When biologists say an animal is "basal" does this mean:
* the animal has more traits found in extinct animals than its close relations
* the animal is taxonomically isolated on a branch with few leaves
* because the animals is taxonomically isolated it may have traits of extinct ancestors of a clade lost in most other clade members
I've seen it used in all of these ways. Is the isolation key? Or is it being similar to the ancestor that matters more?
@futurebird I assumed it referred to species that was unusually closed to the common ancestor.
@futurebird @ekg you bet. The choice of what traits to include and exclude is arbitrary - we usually consider large visible morphological traits, especially those that fossilize, but not things like chemical innovations, behaviors, etc.
For example, one of the basal adaptations ("synapomorphies") of the Caryophyllales is betalain pigments (the red in beets, prickly pear fruit), but the group the order, Caryophyllus (renamed to Dianthus, oops), is named after has lost those pigments! We just didn't consider that important when choosing the exemplar to name the family after.
The same thing can happen when deciding what traits matter for being basal or not.
@futurebird @astory evolution occurs in two steps. Genetic drift, and natural selection. When the gen pool contacts it may center on the same position again, assuming a local maximum has been found. The gen pool will only centre around a new position if their exist a better fit for the environment.
Of course the world is more complicated then the models I base the above assertion on.
@ekg @futurebird the trouble is, what does "close" mean? All living organisms are equally far from a shared common ancestor with another living organism in time.
I'm a ways away from my biology undergrad, but I think the most self-consistent usage is "this organism diverged early and has a constellation of ancestral traits that we find meaningful," like "basal angiosperms" such as water lilies and magnolias having large numbers of less-specialized floral parts
(In part however this is because they convergently evolved for beetle pollination before bees etc existed, and beetles like to eat the flowers a bit)