Great Basin Naturalist Memoirs

Volume 3 The Endangered Species: A Symposium Article 14

12-1-1979 Rare species as examples of plant evolution G. Ledyard Stebbins Department of Genetics, University of California, Davis, California 95616

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Recommended Citation Stebbins, G. Ledyard (1979) "Rare species as examples of plant evolution," Great Basin Naturalist Memoirs: Vol. 3 , Article 14. Available at: https://scholarsarchive.byu.edu/gbnm/vol3/iss1/14

This Article is brought to you for free and open access by the Western North American Naturalist Publications at BYU ScholarsArchive. It has been accepted for inclusion in Great Basin Naturalist Memoirs by an authorized editor of BYU ScholarsArchive. For more information, please contact [email protected], [email protected]. RARE SPECIES AS EXAMPLES OF PLANT EVOLUTION

G. Ledyaid Stebbins'

.\bstract.- Rare species, including endangered ones, can be very valuable sources of information about evolution- arv processes. They may be rare and valuable because: (1) they are evolutionary youngsters and could represent an entirelv new evolutionary strategy of great scientific and practical value; (2) they are evolutionary relicts that have stored enormous amounts of genetic information of great worth; (3) they may represent endemic varieties that har- bor a great deal of the genetic variability in the gene pool that would be of enormous value to a plant geneticist; the

rarity of the plant is not necessarily correlated with the size of its gene pool; (4) they may represent unique ecologi- cal adaptations of great value to future generations. Studies of gene pools and the genetics of adaptation constitute a new and developing field of the future.

This morning I'm talking about a some- Projected on the screen is a chart which what different topic than yesterday evening Ownbey put in the American Journal of Bot- and one more in hne with my usual interest any in 1950. The three species which are list- because, as many of you know, I am primari- ed all have the chromosome number 2n= 12. ly an evolutionist. The theme that I would They are well-known European plants in-

like to develop is that rare species, including troduced as weeds into North America. the endangered ones, can be very valuable There are no species of Tragopogon in the .sources of information about evolutionary Old World that have chromosome numbers

processes. To illustrate that theme, I am go- higher than 2n=12 and n = 6. However, in ing to give you examples of four alternative, the backyards and railroad yards around Pull- but not mutually exclusive, theories that have man, Washington, Ownbey found two differ- been suggested for the reason that species are ent entities having 24 chromosomes, twice rare. One of the oldest, which was cham- the number of all the others in the genus. By pioned in the early part of the 20th century a series of ingenious hybridizations and analy- Willis, was that the rare endemic ses of chromosomes, he established without by J. C. species were youngsters. They appeared on doubt that one of them, described as a new the earth recently and haven't had time to species, T. miscellus, was derived from hy- spread. He wrote a whole book on the sub- bridizations between T. ditbius and T. pra- ject, which he called Age and Area. I first tensis. Its chromosome number has been heard of that book from my systematics pro- doubled either before or after the initial fessor, M. L. Fernald, who was very much in- hybridization, giving us a stable intermediate, terested in rare species. He regarded them as a principle long known to geneticists. A sec- senescent relics, and strongly opposed Wil- ond species, T. minis, has originated in the

lis's theory. However, Willis was partly cor- same way from a cross between T. porrifoUus rect. We have perhaps more direct evidence and T. duhius. Because T. dubiiis did not ex- of this than for any other hypotheses. This is ist in western North America before about expected, because the origin of some new the 1890s, the age of these two species when species would be recent enough that we he first found them was not more than a half

would know when it appeared. A now classic century. They now are spreading. They are

example is that presented by the late Marion known in one or two places in Montana. Ownbey, of Washington State University, There is another locality in Arizona. They with respect to the goatsbeard genus, Trag- are certainly not endangered. A hundred opogon. years from now, they may have become com-

