[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]
Re: Defending grades (Was: Re: Archaeopteryx (rant))
T. Michael Keesey wrote:
>> That one group of animals can give rise to another group that should not be
>> included in the ancestral is a biological fact.
>No, it's a human construct, and, I would argue, not a very useful one.
In my original email the statement above was followed by an example, namely how
a species becomes paraphyletic when a local population becomes reproductively
isolated from the other (inter-reproducing) populations of this species. After
the speciation event the "outbreak population" is no longer a part of the
"parent species". If this didn't happen every organism on this planet would
still belong to the same species, so this phenomenon is rather central to
evolution. And that closely related lineages over time can change so much that
they appear very different is also a fact.
Regarding how we humans treat higher taxa there is of course a lot of
(Once upon a time I studied the various levels of reproductive isolation in a
widespread living superspecies complex, so I know a little about what
"speciation" means. Unfortunately, it is difficult to do the same with fossil
>To draw a line at an arbitrary point and say, "This no longer counts as part of
>the original lineage," is artificial.
About chronospecies I certainly agree that all boundaries are artificial.
Tim Williams wrote:
>My objection is to the use of the word "class". It's typological baggage.
>Linnaean hierarchy (Kingdom, Phylum Class, Order) is not an evolutionary
>concept, but (as pointed out) a human construct, and entirely arbitrary.
>Linnaean taxonomy may have a use in simple bookkeeping - Linnaean
>stamp-collecting, as I said in a previous post - but not in evolutionary
Agree about typological baggage. But: Even if the levels are arbitrary, there
is nothing arbitrary about what they are used to express. As an evolutionary
taxonomist I object to the description of modern Linnean systematics as
"Linnean stamp collecting" to use for "simple book-keeping". The reason we
still use the Linnean system is that it is a very good tool for information
storage. Once you know the system the position of one taxon gives you a lot of
information about its affiliation, anatomical and other characteristica and
what it includes. Certainly useful in evolutionary discussion.
>Uh-huh. Cladograms are based on anatomical characters (and sometimes
>molecular data too, for extant groups). You can have your cake and eat it
A cladogram can be based on any inheritable character. Dealing with fossils you
have no alternative to anatomical details. Dealing with extant taxa (especially
higher levels) analyses are now often almost exlusively based on DNA because it
is more reliable regarding ancestry. Working with extant anatomy you would e.g.
never get the whales dumped in the middle of Artiodactyla. BTW, this is another
example where the anatomy of a group of animals have changed so much that it is
defensible to classify it outside the parent taxon instead of squeezing it
inside, e.g. as a sistertaxon to Hippopotamidae.
Daniel Grossberg wrote:
>Classification is intended to ease our understanding of a species' origins.
>One can note that aves evolved from reptiles, while still maintaining them as
>a separate class.