Humans have a penchant for organizing. We like order. This need for organization certainly drove Karl Linnaeus, a Swedish naturalist, to published the first catalog of life, the Systema Naturae, in 1735. He devised the framework we still use in our taxonomy.
In the last post, we explored the challenges of recognizing species. New knowledge forces us to re-examine our understanding of the limits of variation of species. We regularly gain or lose species on our life lists as former species are divided into two or more new species or others combined into a single species.
Taxonomists do have methods for defining a species. The problem is that there is more than one method and the different approaches do not always get to the same conclusion!
Similar species are placed in a genus. Genera (the plural of genus) that are similar are placed in the same family. And on we go upward to order, then class, then phylum, then kingdom. Thus, the tree of life is organized.
Linnaeus based his taxonomy on similarity of form. In the next century, Charles Darwin saw that Linnaeus’ system could reflect relatedness. Species in the same genus had a more recent ancestor than two species in different genera or families. He said that our classification of life should be a genealogy.
But how does one decide how large a genus or order should be? Surprisingly, the answer is that it is arbitrary, depending on the preferences of the taxonomist. Some genera have a single species like the genus Icteria, containing only the Yellow-breasted Chat. On the other hand, the snail genus Conus contains 750 species and the sedge genus Carex has nearly 1800 species.
Ultimately, the size of the genus or other taxonomic group is not important as long as it can be defended as a natural grouping. Any taxonomic group should be monophyletic (one branch), containing species more closely related to each other than to any species in other groups. Darwin’s desire to have our taxonomy to be a genealogy is really a desire for our classifications to contain only monophyletic groups.
The job of erecting and revising a taxonomic system for any group of organisms had to rely on similarity of structure until the turn of the 21st century. Now, our ability to rapidly sequence and compare the DNA of organisms gives us a second powerful way to assess relatedness.
Some genes change through mutations quite rapidly so DNA comparisons of these genes are useful for exploring closely related, recently separated species. Other genes mutate very slowly so can be used to assess the relationship between distant groups like phyla or classes. Some genes change at intermediate rates so can be used to assess the relatedness of orders and families.
DNA comparisons have shaken the foundation of our bird taxonomy. Such comparisons allow us to avoid the twin pitfalls of species from a common ancestor diverging strongly and species in different groups converging to similar shapes.
The grebe order was formerly placed close to the loon order. DNA comparisons now tell us that the closest relatives of grebes are . . . . . . . flamingoes! Here we have a case where species have strongly diverged from their common ancestor. Another cool example is that the flightless penguins are most closely related to the albatrosses and shearwaters, masters of long-distant flight.
On the other hand, the hawks and the falcons were formerly lumped together in the same order. DNA tells us that convergence to high-speed, sharp-taloned predators has occurred. The two types of raptors are placed in different orders now. The closest relatives of the falcons are the parrots and perching birds. The New World vultures share a common ancestor with the hawks.
[Originally published on August 13, 2017]