Anole Classics: Albert Schwartz (1968) on Geographic Variation in Anolis distichus

I previously characterized Albert Schwartz as one of the five kings of Greater Antillean anole taxonomy for having described eight new species from the region.  Although Schwartz described the fewest species among the five kings, focusing on new species masks Schwartz’s even more important contributions to cataloguing geographic variation within species.  Schwartz’s career-spanning interest in biogeography and geographic variation resulted in a prolific history of describing subspecies in anoles and other taxa.  Anyone who’s looked at Schwartz and Henderson’s classic book on West Indian reptiles and amphibians is familiar with the irregular blobs that designate subspecies boundaries on the range maps for many of the region’s most geographically widespread species.  Many of these blobs were the result of Schwartz’s own efforts.  The pinnacle of Schwartz’s work on geographic variation may be his 1968 monograph on geographic variation in Anolis distichus.

As Schwartz notes in the introduction to this monograph, A. distichus is among the most abundant, widespread, and geographically variable of all anole species.  It is distributed across Hispaniola and most of the Bahamas, can be found from sea level up to 1800 m, and is so abundant that in some regions it “literally swarms.”  At least eight distinct subspecies were recognized at the time Schwartz began working on the group.  The geographic variation in A. distichus is particularly spectacular because it often involves striking differences in dewlap color and pattern; traveling across Hispaniola alone, one might encounter dewlaps ranging from deep wine red to pale chalky white, and just about any shade of yellow or orange in-between.

Schwartz’s monograph includes impressive water color portraits of dewlap color variation in A. distichus by David Leber, who painted in the field from live animals or from fresh specimens, field notes and color photographs.  As impressive as these plates were, dewlap color and pattern are all but impossible to characterize in fluid preserved specimens.  Largely for this reason, Schwartz’s quantitative treatment focused on characterizing 10 meristic traits from 1588 preserved specimens, with discussions of dewlap color and dewlap color variation being limited to more anecdotal observations.

Schwartz knew he was taking on a considerable challenge because the taxonomy of A. distichus and its relatives had long been clouded by the absence of easily coded diagnostic differences among species and geographically distinct populations.  In addition to making it hard to delimit distinct populations, the subtlety of phenotypic differentiation among A. distichus and its relatives also led to conflict about whether certain taxa were best recognized as species or subspecies.  Bahamian and Hispaniolan populations of A. distichus, for example, were recognized as two distinct species for over 60 years before Barbour formally synonomized them in 1937.

Because many decisions about anole taxonomy were made without explicit reference to species or subspecies concepts or criteria, trying to reconstruct exactly how and why specific decisions were made can be a bit complicated.  Barbour’s decision to synonomize A. distichus (Bahamas) and A. dominicensis (Hispaniola) represented a bit of a change in course for Barbour, who had in previous years advocated and applied a more liberal, and, in many ways, more modern approach that recognized populations isolated on different islands or island banks with binomials rather than trinomials.  Although Barbour’s work preceded Mayr’s formal description of the biological species concept, Barbour’s pre-1937 views were largely in line with Mayr’s; Barbour reasoned that populations that were no longer interbreeding due to complete geographic isolation qualified as distinct species.  I’m not sure why he changed his mind in 1937 and would love to hear from any Barbour scholars out there who might know more.

In any case,  it’s important to note that even in 1937 Barbour’s taxonomic practices did not rule out the recognition of distinct species on the basis of slight, and perhaps even unfixed, differences.  For Barbour and most other anole taxonomists, however, such taxa were only recognized when the two slightly distinct forms could be found sympatrically.  This was the case with a second species that is both closely related and morphologically similar to A. distichus: A. brevirostris.  Although these two species are quite distinct in life, they can be rather difficult to distinguish after preservation; in his 1968 Schwartz noted that the meristic traits used to distinguish distichus from brevirostris – most notably the preoccipital scale found in many brevirostris – are polymorphic in both species.  Nevertheless, they were still recognized as species because they occur sympatrically in several areas on Hispaniola, where they do not appear to hybridize with any frequency.

OK, now we can get back to Schwartz, who followed Barbour and others as recognizing distichus and brevirostris as distinct, while not recognizing Bahamian and Hispaniolan populations of distichus as distinct species, even after finding that populations from the two geographically distinct regions were characterized by a number of phenotypic differences.  Schwartz did, however, describe 11 new subspecies of distichus, two from the Bahamas, three from Hispaniolan satellite islands, and six from mainland Hispaniola.  In most cases, these subspecies were diagnosed by a combination of differences in meristic traits and in the color of the body and/or dewlap.  Schwartz also synonimized one subspecies – A. d. albidogularis – with A. d. dominicensis, noting Mertens described it only because he inadvertently compared his specimens to A. brevirostris rather than A. d. dominicensis.  Although Schwartz found traits that distinguished the Florida populations from other populations of distichus and retained us of A. d. floridanus, it now seems clear that this population results from relatively recent invasion by both Bahamian and Dominican species.

In addition to his important work diagnosing and delimiting subspecies in distichus, Schwartz commented frequently about hybridization and potential intergradation between parapatrically distributed Hispaniolan subspecies.  In some regions he reported evidence for intergradation, but in others reportedly more abrupt phenotypic transitions.  Presumably, any observations of two distinct populations existing in sympatry would have led to the diagnosis of distinct species (as it did with distichus and brevirostris).

Schwartz also commented extensively on the biogeography of distichus and reconstructed a crude dendrogram to represent relationships among subspecies (see below).  He suggested that the primary division between Bahamian and Hispaniola taxa resulted from overwater dispersal directly from Hispaniola to the Bahamas, but noted the curious absence of distichus from Cuba and southern Bahamian islands that  might have served as waypoints for colonists.  Although he found it impossible to determine if the Hispaniolan species originated on this island’s north or south progenitor islands, he did recognize a split between the subspecies found today on these two historical landmasses.

Schwartz’s careful work on distichus has inspired generations of subsequent study, initially by Ernest Williams and his students Preston Webster and Susan Case.  Unfortunately, much of Webster’s impressive body of work on distichus was published informally in Anolis Newsletter III after he died tragically in a car accident shortly after completing his thesis.  Case and Williams, for their part, published a number of papers addressing geographic genetic variation in distichus.  Their work recovered some evidence for genetic differentiation of Schwartz’s subspecies, but with a limited number of loci and without comprehensive geographic sampling.

My laboratory group has now picked up where our Case and Williams left off and are actively engaged in a number of projects on geographic variation, adaptation and speciation in distichus that are directly inspired by Schwartz’s work in 1968.  We’ve found evidence that Schwartz’s Dominican subspecies are associated with distinct mitochondrial haplotype clades and that some experience reduced gene flow where they come into contact.  I hope Schwartz would be happy to know that we’re now extending his work to include experimental genetics, genome scans, and detailed quantitative measurements of color and pattern.

• Author
• Recent Posts

Rich Glor

Latest posts by Rich Glor (see all)

• JMIH 2014: Early Records of Fossil Anolis from the Oligocene and Miocene of Florida, USA - August 13, 2014
• JMIH 2014: Relative Contribution of Genetic and Ecological Factors to Morphological Differentiation in Island Populations of Anolis sagrei - August 7, 2014
• JMIH 2014: The Ultrastructure of Spermatid Development within the Anole, Anolis sagrei - August 5, 2014