artmenl of Genetics, University of California. Davis, California (

113 114 Great Basin Naturalist Memoirs No. 3 mon weeds. other two species. Delphinium hesperinm and There are other very young examples, not D. recurvatum have been crossed. The hybrid quite so young as these. Indirect evidence is partly fertile and looks like D. gypsophi- suggests that some species have arisen since lum. The interesting thing is that when the the retreat of the glacial ice from northern F-1 hybrid is crossed with native D. gyp- America and the drying up of the cli- North sophilum it is found to be more compatible in places like California where the mate than the cross with either of its own parents. Pleistocene was a rainy or pluvial period. This is pretty good evidence that the native example is that of three species in the One D. gypsophilum is derived from hybridization Laijia, or "tidy tips," an annual plant genus between the two other species. This again is of the composite family, or Asteraceae. The a recent species. It is too common to be on three species involved are L. jonesii, L. mun- the rare and endangered list, but it is cer- zii, L. leucopappa. They are very closely and tainly recent and uncommon relative to its related, on the l3order line of becoming spe- ancestors. cies. can be hybridized easily and the They An ancient species is the California Big is partly fertile. They are more dis- hybrid Tree, Sequoiadendron giganteum. It is con- tantly related to L. fremontii and L. platij- fined to certain groves in the Sierra Nevada glossa. in California, extending from Sequoia Na- Their distributions are as follows: Laijia jo- tional Forest in the south in Tulare County nesii occurs near the coast of south central into ever-smaller groves, to a very tiny one in California, not far from San Luis Obispo; the northernmost area in Placer County to is in small valleys of Laijia miinzii found the the west of Lake Tahoe. It does not have a the inner south coast ranges; and L. leuco- much-restricted gene pool. Horticulturists pappa, in the central valley of California, on have found they can extract various modifi- or hillsides not far from Bakersfield. one two cations of it by simple inbreeding. Daniel Ax- They are closely related entities occupying elrod, with his brilliant studies of the pa- neighboring areas in the same general region. leobotany of western Nevada, has shown that The climate of this region was drastically California Big Tree was very abundant in likely changed in the Pleistocene. It seems Pliocene forests, six, seven, or eight million that the splitting of these three populations years ago in western Nevada when the Sierra from each other is post-Pleistocene, about six Nevada didn't exist and the climate of Ne- to eight thousand years ago. vada, due to moist winds from the ocean, was is line of Layia miinzii on the border being still relatively mesic. Because of the Pleisto- in season, but is certainly not en- rare a dry cene changes in climate, it has become re- dangered. Layia jonesii, being on the coast, stricted in two ways: (1) the elimination of might be endangered. Layia leucopappa, in the stands to the east of the Sierra Nevada by highly cultivated central valley, definite- the aridity and (2) the reduction of the stands in ly is endangered. A nature conservancy California by the increasing length of the is very group, located in Bakersfield, much rainless dry summer. I say this because of re- interested in it. It occurs on a large private search by the late Woodbridge Metcalf, ex- range area, the manager of which says they tension forester at Berkeley. He showed that are going to preserve it for us. This is a case the big tree would seed itself only in a year is on the rare of a young species which and when the dry season from May till October is list. endangered shorter than usual. If it gets a few of these Another example of a similar nature is a short dry seasons between seed germination species of larkspur. Delphinium hesperinm is and the time when the trees are 8-10 years species of the oak a common widespread old, then it competes with sugar pine, white ranges of Cali- woodlands in the inner coast fir, and other forest species. If young trees fornia. Delphinium recurvatum is foimd in are exposed to a succession of normal dry the central valley and bottom lands and Del- summers, they cannot compete with the implies, phinium gypsophilum, as the name seedlings of the other species. This, then, is lives in gypsumlike soil on the hills bordering an example of an ancient species. the valley. It occupies a habitat between the There are some others. One of the inter- 1979 The Endangered Species: A Symposium 115

