Modern plant classifications is based on common descent groups; the idea being that each group contains all the descendants of the last common ancestor. Groups range from a clade containing many thousands of species to a genus containing half a dozen, but all share this relationship.
Now, what happens when a new species comes along? Let's say a cabbage ( Brassica oleracea) and a radish ( Raphanus raphanistrum ) somehow created a wide cross hybrid which stabilized into a new species, which didn't cross easily with either parent. Let's say it is different enough to place in its own genus in Brassicaceae, so it is now called Raphassica frankensteinia.
So now we have a problem. The genus Brassica no longer contains all descendants of the ancestral brassica, nor does the genus Raphanus contain all descendants of the ancestral Raphanus, whatever they might have been. But classifying the new species Brassica frankensteinia or Raphanus frankensteinia would still not solve the problem. What should be done with this species?
Or let's say that the process did not involve two species. Let's say a Brassica species mutated far enough to "deserve" its own genus (quite different then all other Brassicas). Under the new Cladistic method of plant organization, would this ever be allowed to happen? Or would it just have to stay in that genus? Or will the twigs of the tree of life eventually turn into stems? Will there, millions of years from now, be a vast clade called brassicots, containing dozens of orders and hundreds of families, all descended from the humble Brassica oleracea? (Assuming plant classification is a thing in millions of years.)
So is this a problem for clade systems, since once a wide cross or mutation is given its own genus, both the old and the new genus are not monophyletic groups anymore? Interesting that I stumbled upon a cross that really happens.
Modern plant classifications is based on common descent groups; the idea being that each group contains all the descendants of the last common ancestor.
There might be monophyletic clades, (descended from the same ancestor population), paraphylic clades, (nearly monophyletic, but not quite), or polyphylic clades (having genes that were not inherited from a common ancestor).
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