Isozyme Variation in Echinocereus engelmannii var. munzii (Cacteceae)

Assessing the degree of evolutionary differentiation between a rare taxon and its relatives is critical from a biological perspective. Because funding and time are limited, conservation biologists need to set maintainance of truly unique pieces of biodiversity as a higher priority than maintenance of interesting populations of widespread species. Although many factors go into setting priorities, thorough assessment of evolutionary distinctiveness based on multiple lines of evidence is the most basic. We used isozyme characters to assess the distinctiveness of a purportedly rare taxon, Echinocereus engelmannii var. munzii (Munz's hedgehog cactus), from other common varieties of the same species. We sampled at least 15 individuals from one population in each of the three mountain ranges where E. e. var. munzii is present within the United States plus three corresponding desert populations of a more common variety. To test for clinal variation between mountain and desert populations, we sampled three elevationally intermediate populations in the San Bernardino Mountains. We recognized a total of 104 band patterns for nine enzyme systems. All nine enzyme systems yielded polymorphic zymograms. Although we found substantial intrapopulation variability, no significant isozymic differentiation was apparent among any of the sampled populations. Lack of such differentiation implies that the effects of selection or drift have not been felt or detected in these populations. In terms of these enzyme systems Echinocereus engelmannii var. munzii is not distinct from other members of the species. Although there is no evidence of isozymic distinction, morphological evidence should be included in any decisions to submerge this taxon.