Phylogeny Lab

Parsimony steps are counted with the Fitch method: for each character, the fewest gains or losses that explain the tip states on your tree, added over all characters. The "best possible" value comes from checking every possible tree for these taxa. Characters are simplified to present or absent, and the datasets are small teaching sets; real studies use many more characters, often DNA.

The time axis is broken: the Precambrian (4,600 to 541 million years ago) is squeezed into the left part so the Phanerozoic can be read. Oxygen and diversity curves are smoothed, approximate reconstructions; real estimates vary between studies. Diversity is a relative index of marine animal families, not a species count. Scenes are cartoons, not reconstructions of real places.

Problems are randomly generated. Half-lives used: carbon-14, 5,730 years; potassium-40, 1.25 billion years. Trees use letters for taxa so you read the branching, not what you already know. The worked solution builds step by step below the picture.