This tree is from an analysis of small subunit rRNA sequences sampled
from about 3,000 species from throughout the Tree of Life. The species were chosen based
on their availability, but we attempted to include most of the major groups, sampled
very roughly in proportion to the number of known species in each group (although many
groups remain over- or under-represented). The number of species
represented is approximately the square-root of the number of species thought to exist on Earth
(i.e., three thousand out of an estimated nine million species), or about 0.18% of the 1.7 million
species that have been formally described and named. (Tree of Life pdf, 368 KB)
If biologists do ever succeed in drawing the tree of life, it will look profoundly different from Darwin’s sketch. Lineages do branch as they evolve, but sometimes the branches join back together. […] When biologists draw the relationships of some groups of plant species, their pictures look more like webs than trees.
In other cases, genes don’t have to wait for two species to come together — they simply leap from one branch of life to another.
“Each gene has its own evolution. It’s not inherited from mother to daughter; it’s inherited from a neighbor,” said Peer Bork of the European Molecular Biology Laboratory.
Vedi anche: Homo homini loop.