Someone finds an organism and considers it to be “different enough” from all other organisms that have been described in the literature. This person will collect (or, in the past, sometimes illustrate) the organism and store it somewhere such as in a museum - this is the “holotype”. The person will then write a paper with the description of the organism, compare it to some of the most similar known members, and make an argument for calling this holotype a member of a new species. If the species has particularly unique traits, or substantially different genetics, the author can argue for the description of a new genus - or even a higher rank.
But… The line is indeed extremely blurry. There is no universal agreement about where to draw the lines. The description of a new taxon is an argument, and experts disagree continuously. The tree is being continuously shuffled and it is not uncommon to see different publications using different scientific names for the same species.
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
Someone finds an organism and considers it to be “different enough” from all other organisms that have been described in the literature. This person will collect (or, in the past, sometimes illustrate) the organism and store it somewhere such as in a museum - this is the “holotype”. The person will then write a paper with the description of the organism, compare it to some of the most similar known members, and make an argument for calling this holotype a member of a new species. If the species has particularly unique traits, or substantially different genetics, the author can argue for the description of a new genus - or even a higher rank.
But… The line is indeed extremely blurry. There is no universal agreement about where to draw the lines. The description of a new taxon is an argument, and experts disagree continuously. The tree is being continuously shuffled and it is not uncommon to see different publications using different scientific names for the same species.
and if we get a fixed rate for genetic diference?, if it’s X different, so it’s other specie
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!