Phylogenetic Tree
Phylogenetic Tree
[A circular phylogenetic tree is shown, with various parts of the tree colored red, green, blue, and black.  Text is written on the root of the tree and the first few branches, getting smaller until it becomes unreadable.  The visible text is:] Duke Duke Gonzaga Louisville Duke UVA FSU Dayton UVA Kansas FSU
[Caption below the panel:] I was kicked off the biology project after I secretly replaced all the phylogenetic trees in our new paper with March Madness brackets.

In biology, phylogenetic trees are a way of showing evolutionary relationships between species. Each split in the tree represents a species that was the common ancestor of the two species beneath it, resulting in a bifurcating structure that can be followed all the way back to a single root - the most recent common ancestor of all species in the tree.
In sport, a tournament tree is a diagrammatic way of showing the progress of competitors in an elimination tournament. Each split in the tree represents the winner of a match between the two competitors beneath it. This too results in a bifurcating tree structure, which eventually terminates at a single root representing the champion of the tournament. Tournament brackets are a recurring theme at xkcd.
In this comic, Randall has taken advantage of the similarity between these two diagrams in order to prank his fellow biologists.
Each year in the United States, in March and early April, 68 National Collegiate Athletic Association (NCAA) Division I college basketball teams play in a single elimination postseason tournament to decide the national champion of college basketball. This tournament is colloquially known as March Madness . Associated with this tournament, it has become commonplace to guess the outcomes of each game, and predict who will win the tournament. A diagram illustrating the progress and elimination of teams through the tournament is called a bracket . Presumably Randall is referring to the men's college basketball tournament here, though there is a separate women's college basketball tournament that is also referred to as "March Madness".
Randall has replaced the trees in a biology paper with a basketball March Madness bracket , which is not related to biology. The 2019-20 NCAA college basketball regular season had not ended yet at the time of this comic's publication, so the partial bracket shown is a fictional bracket. Compared to a phylogenetic tree, the 'root' of a tournament tree is the final result (once known), rather than the common ancestor that was prior in time to all those that came after; the 'leaves' are all the initially hopeful competitors, rather than the latest extant (or unsucceeded extinct) organisms.
The title text shows the inverse of what the comic says: Apparently the March Madness bracket pool removed Randall after he tried to introduce biology-related evidence comparing the National Basketball Association (NBA) and American Basketball Association (ABA) to organisms and claiming the ABA is an endosymbiont living inside the NBA. An endosymbiont is an organism living inside another organism. In a way, this can be considered true of these two leagues, as the NBA and ABA merged in 1976 after which the ABA ceased to exist. 4 teams from the ABA, the Denver Nuggets , Indiana Pacers , Brooklyn Nets and San Antonio Spurs , continue to exist today as NBA teams. It is additionally humorous that Randall brings up the ABA/NBA merger in a March Madness bracket group, as March Madness is a college basketball tournament, as opposed to professional basketball played by the NBA and ABA.
A March Madness bracket was also the topic of 1819: Sweet 16 .
The bracket shows the Duke University basketball team winning the NCAA college basketball tournament. Strangely, it shows Gonzaga University linked only to explicitly non-Gonzaga branches, suddenly appearing out of the bottom section, which is not possible in a sports bracket context, but possible in biology if Gonzaga is an identified ancestral root with all descendant evolved species identified by a new term. In fact, the implied unchanged continuity of Duke from 'universal ancestor' to niche population sharing the world with all of its diverged and re-evolved outbranchings (rather than perhaps used as a term for a typically broad cladistic group of branches, such as Archaea) would be more curious - or just imply an inherent of available precision in the necessary paleobiological studies that classify the proposed UA and its descendency.
As of the publish date of this comic, all of the college basketball teams mentioned (except the University of Virginia) were ranked in the top 25 of the Associated Press poll . The University of Virginia was the 2019 national champion (winner of the tournament), so that may have been why they were mentioned.
[A circular phylogenetic tree is shown, with various parts of the tree colored red, green, blue, and black.  Text is written on the root of the tree and the first few branches, getting smaller until it becomes unreadable.  The visible text is:] Duke Duke Gonzaga Louisville Duke UVA FSU Dayton UVA Kansas FSU
[Caption below the panel:] I was kicked off the biology project after I secretly replaced all the phylogenetic trees in our new paper with March Madness brackets.
