Before the era of molecular biology, species were organised based on their physical characteristics.
This, however, has turned out to be somewhat incorrect, as many lineages converge to the same outcome after initially being separated. For example, there are different species of flying squirrel which live on different continents. Their last common ancestor was not a flying squirrel, and following separation onto different parts of the world, they continued evolving separately.
Due to similar niches in their environment, they happen to have both evolved their flying ability further down the line. This is called convergent evolution and it led scientists to assume these species must be closely related since they look similar. Similarly, archaea used to be lumped in with bacteria due to their apparent similarities, only to later be found to be separate altogether.
Molecular biology techniques such as genetic fingerprinting have enabled researchers to compare organisms and species on an unprecedented level with much more data and detail, to be able to ascertain their relatedness.
Any 2 given people share 99.9% of their DNA code. But the differences present in the remaining 0.01% of it are enough to enable reliable identification, with the exception of monozygotic twins. The DNA containing this is called variable number tandem repeats (VNTRs) because they are just sequences of DNA repeated many times.
Aside from genes, or coding DNA, there are non-coding regions which repeat themselves many times over in each individual, with some sequences contained within varying. This variability is less in closely related individuals. This is where the usefulness of genetic fingerprinting comes in. This covers medicine, criminology and biodiversity conservation among other things.
1. The sample DNA undergoes PCR then cleavage at multiple sites with restriction endonucleases
2. The resulting many small fragments are tagged using a radioactive molecule
3. They’re separated using gel electrophoresis and viewed using a developed photographic film
The bands exposed then undergo simple visual analysis by matching up the template DNA with other DNA that could be similar more or less, depending on situation. Above, the DNA found at a crime scene is compared with that of 3 suspects. The bands of suspect 2 are perfectly aligned with the crime scene DNA.
Different species may be compared in this way for the purpose of determining how closely they are related to one another. Another molecular biology technique which can be used to this end is enzyme comparison. Enzymes are the direct products of DNA sequences, so the similarity in amino acid sequence in the enzyme can be used to compare different species. Enzymes are proteins, so any protein could theoretically be used to test this, such as haemoglobin.
From this short sequence of amino acids in the haemoglobin of these different species we can infer several things. Let’s do humans and chimps first! How many differences are there? Lys, Glu, His, Iso and… Lys, Glu, His, Iso. Right. Absolutely no difference. Humans and gorillas have one difference, zebras and horses have one difference and zebras and humans have 3 differences!
We can infer a lot of different information from this table, and it’s just a very small sequence in just one protein looking at just five different species. The potential of investigating diversity with molecular biology tools is astounding.
DNA can be studied similarly, and a lot of creativity can be employed to come up with ways to twist and turn heaps of genetic data in such a way that interesting information can be pulled out. In this example, it’s a fairly straightforward, run of the mill comparison between the DNA sequence itself of a mouse gene versus a fly gene.
We can see that the sequence itself is 76.66% identical, while the protein product resulting from the exons only, is actually identical in its entirety at 100% between the two sequences (highlighted in green).