Before middle Miocene times, Baja California was attached to the rest of the North American continent. Consequently, most of the terrestrial fauna and flora of the peninsula had its origins in mainland Mexico. However, the separation of the peninsula and its northwestward displacement resulted in a variety of distribution patterns, isolations, extinctions, origins and ultimate evolution of fauna and flora in several ways. The islands in the Gulf of California have been colonized by species from Baja California and mainland Mexico. Some workers (Soulé and Sloan 1966; Wilcox 1978) consider that many of these islands originated as landbridges. Geographically, most of the islands are closer to the peninsula than to the mainland. Therefore, it has been assumed that the Baja California Peninsula was the origin of most of the organisms inhabiting them (Murphy 1983). Islands separated by depths of 110 m or less from the peninsula or mainland Mexico apparently owe their current insular existence to a rise in sea level during the current interglacial period (Soulé and Sloan 1966). In contrast, little information exists for deep-water islands. Any complete analysis of the distribution and origin of several organic groups inhabiting the Gulf of California islands should involve the consideration of several contrasting models arguing in favor of or against the equilibrium theory (MacArthur and Wilson 1967). In any model, one of the most important features to consider is the relationship between the species inhabiting the gulf islands and the physical and geological processes of formation of the islands, as well as their age, size, and distance from either the peninsula or the mainland. Understanding colonization, migration, and distribution, particularly in some groups, requires information on whether a particular island was ever connected to a continental source. For example, to explain some characteristics of the populations of any island, which presumably had a recent (<10,000-15,000 years) connection to a continental source, it is necessary to evaluate the coastal erosion or the relative rise in the sea level. These factors might contribute to effectively isolating an insular habit or to forming landbridges.
ORIGIN OF GLOBAL DEEP FAULTS AND THEIR SIGNIFICANCE IN EARTH DEGASIFICATION