A family tree turns into a tangle when you factor in adopted children and remarriages. Evolution often resembles such a tangle: although the standard model draws a tidy tree of species, in reality genes travel not only through inheritance but also directly between unrelated organisms. The result is a messy network of interwoven lineages.
To measure how far a network is from a simple tree, scientists devised a yardstick: how many extra links must be severed or imaginary branches added to untangle the knot. They studied three approaches: one severs only 'wrong' links, another any links, and the third adds ancestors. It turned out that these yardsticks don't always agree: a network might look simple but require more fixes due to tricky knots.
Horizontal gene transfer is like suddenly getting a gene from your neighbor that lets you digest milk better. In nature, this happens all the time, especially among microbes.