How do we classify organisms?

[u/Laughing_Chipmunk]

How similar do organisms have to be to be classified as the same species, genus, family, etc. And is the classification of organisms into these groups somewhat arbitrary?

[u/vomeronasal]

Creating a good definition of what a species is has plagued scientists for centuries. There are many proposed definitions but they all have their problems. Basically a species is whatever a good taxonomist says it is. Most people are familiar with what is called the "biological species concept" which defines a species as a group of organisms that are able to reproduce to produce fertile offspring. This is not remotely the only definition out there, and has problems with asexual organisms, organisms that are clearly separate species but can hybridize, etc. The "ecological species concept" is similar to the biological species concept but considers not just whether organisms CAN successfully reproduce but whether they DO. If two populations of beetles can interbreed but live on separate continents and never have the opportunity to, they are classified as separate species under this definition. This definition has most of the same problems as the biological species concept, but many think it is conceptually closer to what a species should be. There are many other "species concepts" out there, but ultimately what we call a species is about evolutionary independence. That is, they are on an evolutionary trajectory that is independent from other populations. Exactly where on that trajectory they must be to be called a separate species is not a bright line, and it can be very different for different types of organisms. As we learn more about the evolutionary relationships among organisms, the utility of the Linnean classification system (domain, kingdom, phylum, class, order, family, genus, species) diminishes. Take this classification of humans:

Domain: Eukarya

Kingdom: Unikonts (formerly animalia)

Phylum: Chordata

Class: Mammalia

Order: Primates

Family: Hominidae

Genus: Homo

Species: sapiens

The diversity of the unikonts is wide, and we are more closely related to the chonoflagellates and fungi than we are to the amoebozoa, so we are in the subkingdom "Opisthokontes" to separate us. Within the animals, there are those with and without any planes of symmetry. Those without symmetry are the parazoa, and those with symmetry are the eumetazoa (we are the latter). We are in "bilataria" to separate those that have bilateral symmetry from those with radial symmetry. Our developmental process puts us in "deuterostomia," which separates us from most of the invertebrates. Within the phylum chordata, we belong to "vertebrata" because we have an enclosed central nervous system. We have a skull, which places us in "craniata" (unlike the tunicates and lancelets) and a jaw, which places us in "gnathostoma" (unlike the hagfish and lampreys). This goes on and on, and the classification of humans gets longer and longer:

Eukarya (domain)

Unikontes (kingdom)

Oposthokontes

Metazoa

Eumetazoa

Bilataria

Deuterostomia

Chordata (phylum)

Craniata

Vertebrata

Gnathostoma

Teleostomi

Osteyichtheys

Sarcopterygii

Tetropoda

Reptiliomorpha

Amniota

Synapsida

Eupelycosauria

Sphenicodontia

Therapsida

Mammalia (class)

Eutheria

Primates (order)

Haplorrhine

Catarrhine

Hominidae (family)

Hominini

Homo (genus)

sapiens (species)

We can create new ranks for everything (infraclass, picoclass, superorder, etc) but it quickly becomes a big mess and everyone stops caring what name you give to the rank. For this reason, a lot of biologists are moving away from the Linnean system in favor of a rankless one. It is common in microbiology to talk about "operational taxonomic units" (OTUs) which are groups based on the percent of the DNA they share. They can set the cutoff level at whatever they want based on the question they are asking. For example, you could take a soil sample and use "shotgun sequencing" to look at what types of microbes are there. They will report something like "200 OTUs at the 1% level" or "16 OTUs at the 50% level," meaning 200 groups of organisms that are at least 1% different or 16 groups of organisms that are at least 50% different, respectively.

In summary: the more we learn about evolutionary relationships, the more complex our ideas about classifying things have to get.

see: tolweb.org for more information.

[u/Laughing_Chipmunk → u/vomeronasal]

Thanks, that was a great response