esting things that Professor Jack Major and I etic diversity. At that time I was influenced worked out some years ago is that the con- by the work of Sewall Wright on inbreeding centration of rare rehctual species, which and its ramifications, which was very popular were more common in the past, is bipolar in at that time. Now we have data on restricted Cahfornia. It represents two elements, first a species like Clarkia franciscana, which grows northwestern element which is related to the in only a single hillside in the city of San Pacific Northwest and Asia. The species in- Francisco and which, as my colleague Leslie volved are mainly trees or shrubs related to Gottlieb has shown, has as much biochemical holarctic members of the same genera. Exam- variability in it in terms of enzyme alleles as ples are the Weeping Spruce {Picea brewe- Clarkia ruhicunda, which is much more riana), which is narrowly restricted to this widespread throughout the San Francisco area. The Sadler Oak {Qiiercus sadleriana) is Peninsula. I now reduce the category of spe- related to the Chestnut Oaks of the eastern cies that are rare because of restricted gene United States, and is restricted to the Sis- pool to a rather small one consisting of spe- kivoii Mountain area. The Port Orford Cy- cies that have not only become inbred be- press (Cliamaecijparis lawsoniana) is in this cause of small population size, but, in addi- area. It contains several endemic species of tion, because of the shift from cross- mesic genera like Vancouveria and some gen- fertilization to self-fertilization or pre- era of the saxifrage family. dominant fertilization. Again, Leslie Gottlieb In the southern area occur rare, endemic has shown with his work on enzyme varia- species like Hesperocallis of the Lily family, bility that in eastern Oregon there is a wide- the jojoba shrub, Simmondsia, not particu- spread species of the composite genus Steph- larly rare but certainly relictual. anomeria {Stephanomeria coronaria), which Bv contrast, central California contains en- has an enormous amount of genetic varia- demic species, which seem to be new like bility, but right next to it there is a very re- those of ArctostapJnjlos or Manzanita, al- stricted species along Malheur Lake in east- ready discussed, and other annuals. Because em Oregon, Stephanomeria malheurensis, central California has received the greatest which has very little variability. One differ- disturbance in relatively recent times, one ence between the species is that S. coronaria can get some idea of whether a species is re- is largely outcrossed and S. malheurensis is lictual or not, both by its taxonomic affinities almost completely inbred. The inbreeding as and by the geographic areas in which it much as the restriction in size of the popu- grows. lation and habitat was responsible for the re- An example of an herbaceous species from duction in size of the gene pool. The same the northern relictual area is Darlingtonia situation exists in some species of animals like caUfornica. It does not have official preserva- the burrowing rodents of the Middle East, tion that I know of in California. It should, studied by several Israeli zoologists, and cer- because it is an attractive species, an insect- tain fishes in Mexican caves which have very digesting pitcher plant. Every high school bi- restricted gene pools. One that is rather ology teacher wants one in the classroom, widespread, the southern alligator, apparent- and people also like them in their homes. ly has extremely low variability in its gene

There is danger to them from vandalism. I pool. The whole hypothesis that there is a sometimes blush when I go up the Oregon strong correlation between rarities and en- Coast Highway and see that Oregon has a dangeredness and the size of the gene pool preserved area of Darlingtonia labeled as must be greatly modified, if not entirely re- such, while California doesn't. jected. In my own thinking, I am substituting The next group of rare and endangered the concept of ecological traps. Plant species species are of an entirely different nature. may be hemmed in by environments that are Here, I want to state publicly that one of my so different from the ones in which they articles written in 1942, and often quoted, grow that they do not have the genetic po- apparently is not correct. In 1942, I made the tential to colonize those habitats. Sometimes speculation that rare and restricted species the "traps" are quite clearly defined so that I would usually be so because of restricted gen- call them ecological islands. Usually the soil 116 Great Basin Naturalist Memoirs No. 3 is so different on the islands, relative to that pine seedlings invading a forest of native Eu- of the surrounding islands, that the plants are calyptus. as if they were on an oceanic island sur- A well-known ecological island exists in rounded by a sea of unfavorable soil condi- the Sierra foothills of California, near the tions. Such an island is Pine Hill, 25 miles town of lone. It is based on a hard pan soil intru- east of Sacramento. It consists of basic which is of Eocene age, about 40 million sives, a very distinctive type of rock, sur- years old. It was a sea-beach terrace facing rounded by the various metamorphic rocks the Pacific Ocean, which at that time cov- in Sierra foot- commonly found the Nevada ered all of the present central valley. It is hills. Interestingly enough, this particular is- dominated in many areas by an olive-colored land is believed to have once been an oceanic shrub, Arctostoplujios mijiiifolia, the lone island. Dr. Eldredge Moore of our geology manzanita. There is a margin of the common department at Davis, in association with the gray manzanita, Arctostoplujios viscida, and new theory of Plate Tectonics, points out interior live oak, but most of the area sur- that basic intrusives are associated with the rounding it is grassland consisting of in- roots of volcanos. The situation could be ex- troduced annual grasses and scattered blue plained if the rocks now exposed at Pine Hill oaks and digger pines that are the normal were the roots of an ancient submarine vol- dominant vegetation of the area. The plants cano that arose in the Pacific Ocean to the that are rare and endangered are held in west of what then was the seashore, but is check by the very special ecological condi- now the eastern edge of the central valley. tions that prevail here. There is really an is- Because of cnistal movements, this ancient is- land within an island because a buckwheat, land jammed itself against the older rocks. Eriogonum apricum, which is confined to the A striking endemic on Pine Hill is Fe- lone manzanita island, grows only in the few montodendron californicum ssp. decumhens, most barren parts of it. a prostrate shrub that bears highly distinctive The gene pool of E. apricum is most inter- copper-colored flowers. esting. There are three patches of it. The

