by James E. Low
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ECHEVERIA VS. DUDLEYA, sorting out the confusion:
Plants in genus Dudleya closely resemble many Echeveria species. Both do, indeed, belong to the same family. Dudleyas are common in parts of Mexico too; so most people assume that they must be very closely related as they look so much alike. Oddly enough, this is not the case. First, the 2 ranges are not really that close together; overlapping only just a bit, if at all. Thus, rarely would plants of these two genera ever grow near each other in nature.
The Dudleyas are mostly from the Pacific coast. This includes Baja Calif. (where not a single Echeveria species is found native). It continues northward over the entire Calif. coast within the USA and a bit north into Oregon. These Dudleya habitats are places where no Echeveria species are native. The Dudleya range goes inland to the western foothills of the Sierra Nevada Mtns. of Calif. and a few are native as far east as central Arizona, and southern Nevada. Maybe a few are found on south into Sonora state of mainland Mexico. These places constitute the total range for Dudleya.
The most important factor which proves that the relationship is distant concerns compatibility. Hybrids have been attempted using species from these 2 genera by some of the best horticulturists in the world ever since the two genera have been known. None have ever been known to succeed. It must be concluded that species in these 2 genera are totally infertile to each other. Why? Both are probably descended from ancient Sedum parents, but it seems that each genus came from different Sedum parents, and by chance these two ancient Sedums were not closely related to start with. The apparent similarity seems to be a classic case of “parallel evolution”.
The chromosome counts of species within Dudleya are based on n=17. Several species have multiples of this; the “polyploid forms”. These same counts are almost unknown in the Echeverias, except for one species, and even it is not related. The internal DNA code sequence is basically different between all species of these 2 genera. Detached leaves of Dudleyas never grow new plants. Detached leaves of all Echeverias easily grow 2 new plants each, so this also is a very basic difference. Prof. Uhl reports that it appears the Dudleyas originated on a different tectonic plate than that of mainland Mexico where the Echeverias originated. At the time of evolution of the Dudleya genus from its Sedum parent these plates may have been in a far different location than where we know them to be today. Thus, the similarity of appearance seems to be more a matter of mere chance than of any true relationship. This seems to be a classic case of "parallel evolution".
More recently (1995) botanist Henk ‘t Hart of Holland has determined that there are 2 main lines of evolution of European Sedums called the genuina types. Mexican Sedums merely continue both of these two lines. They got to the New World via “continental drift” and Plate Tectonics. This is a relatively new theory about 50 years old, but it is so compelling based on collected evidence that it has become almost universally accepted, unusual for a radical new theory. This states that all continents were once together, and later they slowly split apart. Both of these 2 Sedum lines thus crossed over the Atlantic Ocean riding on the American plates. They did this simply because when the Americas separated from Europe, the western part already had species of both lines of Sedums growing on it. The Atlantic Ocean came into existence between these 2 parts, so the American species were riding on that section as if on a slow ship. Thus, some of both types ended up far to the west, especially in the land that would become Mexico. The breaking apart of these two lands is believed to have occurred about 200 million years ago. Thus, any new genera that descended from of these 2 ancient lines in the New World would still be essentially unrelated to each other because they were unrelated even before this split occurred. Mr. Henk ‘t Hart used a new technique called “DNA sequencing” which involves difficult technical work using the electron microscope to decipher parts of the code. When applied to the Echeverias and the Dudleyas this method did indeed prove that these two genera evolved from these early and unrelated Sedum parents. One genus came from one ancient line, and the other from the other line of Sedum evolution. Such lines are called “Clades”. For example, in Europe two very similar looking but common Sedums were found to fall into these 2 different and totally unrelated Clades. These are Sedum acre (yellow flowering), and Sedum anglicum (white flowering). The very misleading similarity of appearance between the Dudleyas and the Echeverias shows just how tricky botany can sometimes be. Probably, if the electron microscopes had not been invented we would still have little or no idea of these facts even today.
Next: Some personnal comments about this listing and personal commentary.