When the same habitat types (forests, oceans, grasslands etc.) in regions of different latitudes are compared, it becomes apparent that the overall number of species increases from pole to equator. This latitudinal gradient is probably even more pronounced than current records indicate, since researchers believe that most undiscovered species live in the tropics.
One hypothesis to explain this phenomenon, the “time theory” holds that diverse species adapted to today’s climatic conditions have had more time to emerge in the tropical regions, which, unlike the temperate and arctic zones, have been unaffected by a succession of ice ages. However, ice ages have caused less disruption in some temperate regions than in others and have not interrupted arctic conditions.
Alternatively, the species-energy hypothesis proposes the following positive correlations: incoming energy from the Sun correlated with rates of growth and reproduction; rates of growth and reproduction with the amount of living matter (biomass) at a given moment; and the amount of biomass with number of species. However, since organisms may die rapidly, high production rates can exist with low biomass. And high biomass can exist with few species. Moreover, the mechanism proposed—greater energy influx leading to bigger populations, thereby lowering the probability of local extinction—remains untested.
A third hypothesis centers on the tropics’ climatic stability, which provides a more reliable supply of resources. Species can thus survive even with few types of food, and competing species can tolerate greater overlap between their respective niches. Both capabilities enable more species to exist on the same resources. However, the ecology of local communities cannot account for the origin of the latitudinal gradient. Localized ecological processes such as competition do not generate regional pools of species, and it is the total number of species available regionally for colonizing any particular area that makes the difference between, for example, a forest at the equator and one at higher latitude.
A fourth and most plausible hypothesis focuses on regional speciation, and in particular on rates of speciation and extinction. According to this hypothesis, if speciation rates become higher toward the tropics, and are not negated by extinction rates, then the latitudinal gradient would result—and become increasingly steep.
The mechanism for this rate-of-speciation hypothesis is that most new animal species, and perhaps plant species, arise because a population subgroup becomes isolated. This subgroup evolves differently and eventually cannot interbreed with members of the original population. The uneven spread of a species over a large geographic area promotes this mechanism: at the edges, small populations spread out and form isolated groups. Since subgroups in an arctic environment are more likely to face extinction than those in the tropics, the latter are more likely to survive long enough to adapt to local conditions and ultimately become new species.
15. Which one of the following most accurately expressed the main idea of the passage?
(A) At present, no single hypothesis explaining the latitudinal gradient in numbers of species is more widely accepted than any other.
(B) The tropical climate is more conductive to promoting species diversity than are arctic or temperate climates.
(C) Several explanations have been suggested for global patterns in species distribution, but a hypothesis involving rates of speciation seems most promising.
(D) Despite their differences, the various hypotheses regarding a latitudinal gradient in species diversity concur in prediction that the gradient can be expected to increase.
(E) In distinguishing among the current hypotheses for distribution of species, the most important criterion is whether a hypothesis proposes a mechanism that can be tested and validated.
16. Which one of the following situations is most consistent with the species-energy hypothesis as described in the passage?
(A) The many plants in a large agricultural tract represent a limited range of species.
(B) An animal species experiences a death rate almost as rapid as its rate of growth and reproduction.
(C) Within the small number of living organisms in a desert habitat, many different species are represented.
(D) In a tropical rain forest, a species with a large population is found to exhibit instances of local extinction.
(E) In an arctic tundra, the plants and animals exhibit a slow rate of growth and reproduction.
17. As presented in the passage, the principles of the time theory most strongly support which one of the following predictions?
(A) In the absence of additional ice ages, the number of species at high latitudes could eventually increase significantly.
(B) No future ice ages are likely to change the climatic conditions that currently characterize temperate regions.
(C) If no further ice ages occur, climatic conditions at high latitudes might eventually resemble those at today’s tropical latitudes.
(D) Researchers will continue to find many more new species in the tropics than in the arctic and temperate zones.
(E) Future ice ages are likely to interrupt the climatic conditions that now characterize high-latitude regions.
18. Which one of the following, if true, most clearly weakens the rate-of-speciation hypothesis as it is described in the passage?
(A) A remote subgroup of a tropical species is reunited with the original population and proves unable to interbreed with members of this original population.
(B) Investigation of a small area of a tropical rain forest reveals that many competing species are able to coexist on the same range of resources.
(C) A correlation between higher energy influx, larger populations and lower probability of local extinction is definitively established.
(D) Researchers find more undiscovered species during an investigation of an arctic region than they had anticipated.
(E) Most of the isolated subgroups of mammalian life within a tropical zone are found to experience rapid extinction.
19. Which one of the following inferences about the biological characteristics of a temperate-zone grassland is most strongly supported by the passage?
(A) It has more different species than does a tropical-zone forest.
(B) Its climatic conditions have been severely interrupted in the past by succession of ice ages.
(C) If it has a large amount of biomass, it also has a large number of different species.
(D) It has a larger regional pool of species than does an arctic grassland.
(E) If population groups become isolated at its edges, they are likely to adapt to local conditions and become new species.
20. With which one of the following statements concerning possible explanations for the latitudinal gradient in number of species would the author be most likely to agree?
(A) The time theory is the least plausible of proposed hypotheses, since it does not correctly assess the impact of ice ages upon tropical conditions.
(B) The rate-of-speciation hypothesis addresses a principal objection to the climatic-stability hypothesis.
(C) The major objection to the time theory is that it does not accurately reflect the degree to which the latitudinal gradient exists, especially when undiscovered species are taken into account.
(D) Despite the claims of the species-energy hypothesis, a high rate of biological growth and reproduction is more likely to exist with low biomass than with high biomass.
(E) An important advantage of the rate-of-speciation theory is that it considers species competition in a regional rather than local context.