Final answer:
The most important factor for explaining the equal numbers of snapping shrimp species on either side of the isthmus is the relative shortness of time they have been separated. Other factors like the depth of the ocean, number of depth habitats, and the elevation of the isthmus are less directly related to the speciation rate in this context.
Step-by-step explanation:
When considering why there are equal numbers of snapping shrimp species on either side of the isthmus, it is important to understand the roles of environmental conditions in affecting speciation rates. A significant factor explaining this phenomenon is the relative shortness of time these species have been separated. The primary impact of the isthmus is the environmental barriers it creates, leading to separate populations that could diverge into different species. However, the shortness of time might have been insufficient for significant speciation to occur. Option number 1 is correct.
When populations are separated abruptly, such as due to the rise of an isthmus, a period of isolation could lead to new environmental conditions and thus, new selective pressures. Over time, this can lead to speciation, where new species emerge that are adapted to their particular environments. However, if not enough time has passed, the genetic differentiation between the populations may not be significant enough to result in different species on each side.
Other factors mentioned, such as the depth of the ocean, number of depth habitats, and elevation of the isthmus above sea level, while they may affect biodiversity or species distribution to some extent, are not as directly related to the short-term balance of species numbers on either side of the isthmus.