The Joshua tree's range in the Mojave is an archipelago, constrained by altitude and available moisture. The trees mainly grow at altitudes between 2,000 and 6,000 feet, most happily in the upper middle part of that range. Map the range of the tree and you will draw a set of disconnected splotches across the Mojave. One very large splotch covers the west end of the desert, where the rising of the Sierra Nevada and the Transverse Ranges drags the valley floors up into the tree's preferred range, and you can travel from Twentynine Palms to Gorman to Ridgecrest mainly within sight of a Joshua tree. Somewhat smaller splotches cover the Cima Dome-Ivanpah-Lanfair area and Pearce Ferry Road, Wickenberg and the Bill Williams River and Goldfield. The rest of the range is in small patches on the sides of mountains.
And notable on this map you've drawn is a broad Joshua-less swath through the middle of the desert, curving in from Needles like a lowercase "C" a hundred miles thick, passing through Barstow and Death Valley with only a couple patches of trees in Fort Irwin to interfere with the nice clean lines. Only in the northern limit of the Joshua tree's range does this treeless swath close up, and the east and west populations of Joshua trees meet in Nevada's Tikaboo Valley.
Living in isolation from their cousins, the eastern and western groups of Joshua trees have evolved different growth habits, a different appearance. But just how different are they?
The eastern Joshua trees are generally referred to as Yucca brevifolia var. jaegeriana, named by Susan D. McKelvey in 1935 after Edmund C. Jaeger, who later became dean of American desert studies in the mid-20th Century. Out of courtesy to their namesake, let us consider Jaeger's 1957 description of the trees:
"Near Cima and again in close-by Lanfair Valley is found the largest concentration of yuccas in the United States. It is a giant forest of tree yuccas of the subspecies jaegeriana (Yucca brevifolia jaegeriana), with trees in places so thickly set that it is difficult to see far among them. This yucca differs from the tree yuccas of the western and northern Mohave Desert in having an average shorter main trunk, shorter leaves, a greater tendency to freely branch, and a different chromosome constitution."
That split-infinitive "greater tendency to freely branch" is the main way you can tell which group a Joshua tree belongs to, and I'll discuss it in some detail in a post to come. For now we'll leave it at this: the buds at the ends of "jaegeriana" Joshua trees' branches sometimes split spontaneously, becoming two buds that grow into two branches. No one knows just why, but we do know that the trees' western cousins almost never do this. That means the eastern trees tend to branch earlier and more often than their cousins, which branch mainly after flowering or injury.
What I want to discuss now is Jaeger's reference to the trees being a subspecies.
A subspecies is a group of individuals within a species that are distinct from the rest of the species in appearance and behavior, and which usually occupies a separate geographical range. A variety is a similar subgroup within a species, albeit with less distinct differences. The line between what constitutes a subspecies and what constitutes a variety is often as indistinct as the line separating Joshua tree forests from the piñon-juniper forests into which they merge at altitude.
Pretty much since the differences between the two populations of Joshua trees were discovered, botanists have had different opinions about those populations' proper taxonomic rank.
Pioneering desert botanist Susan Delano McKelvey first described the eastern type in 1935, naming it for Jaeger and designating it as a variety. While that assessment has more or less stood up to this day, a number of authorities over the last century -- apparently including Jaeger -- felt the distinctions between the two populations merited a subspecies division. (Jaeger can perhaps be excused for apparently having wanted "his" Joshua trees to be a full subspecies.)
Part of the issue is that even with our fuzzy boundaries between our definitions of varieties, subspecies, and species, the real world is even fuzzier. One of the classic delineators between species is reproductive isolation: if two populations are incapable of cross-breeding and producing reproductively fertile offspring, that's generally taken as an indication that the two populations should be considered distinct species. It's not a hard and fast rule, and there are plenty of exceptions, especially in the plant world. But it remains a pretty good rule of thumb: If two groups of plants can't pollinate each other and set viable seed that will grow into a fertile "adult," that's a good basis for calling them two different species.
