A biological continuum

Thu Mar 30 07:50:39 CST 1995

>>Mammals and birds are the only major groups of eukaryotic organisms
>>that reproduce exclusively by sexual means (a rather small proportion
>>of eukaryotes), and even they are not free from phylogenetic
>I believe that my herpetologist colleagues view such things as partheno-
>genesis in lizards as the exception, rather than the rule.  A group
>does not need to reproduce exclusively by sexual means in order for
>sexual reproduction to have a major influence on its evolution.
>And certainly the fact that cladistics works, and in many cases works
>well, argues that reticulation, while perhaps important in certain
>cases, is not an overriding factor in evolution.

There are several points here.

     A) Curtis, you're still stuck on vertebrates.  While I can't
     be bothered to do the guess-work calculations, I'll bet that
     the proportion of lizards that are parthenogenic is orders
     of magnitude greater than the proportion of eukaryotes that
     are vertebrates.  Cases of continua between `species' of
     sexually reproducing vertebrates can be found in a good text
     on animal evolution.

     B) I do not know what you mean by "cladistics...works well".
     Cladistics as practiced today often produces one (or many
     more) *hypothetical* phylogenies that "in many cases"
     appeals to the subjective conceptions of the worker.  Even
     if one accepts the theory of cladistics (presumably we are
     talking about the Hennig derivative procedures here, which
     do not permit the consideration of the continuous nature of
     organismal phylogeny through time), the methodology has
     serious flaws and is highly subjective (in practice), and
     the results of a particular analysis are to be viewed only
     as an evolutionary hypothesis.  It may work "well" because
     one sees what one wants in the results.  The presence of
     reticulation cannot be determined by popular cladistic
     procedures, in fact one starts with the assumption that
     reticulation does not exist.

     C) Reticulation is very real, even in vertebrates; to
     mention two cases: the complex of salamanders in the
     genus _Ambystoma_  from eastern North America.  This group
     has been studied intensively since the early 1960s only to
     reveal more and more complexity to the reticulation.  Four
     diploid species are known to be hybridizing and exchanging
     genes (A. laterale, A. jeffersonianum, A. texanum, A.
     tigrinum) and other species are suspected to be involved.
     Diploid, triploid, tetraploid and even pentaploids are
     known; polyploids being mostly female.  Also among the most
     heavily studied organisms are cereal grasses and their
     relatives.  Wheat, barley and rye belong to the Triticeae
     tribe, in which a vast array of `intergeneric' hybrids have
     been documented where two or more distinct genomes have
     combined in hybrids to give an incredible taxonomic tangle
     of intergrading species and genera.

>>As J. K. Veldkamp has pointed out: All plants are hybrids, but some are
>>greater bastards than others.
>>There is a continuum between taxon pairs where the taxa are discretely
>>distinct and pairs where the taxa are continuously variable.
>One of my research specializations is homoploid hybrid speciation in
>flowering plants.  It is abundantly clear that hybrid speciation does not
>of itself blur the boundaries between taxa.  In fact hybrid species are
>very often as distinct as their parents, simply because the selection
>factors that allowed them to form in the first place act to constrain
>their morphology.

Are these plants asexual F1 populations; no segregation at F2
generation, no chance for backcrossing?  Are these plants which
normally only reproduce through chasmogamous, outcrossing
flowers?  What about species that are wide-spread with
combinations of populations/individuals/flowers that are
outcrossing, selfing and/or apomictic (apart from more
traditional vegetative reproduction means)?

>It is true that introgressive hybridization can
>produce some strange intermediates, but introgression does not result
>in new species (although it can "inject" new features in existing ones),
>and it is often of limited scope in relation to the total ranges of the

Introgression does not result in new species, agreed, it results
in continua.

>Just because intermediates exist, that doesn't mean that they are important,
>either statistically or biologically. I still contend that we blind ourselves
>to the actual variation that exists by assuming a continuum based on the
>evidence of a few intermediates.

OK, in some cases intermediates are not important "statistically
or biologically", but do not restricting sampling to specific
cases or in time or in space or in numbers, so that few
intermediates are detected.  To say it again, the extent of
variation between forms is continuously variable, from uniform
continua to wide hiatuses.  To deny the existence/significance of
continua is to deny an important aspect of biological variation.
Maybe the difficulty is that your work is done on California
plants (?).  I suggest that you might try to examine a
circumboreal species complex (animal or plant) *throughout it's

Please note that the quote of the previous post should be
attributed to J. F. Veldkamp (not J. K.).

More information about the Taxacom mailing list