This interesting genus began its taxonomic history with the publication by Gleason in 1931 of four species, all collected on (Jerro Duida, viz. Tyleria floribunda, T. grandiflora, T. linearis, and T. spathulata. Only the last has been found elsewhere, being a prominent element of the cumbre flora of Cerro Huachamacari, a sandstone mountain that lies to the north of and adjacent to Cerro Duida, and on Serrania Paru, which lies some 150 k m to the north. Our more recent visits to Cerro de la Neblina, more than 300 k m to the south of Duida, have resulted in the discovery of four additional species. It is not to be unexpected that further species of Tyleria will be found as the great cerros of Neblina and Duida and the immediately associated tabular mountains as yet unvisited by botanists become more fully explored or initially visited.
For those presently known, a curious relationship develops. The four newly discovered species of Neblina stand as morphologic and geographic analogues of their Duida congeners; the eight species thus stand as four mutually bracketed and related pairs.
Tyleria floribunda of Duida and T. spectabilis of Neblina bear overwhelming morphologic evidence of immediate affinity in habit. In form of leaf and inflorescence they are exceedingly similar, differing largely in leaf apex and margin and critical characters of the flower. T. floribunda, the largest tree of the genus on Duida, is strikingly fastigiate. T. spectabilis, the largest tree of the genus on Neblina, is fastigiate-candelabriform.
Tyleria pendula is certainly the analogue of T. spathulata, the inflorescence of the first being pendulous and more nearly racemiform, and the leaves more strongly petiolate and strongly aristate. The two occupy comparable ecological niches in their respective geographical areas.
Tyleria linearis of Duida and T. aristata of Neblina are botli wiry-stemmed shrubs of closely similar habit, forming dense thickets along water-courses. Their flowers are solitary and axillary; yet the leaves of T. linearis are, as the name indicates, narrowly linear and sessile and quite devoid of an arista, while the leaves of its Neblina counterpart are elliptic, petiolate, and strongly aristate. The fourth pair, Tyleria grandiflora and T. tremuloidea, are less obvious analogues, yet they are probably most nearly interrelated between themselves than to other species of the genus.
It is thus most interesting to postulate a parallel evolution of each of the pairs from a common forerunner, or the segregation of one of each set directly from its analogue. If such a postulation holds, and it is abundantly supported by morphologic analogy, it is exceedingly difficult to give basis to genetic explanation for the presence of the strong aristate leaf-tip that obtains absolutely for the Neblina series, and is absolutely absent from the Duida series. This character, certainly of genetic significance, cuts directly across otherwise clear lines of affinity of the eight species so involved. An alternative explanation is the evolution from two ancestors, with parallel adaptation to similar habitats.