Expert: Dana Krempels, Ph.D. - 6/14/2007

Question
Dr. Krempels,

I was hoping you could shed some light on an argument a friend and I were having on genetics and evolution. We were talking about the links between the genus Australopithecus and the genus Homo. First, could two individuals from different genera interbreed? If not, how does a mutation separate itself from its species/genus? In other words, how did evolution progress though different genera? As a modern example, since its been proven that people with down syndrome can reproduce, if they did so for a number of generations (only interbreeding with other individuals with down syndrome) would a new species/genus eventual emerge that could no longer interbreed with Homo Sapiens (I only chose down syndrome as an example because it is a chromosomal mutation)? I'm basically just trying to understand how evolution works on the genetic level. Any information you could provide would be greatly appreciated. Thanks!

Answer
Dear Chris,

By definition, individuals of the same species must be able to breed to produce fertile, viable offspring.  If members of different species cannot do this (and there is some wobble there, of course...), then members of different genera certainly cannot.

As you probably know, Australopithecus and Homo did not exist anywhere near the same historical time, so the question in their particular case is moot.  The most you can say is that they have a common ancestor, and that's where it ends. We don't even have DNA samples from Austalopithecus to see just how much genetic similarity there might be between that ancient species and our own.

That said, the only real unit of evolution is the *population*--which consists of members of the same species.    One genus does not give rise to another.  In modern cladistic systematics (classification system), one taxon (i.e., a known phylogenetic unit at any taxonomic level, such as "Homo sapiens" or "Phylum Arthropoda") does not change into another one.  Rather, related taxa diverge from a common ancestor.

Hence, one cannot say that Australopithecus evolved into Homo sapiens any more than one can say that humans evolved from monkeys.  One can say ONLY that Australopithecus and Homo shared a hypothetical common ancestor somewhere in the mists of time as their populations evolved.

How do populations evolve into new species?  That can happen many different ways, and for an excellent primer on that, I'd visit the *fantastic* web site of the University of California at Berkeley Museum of Paleontology here:

http://evolution.berkeley.edu/

For an overview of the mechanisms of speciation, try this area:

http://evolution.berkeley.edu/evolibrary/search/topicbrowse2.php?topic_id=51

This is by far the best and most comprehensive overview of modern evolutionary theory I've ever seen, and it is SO user friendly and easy to understand that it will bring tears to your eyes!  (It chokes me up every time!  ;)  )

Once you've wandered around that site a bit, and if you find you still have questions, then please feel free to write again.

I hope this helps.

Dana