As I've written here before, Joshua trees rely on yucca moth partners -- in the genus Tegeticula -- to pollinate their flowers and produce seeds to grow new Joshua trees. Without the yucca moth's intervention, the Joshua trees can't reproduce.
In 2003 Olle Pellmyr, a lepidopeterist from the University of Idaho, found that the yucca moths pollinating "jaegeriana" Joshua trees were of a different species than the moths working the trees to the west. The western Joshua tree moths had been classified as the species Tegeticula synthetica, so Pellmyr, in a nod to Hegelian dialectics, named the eastern moths Tegeticula antithetica.
The eastern moths are distinct from their western cousins in a number of ways, but the most relevant difference here is that female eastern Joshua tree moths have longer ovipositors -- the organs they use to inject eggs into the trees' developing fruit -- than their western counterparts. If you compare the flower structure of the two populations of trees, a reason for this difference becomes clear. To lay her eggs in a Joshua tree flower's ovary, a female moth inserts her ovipositor through a tube called a "stylar canal." The eastern trees' flowers have longer stylar canals, and that's meant the eastern moths have had pressure to evolve longer ovipositors.
In 2007 Lee Lenz of the Rancho Santa Ana Botanic Garden proposed that based on these differences in morphology and reproduction between the two populations of Joshua trees, Yucca brevifolia var. jaegeriana should be elevated to full species rank. That would make the eastern trees members of the new species Yucca jaegeriana.
Lenz's assessment wasn't accepted overnight, but few taxonomical pronouncements are. References to "Yucca jaegeriana" in the literature since are sparse. Chris Smith, an evolutionary biologist from Willamette University who studies the differences between the two populations, still referred to them as varieties in 2009. In a 2012 paper on yucca evolution, he and his co-authors stated that Joshua trees "may be two species situated along a north-south transect in the Mojave desert region." A 2013 paper in the journal Molecular Ecology by Tyler N. Starr and five colleagues, including Smith, elevates the varieties to subspecies, at least for the purposes of discussion and for the time being.
That 2013 paper bent my mind a little when I first read it. In Nevada's Tikaboo Valley, where both kinds of Joshua tree grow side-by-side, there are trees that are clearly the result of cross-breeding between the two kinds. But those hybrid trees all came about when pollen from a jaegeriana was packed into the ovary of a brevifolia, and not the other way around.
How did the paper's authors know? Plant cells have organelles called chloroplasts, which do the work of photosynthesis. Chloroplasts have their own genome, little bits of DNA that provide instructions for things like photosynthesis and building proteins and making new chloroplasts. Most flowering plant species' pollen cells don't have chloroplasts. There are some exceptions, but Joshua trees aren't one of them. And that means that Joshua trees get their chloroplasts solely from their female parent's ovum. Which means in turn that by examining a tree's chloroplast DNA, you can find out who its mother was. It's an almost exact analogue of using human mitochondrial DNA to trace a person's maternal ancestry.
Of the hybrid trees examined by Tyler et al, none had jaegeriana chloroplasts. All the hybrids had jaegeriana fathers and brevifolia mothers. None came from brevifolia pollen packed into a jaegeriana flower. There's an obvious possible reason for this: with their longer ovipositors, the eastern moths do just fine at pollinating the western trees. The western moths just don't fit in the eastern flowers.
Which means that there is indeed reproductive isolation between the two populations of Joshua trees, but only in one direction. Taxonomize that.
How botanists will eventually decide to describe the relationship between the two groups of Joshua trees is a question I find fascinating. Imagine having two species of Joshua trees instead of one! It would be a handy lesson about how life endlessly confounds our attempts at tidy categorization. There might be conservation implications. It would be a reminder that the desert is almost always more biologically diverse than we think it is. It would be a nice tribute to an important biologist. And it will almost certainly make no difference whatsoever to the trees.
This is an excerpt from the author's forthcoming book on Joshua trees.