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234 IGUANA  •  VOLUME 12, NUMBER 4  •  DECEMBER 2005 GIFFORD

An adult male Leiocephalus personatus from near Juanillo, La Altagracia Province. This wide-ranging species reaches the westernmost
extent of its range along the southern coast in association with a sharp ecotone near Baní.

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Hybrid zones have been of great interest to evolutionary biol-
ogists for quite some time. However, only in the last few

decades have they received much attention. This change follows
a paradigm shift from viewing hybrid zones as rare occurrences in
the general distribution of “pure” forms to their potential impor-
tance as regulators and creators of biodiversity (Harrison 1993).
The resultant research effort has created a realization that the
analysis of hybrid zones can provide information with far-reach-
ing implications for speciation, adaptation, and even conservation

(Arnold 1997, Harrison 1993). With this in mind, the following
article discusses hybrid zones in the context of their consequences
for speciation and conservation, focusing on an area identified as
a zone of secondary contact between two lineages of the lizard
Ameiva chrysolaema in the Dominican Republic.

Hybrid zones can be defined as regions (either narrow or
broad) of genetic or anatomical changes that separate otherwise
continuous and homogeneous “taxa.” “Taxa” is placed in quotes
here to represent not only recognized species but also divergent

Ecotones and Hybrid Zones:
Implications for Conservation Illustrated by
Ground-dwelling Lizards from Hispaniola

Matthew E. Gifford

Department of Biology, Washington University
Saint Louis, Missouri

Photographs by the author except where indicated.

IGUANA  •  VOLUME 12, NUMBER 4  •  DECEMBER 2005 235ECOTONES AND HYBRID ZONES

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Ameiva chrysolaema from 5 km west of Duvergé, Independencia Province. Two distinct lineages of this species meet at the ecotone near Baní.



lineages that may or may not be formally recognized. The advent
of new methodologies and availability of increasingly detailed
genetic data has revealed that previously unknown lineages are
common and characterize most widely distributed groups of
organisms. Areas of contact between lineages many times result
in interbreeding among members of these lineages. Such zones
provide unique opportunities to study the process of speciation
and adaptation. Combined analyses of different types of char-
acters (genetic and anatomical) provide information on the
processes that maintain hybrid zones or contribute to their col-
lapse (Barton and Hewitt 1985). Hybrid zones can form in areas
of continuous habitat or at habitat transitions (i.e., ecotones).
The hybrid zone discussed herein is associated with a prominent
ecotone in the south-central Dominican Republic near the city
of Baní.

The island of Hispaniola has had a dynamic geological his-
tory highlighted by periodic inundation of the Cul de Sac-Neiba
Valley in the central portion of the island. These inundations sep-
arated Hispaniola into two paleoislands at various times during
the Miocene to Pleistocene epochs (~15–1 million years ago
[mya]; Mann et al. 1999, McLaughlin et al. 1991, Taylor et al.
1985). After recession of seawater, habitable land likely became
emergent during the late Pleistocene and Holocene. Genetic data
suggest that these seawater inundations fragmented populations
of Ameiva chrysolaema, forming eastern and western lineages
along the southern coast of the island (Gifford et al. 2004). This

study also identified a restricted area (in the vicinity of Baní)
where these two lineages have come into secondary contact. 

The diverse topography of the island contributes to a vast
array of environmental conditions and creates near Baní a sharp
environmental shift from relatively moist conditions in the east
to extremely dry conditions in the west. Character shifts in A.
chrysolaema associated with this ecotone include body size
(SVL), dorsal scale size, and both nuclear and mitochondrial
genetic characters (unpubl. data). In the following section, I con-
sider what these character shifts imply about the nature of the
hybrid zone and discuss the implications of these shifts for spe-
ciation, adaptation, and conservation in the group. Please note
that this discussion is based on preliminary data and is used only
to illustrate a thesis. Additional data collected during future field
seasons will be necessary to further characterize this hybrid zone
and test the hypotheses discussed here.

