In developing language, regular people use words for things that we can recognize and agree on, like what a “tree” is. Then scientists got involved and tried to identify and categorize every species of tree. And once genes began to be studied, scientists realized that trees are not all related to each other at any level, and maybe even our definition of what a tree is cannot be trusted.
“Trees” are not a coherent phylogenetic category. On the evolutionary tree of plants, trees are regularly interspersed with things that are absolutely, 100% not trees. This means that, for instance, either:
The common ancestor of a maple and a mulberry tree was not a tree.
The common ancestor of a stinging nettle and a strawberry plant was a tree.
And this is true for most trees or non-trees that you can think of.
I thought I had a pretty good guess at this, but the situation is far worse than I could have imagined.
So, what is a tree? You might say it’s a plant made of wood, but our definition of wood is pretty muddy, too. It turns out that an awful lot of non-woody plants have genes to make wood, and could be trees if conditions are right. And it gets weirder from there. Read how nature keeps making trees out of other plants, and vice versa, at Eukaryote Writes Blog. -via Metafilter