Another ecological island is in the area by maximum distance from one to another is

Monterey and Pacific Grove. There are two about 10 miles and there is one about half- species of rare endemic cypresses of the area. way in between. Rod Myatt of UC Davis did Neither of them is in danger now, because a master's study on morphological variability both are preserved. One is the well-known and found that the different patches can be Monterey cypress {Ciipres.sus macrocwpa), distinguished morphologically. They are found only on the granite ledges near the races. There is not only a lot of variation shore. The other is the Gowen cypress, found within each of these patches, but distinctive only in the hard pan of the raised beach, in differences between patches. In other words, the interior of the peninsula. it appears as if the lone manzanita within its Monterey pine is confined to the Monterey area of 10 miles has as much morphological ecological island plus two others in Califor- variability as does another buckwheat, nia, one 50 miles north, the other 120 miles Eriogonum nudum, within an area of equal south. There is evidence that it is not restrict- size, 10 miles in diameter in the Sierra foot- ed in its colonizing ability, if it has the right hills. The difference is that Eriogonum nu- conditions, so that it could get out of its eco- dum, one of the most common buckwheats in logical trap. This is evident from what the California, has a multitude of races that are Monterey pine has done in the southern hem- adapted to all sorts of climatic conditions isphere, in Chile, New Zealand, and Austra- over this extensive area. Another fact is that lia. In all three regions, extensive forests of E. nudum is a much bigger species and its this species have been planted that in many seeds are much bigger. Its seedlings are prob- places look quite natural. Some trees are far ably much more competitive so that it could taller than those in California, reaching colonize new areas more easily than could heights of 100 to 150 feet. Eriogonum apricum. Perhaps E. apricum is a Most interesting of all, in the vicinity of relic of the days when annuals, even native Canberra, Australia, I have seen Monterey annuals, were much fewer than they are now. 1979 The Endangered Species: A Symposium 117

It may have colonized the barren places that Now, why do these rare plants grow here? no other perennials could live in, when there It isn't because they are lime-loving calco- wasn't so much annual competition. That's philes. The Kobresia grows on Mount Evans pure speculation. This again emphasizes the in Colorado which is acidic granite. Certainly ecological entrapment which I believe is the Scirpus rollandii is to a certain extent a cal- basis for imderstanding the distribution of ceophile, but it doesn't seem as though the most of our rare and endangered species, in- limestone is the basic reason. The other fac- dependent of age and independent of quan- tor is this—limestone is porous. Because it titative sizes of gene pools. holds water, on a steep slope like the one which supports the rare plants, water oozes The final example is similar. It is Convict out from the ground throughout the summer. Canyon in the eastern Sierra Nevadas. It is Remember that the Sierra Nevada, in con- distinctive, because while most of the Sierra trast to the Rocky Mountains or the Wasatch is either granitic or ancient acidic crystalline Range, has very few summer storms. They ex- volcanics of Mesozoic age or earlier. Convict ist, but they're small and most of them hardly Canyon has a vein of limestone running wet the ground. Much of the sierran area in through it. It is the only sizable part of the the well-drained slopes becomes very dry in Sierra Nevada that has limestone in associ- the summer, and the mesic plants have to ation with alpine or subalpine conditions. grow where they get heavy snowfall during Mount Baldwin, 12,000 feet high, supports the winter. The rare plants, however, grow a rare rock cress, Draba nivalis, and is the on a bench area that gets relatively less snow in the Sierra Nevada. The near- only locality during the winter which is constantly coming it, as I are the est to as far know, Ruby out of the limestone formation. This, I be- in eastern Nevada. Mountains lieve is responsible for the unusual nature of One of the most remarkable plants, how- the environment. ever, in this area, is a willow which is related My final remark has become obvious from to a Rocky Mountain species. Salix brachyo- what I've had to say. Ecological genetics is a carpa. Another species is a relative of the relatively new field. The combination be- sedges, perhaps ancestral to the genus carex tween studies of gene pools and the genetics Kobresia mijosuroides and an extremely local- of adaptation, I predict, is a field which is ized dwarf bullrush, Scirpus rollandii, some- just beginning to emerge and will be explo- times put in the species Scirpus pumilus, sive in the next half century. Young scientists which has been the subject of interest for arc- who are interested in native environments tic alpine botanists for many years. The two and wish to study them in depth from an ana- discoverers of these species were Jack Major lytical point of view will have an exciting ca- and Sam Bamberg. reer of discovery ahead of them.