When creating a set of rules to categorize living things we get to decide how rigid we want the rules to be. The more rigid the rules, the easier it is to draw lines. The more flexible they are, the greater the room for ambiguity.
Taxonomy would be much simpler if we could create a rigid rules. But evolution is a rather chaotic process, and living things are very diverse. Nature is very flexible. Rigid rules won’t let us categorize this diversity in meaningful ways.
For example, if we want to set a “fixed rate for genetic difference”, we will run into a problem that different species have different genetics. It could be that you find that the genetic difference between a human and a banana is less than the difference between two fungi species that we consider to be in the same genus. So, seeing this, you have to either group humans and bananas together, split the fungi into many tiny groups, or lift this global “genetic difference” constraint and focus instead in making more local comparisons.
The last option is what taxonomists usually do. There are several regions in the genome that are used that are used as “barcodes” and are also used to establish the “distance” between organisms in evolutionary terms. These are very useful for building a case for a new species. But it is only part of the argument, and we can’t define an absolute reliable number that always works. Here is a review that covers the topic of DNA barcoding and species delineation that you may look into if you want more detailed information: https://hal.science/hal-01958691/file/dna-barcoding-species-delineation-and-taxonomy-a-historical-perspective.pdf
When creating a set of rules to categorize living things we get to decide how rigid we want the rules to be. The more rigid the rules, the easier it is to draw lines. The more flexible they are, the greater the room for ambiguity.
Taxonomy would be much simpler if we could create a rigid rules. But evolution is a rather chaotic process, and living things are very diverse. Nature is very flexible. Rigid rules won’t let us categorize this diversity in meaningful ways.
For example, if we want to set a “fixed rate for genetic difference”, we will run into a problem that different species have different genetics. It could be that you find that the genetic difference between a human and a banana is less than the difference between two fungi species that we consider to be in the same genus. So, seeing this, you have to either group humans and bananas together, split the fungi into many tiny groups, or lift this global “genetic difference” constraint and focus instead in making more local comparisons.
The last option is what taxonomists usually do. There are several regions in the genome that are used that are used as “barcodes” and are also used to establish the “distance” between organisms in evolutionary terms. These are very useful for building a case for a new species. But it is only part of the argument, and we can’t define an absolute reliable number that always works. Here is a review that covers the topic of DNA barcoding and species delineation that you may look into if you want more detailed information: https://hal.science/hal-01958691/file/dna-barcoding-species-delineation-and-taxonomy-a-historical-perspective.pdf
wow, thanks for the explanation! the banana and human example was very clever, thanks for the link too!