Patterns of character variation across the zone
Mitochondrial genetic characters show a sharp change in fre-
quency from those characterizing the western lineage being
found in essentially all individuals in those populations west of
the ecotone, to being absent from populations on the east side
of the ecotone. However, two populations within the ecotone
contain characters of both lineages (Gifford et al. 2004).
Preliminary data from studies of nuclear genetic material show
a similar pattern, but the area over which the change occurs

236 IGUANA  •  VOLUME 12, NUMBER 4  •  DECEMBER 2005 GIFFORD

Map of mean annual precipitation (mm) for the Dominican Republic. The ecotone discussed in the text refers to the area in the vicinity of Baní.



appears to be wider. Anatomical characters also seem to exhibit
comparable relationships across the contact zone. In the case of
body size, populations in dry habitats to the west tend to con-
tain smaller individuals than populations in the moister eastern
environments. This is illustrated clearly by a positive correlation
between precipitation and SVL across the contact zone.

Implications of contact zones for 
speciation and adaptation

The coincidence of contact zones (i.e., the occurrence of zones
corresponding to several characters in similar geographic locales)
and the concordance of changes (i.e., similar rates of change over
the contact zones for different characters) have been used as evi-
dence that natural selection can maintain a hybrid zone (Endler
1977, Barton and Hewitt 1985). However, two types of selec-
tion could be responsible and are indistinguishable from one
another. Selection may work against individual hybrids, which
would suffer a decrease in fitness relative to “pure” forms.
Alternately, selection may favor local adaptation to different
selective pressures on either side of the zone. One may be able
to distinguish between these types of selection by using addi-
tional data. For example, the first hypothesis may be supported
if populations in the hybrid zone are found in a population den-
sity trough (i.e., have decreased population densities), which is
a testable prediction. That hypothesis would be further strength-
ened if reciprocal transplant experiments indicated a decreased
fitness of hybrids in all transplanted habitats. Although testing
these predictions takes considerable time and effort, the impor-
tant point here is that natural selection is important in structur-
ing the hybrid zone.

Variation among transition zones for anatomical characters
could arise in several ways. Some characters may be under selec-
tion while others are effectively neutral. That could lead to tran-
sition zones for neutral characters governed most strongly by
totally random factors (as opposed to selection). Variation
among transition zones for genetic characters might be gener-

IGUANA  •  VOLUME 12, NUMBER 4  •  DECEMBER 2005 237ECOTONES AND HYBRID ZONES

Morphological (dashed line) and environmental (solid line) clines for
populations of Ameiva chrysolaema sampled across the ecotone. Boxes
represent mean SVL (mm) for each sampled population along the
transect. Crosses represent mean annual precipitation (mm) for each
sampling point along the transect. The x-axis represents distance (in
km) from an arbitrary locality on the western side of the ecotone.

Leiocephalus semilineatus from 5 km west of Duvergé, Independencia Province. This very drought-tolerant species is largely confined to the very
dry Cul de Sac-Neiba Valley and reaches the easternmost extent of its range at the ecotone near Baní.



ated by the different properties of mitochondrial versus nuclear
genes. The latter generally evolve more rapidly, so differences
might be coupled with the age of the hybrid zone. If genetic
variation is minimal and anatomical variation pronounced, nat-
ural selection on anatomical features may be very strong.
Different patterns may suggest that the lineages are diverging
while others might indicate that they are merging in a fashion
that could result eventually in a single genetic lineage.

Implications of hybrid zone studies 
for conservation

At first glance, the presence of a hybrid zone may cause prob-
lems for conservationists. These problems likely stem from the
lack of consensus about the appropriate methods by which we
should identify species. By viewing speciation as an extended
process and species as elements of populations-level lineages
(sensu de Queiroz 1998), some of these problems may be

238 IGUANA  •  VOLUME 12, NUMBER 4  •  DECEMBER 2005 GIFFORD

Leiocephalus schreibersii from 5 km west of Duvergé, Independencia Province. This species is largely associated with very dry habitats; like L. semi-
lineatus, it reaches the easternmost extent of its range at the ecotone near Baní.

