Most species of Nolana are narrow endemics, with small, restricted geographic ranges and specific ecological requirements. Only one species, Nolana galapagensis (Christoph.) I.M.Johnst., has a distribution outside of continental South America. The Galápagos Islands are located on the Equator, ca. 500 miles (~840 kms) west of mainland Ecuador (Fig. 1). N. galapagensis is recorded from ocean on six islands within the Galápagos Island chain (Fig. 2): Isabela, San Cristobal, Santa Cruz, Santa María, Seymour, and Tortuga (Wiggins & Porter, 1971; Eliasson, 1970; A. Tye, pers.). The plants are found in near beach localities within a few meters of the ocean (Fig. 3).
Figure 1. Galápagos Islands are separated from mainland Ecuador by over 500 miles or 840 kms of Pacific Ocean.
|Figure 2. Nolana galapagensis population distributions [+}|
|Figure 3. Nolana galapagensis A. Habit. B. Flowers. (click photo for enlargement)|
| Our preliminary divergence
data allows the calculation of a range of potential dates for the arrival
of Nolana within the Galapagos Islands. In the ITS analysis, divergence
values for N. galapagensis approach those of the outgroup taxon,
Grabowskia. This high level of sequence divergence may be the product
of long-term isolation on islands from continental relatives or some other
phenomenon such as founder effect or genetic drift. Reported values suggest
an average rate for ITS calculated at 5.20 x 10-9 per site per year (Tago,
1999). If this rate is adopted for Nolana, the divergence time of
Nolana from the closest taxon in the Solanaceae is estimated to be
11.6 million years ago (Fig. 6). Further, ITS sequence data suggests that
the divergence time of N. galapagensis from the rest of Nolana
species is ca. 8.1 million years ago. In comparison, the nucleotide substitution
in the coding region of the matK gene provided an average substitution
rate of 4.10 x 10-10, with the divergence time of Nolana from the
closest taxon in the Solanaceae is estimated at 11.0 million years ago.
With matK sequence data, the divergence time of N. galapagensis
from the rest of Nolana is estimated to be 4.0 million years ago.
Geological evidence points to an age for the current Galápagos archipelago
at 4-5 million years, but underwater seamounts may date to 15-20 million
years (Grehan, 2001). Molecular clock calibrations for determining colonization
events may provide estimates that exceed the age of the modern islands.