequence of haemoglobins in different mammals (the following amino acid sequences are completely made up):

From this you can see that humans are most closely related to chimpanzees, but least closely related to mice. (When asked to make inferences about relationships from these tables, you should state which species are most and least closely related.)

There are, however, limitations of investigating evolutionary relationships by comparing protein structures. Comparing DNA sequence can provide more clues than comparing protein structures because:

The triplet code in DNA is degenerate. Because more than one triplet can code for the same amino acid, a change in DNA sequence might not result in a change in protein structure.

Some DNA is non-coding (e.g. introns).

DNA hybridisation

DNA hybridisation is a method of comparing evolutionary relationships which builds on the concept of complementary base pairing.

Recall that complementary bases (A and T, or C and G) are held together by hydrogen bonds which stabilise the double helix.

The method is as follows:

=> Take strands of DNA from two species you want to compare. Heat the DNA to separate the double helices.

=> Place the strands together in solution. They will automatically pair back up again.

=> Some of these helices will be hybrids. They will be made of one strand from one species and one strand from the other.

=> The closer the relationship between the organisms, the more complementary base pairs there will be.

=> Thus more hydrogen bonds will form, so more heat will be required to separate the strands.

=> The hybridised helices are heated until the strands separate, and the heat needed to do so is recorded. The higher the temperature, the closer the relationship between the organisms.

In the animal kingdom, courtship is a necessary precursor to mating because:

It allows individuals to recognise a mate of the same species.

It allows synchronisation of fertility (female may not be fertile at all times - an infertile female will reject the male's advance).

It stimulates the release of gametes.

Usually it is the male who initiates courtship. Because courtship allows individuals to recognise a mate of the same species, courting rituals must be specific to a species.

This means biologists can investigate how closely species are related by comparing their courtship rituals. More similar courting rituals means closer evolutionary relationship!

Comparing protein structures

DNA hybridisation

Courtship