In biological classification, what's the largest and most inclusive rank for living things?

And the answer: Domain.    

In taxonomy, there are three domains – Archaea, Bacteria, and Eukarya – and the first two involve microorganisms. The third, Eukarya, involves most plants and animals. These organisms are sorted into seven more categories: Kingdom, Phylum, Class, Order, Family, Genus, and Species.

Taxonomy, or the science of classifying living things, tells the story of all life on Earth. Sometimes called the Phylogenetic Tree, taxonomy illustrates the relationships between living things.

There are currently around 2 million known species on Earth. However, that is by no means a complete list: scientists discover new species nearly every day. This means that every new discovery adds to ever-enlarging categories of classification – some of which eventually require reordering.

Shortcomings aside, taxonomy has been the dominant biological classification process for around 250 years. Its founder, botanist Carl Linneaus, invented the methodology in the 1700s when he realized the botanical nomenclature of 18th century Europe was highly confusing and inefficient. The method he eventually established was based on morphology, or the study of physical form and structure. This was revolutionary for a study that had been operating off of classification using analogous traits: structures in animals that appear similar, but actually come from very different origins. Under this form of classification, birds would be more closely related to butterflies than reptiles, based on the power of flight.

To learn more about the history and practice of taxonomy, check out the video below.

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