Leiocephalus lunatus from Catuano, Isla Saona, La Altagracia Province, Dominican Republic. Although this individual is not from the main island
of Hispaniola, the species is fairly uniform in pattern across its range. This species has a largely coastal distribution that reaches its westernmost
extent at the ecotone near Baní.



IGUANA  •  VOLUME 12, NUMBER 4  •  DECEMBER 2005 239ECOTONES AND HYBRID ZONES

addressed more effectively. The goal of conservation is to pre-
serve biodiversity. Biodiversity can be defined as the natural
diversity of independently evolving lineages (as opposed to for-
mally recognized species or even higher taxa; see, for example,
Powell 2004 and references therein). Consequently, we must
understand the evolutionary consequences of lineage contact
and the processes acting to either maintain lineage differences or
facilitate lineage merger. Ecotonal contact zones mark the con-
fluence of different environmental conditions that may also rep-
resent the limits of other species adapted to the alternative envi-
ronments. As such, these ecotones represent areas of high faunal
turnover and, hence, high diversity. By conserving these areas,
we may preserve the selective pressures (environmental hetero-
geneity) that have shaped the current distribution of taxa and
the processes that maintain and structure biodiversity. Other
ground-dwelling lizards whose distributional limits are located
at this particular ecotone include Leiocephalus semilineatus, L.
schreibersii, and Ameiva lineolata on the dry western side and L.
personatus and L. lunatus on the moister eastern side.

Literature Cited
Arnold, M.L. 1997. Natural Hybridization and Evolution. Oxford University

Press, New York.

Barton, N.H. and G.M. Hewitt. 1985. Analysis of hybrid zones. Annual
Review of Ecology and Systematics 16:113–148.

de Queiroz, K. 1998. The general lineage concept of species, species criteria,
and the process of speciation: A conceptual unification and terminolog-
ical recommendations, pp. 57–75. In: D.J. Howard and S.H. Berlocher
(eds.), Endless Forms: Species and Speciation. Oxford University Press, New
York.

Endler, J.A. 1977. Geographic Variation, Speciation, and Clines. Princeton
University Press, Princeton, NJ.

Gifford, M.E., R. Powell, A. Larson, and R.L. Gutberlet, Jr. 2004. Population
structure and history of a phenotypically variable teiid lizard (Ameiva
chrysolaema) from Hispaniola: The influence of a geologically complex
island. Molecular Phylogenetics and Evolution 32:735–748.

Harrison, R.G. 1993. Hybrids and hybrid zones: Historical perspectives, pp.
3–12. In: R.G. Harrison (ed.), Hybrid Zones and the Evolutionary Process.
Oxford University Press, New York.

Mann, P., P.P. McLaughlin, W.A. van den Bold, S.R. Lawrence, and M.E.
Lamar. 1999. Tectonic and eustatic controls on Neogene evaporitic dep-
osition in the Enriquillo basin, Dominican Republic, pp. 287 – 341. In:
P. Mann and K.J. Hsü (eds.), Sedimentary Basins of the World: Caribbean
Basins. Elsevier, Amsterdam, The Netherlands.

McLaughlin, P.P., W.A. van den Bold, and P. Mann. 1991. Geology of the
Azua and Enriquillo basins, Dominican Republic; 1, Neogene lithofacies,
biostratigraphy, biofacies, and paleogeography, pp. 337–366. In: P. Mann,
G. Draper, and J.F. Lewis (eds.), Geologic and Tectonic Development of the
North America-Caribbean Plate Boundary in Hispaniola. Geological
Society of America Special Papers, vol. 262.

Powell, R. 2004. Species and subspecies: What do they mean and why should
we care? Iguana 11:108–113.

Taylor, F.W., P. Mann, S. Valastro, Jr., and K. Burke. 1985. Stratigraphy and
radiocarbon chronology of a subaerially exposed Holocene coral reef,
Dominican Republic. Journal of Geology 93:311–332.

Ameiva lineolata from Isla Catalina, La Romana Province. Although most populations of this drought-tolerant species are found in the very dry
Cul de Sac-Neiba Valley west of the ecotone near Baní, some apparently relict isolates are found far to the east.

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