For biologists, the term “eye” describes any light sensitive organ

For biologists, the term “eye” describes any lightsensitive organ consisting of more than one cell. Although most animals have eyes, eye structures vary widely. The compound eyes of insects and other arthropods, for example, have an architecture strikingly different from the single-lens eyes of vertebrates and mollusks. Until recently, most biologists believed that all the different kinds of evolved independently from as many as forty ancestral prototypes, and not from a single ancestral prototype eye.

Traditional means of tracking the evolutionary development of eyes included examinations of internal eye structures, which tended to support the multiple origin theory despite some anomalies such as the resemblance between mammals’ eyes and the eyes of the nautilus mollusk—animals that are not closely related. Proponents of the multiple origin theory dismissed such examples as textbook cases of evolutionary convergence: the idea that even strikingly different prototype eyes could evolve into kinds of eyes remarkably similar to each other.

In support of their theory, these biologists point to the fact that different species inhabiting the same environment frequently have very different eye structures from one another. This lack of correlation between eye structures and physical environments has provided what is perhaps the strongest support for the multiple origin theory. Adherents of the theory argue that if eyes originated from a single ancestral prototype, then there should be similarity in the patterns of eye evolution in species that evolved under the same environmental conditions. The inability of biologists to identify such evolutionary patterns is arguably the primary reason for this theory’s widespread acceptance.

In 1993, however, a crucial link was discovered: a control gene that activates the many genes needed for complete eye formation in fruit flies. Analogues to this gene have since been identified in many organisms, including earthworms, mice, and humans, and are expected to exist in all eye-bearing organisms. Researchers discovered that inserting the control gene present in mice into fruit flies results in the formation of functional fruit fly eyes. This suggests that the control genes in mice and in fruit flies are interchangeable and hence evolved from a single, common ancestral gene. The same may be true of all of these control genes, which would argue convincingly against the multiple origin theory and call for a reevaluation of the evidence that seemed to support the theory. For example, the lack of correlation between eye structures and physical environments may have resulted from the advantage bestowed by eyes. The survival value of sight is perhaps so great that even variations in eyes that might be less than optimal for some individuals in a particular environment are sufficiently advantageous that they allow the individuals to survive and propagate the variation, thus facilitating the proliferation of variations in eyes even in the absence of an environmental difference.

1. The author mentions that biologists accepted which one of the following as evidence for the theory that eyes evolved from multiple origins?

(A) the lack of fossil evidence of a common ancestor for all eye-bearing species
(B) the lack of correlation between eye structures and physical environments
(C) the lack of correlation between physical environments and control genes for eye development
(D) the resemblance between the eyes of mammals and the nautilus mollusk, species that are not closely related
(E) the obvious evolutionary advantage bestowed by some kinds of eyes as compared with others



2. The primary purpose of the passage is to

(A) distinguish between two theories and explain the theoretical basis for each of those theories
(B) argue that a particular discovery provides insufficient evidence for the rejection of a particular widely accepted theory
(C) explain how a particular piece of evidence challenges a particular theory that has been widely accepted
(D) provide grounds for a reexamination of the assumptions underlying a recent challenge to a commonly held theory
(E) suggest some practical implications of a particular theoretical finding that conflicts with a particular commonly held theory



3. The passage provides the strongest support for the inference that the author’s attitude regarding the discovery of a control gene responsible for activating eye formation in fruit flies can be accurately described as

(A) concern that biologists have accepted the single origin theory without carefully evaluating the assumptions it is based on
(B) concern that biologists may have prematurely abandoned the multiple origin theory
(C) confidence that the discovery will imminently lead to the complete abandonment of the multiple origin theory
(D) optimism that its discovery might foster a reevaluation of the assumptions biologists utilize when researching evolution in general
(E) anticipation that the discovery will foster further efforts to determine whether all varieties of eyes have evolved from a single ancestral prototype eye



4. According to the author, control genes have been found that
(A) determine how each species’ characteristic eye structure will differ from that of other species
(B) probably occur in all animals but only activate eye formation in just a few
(C) may be interchangeable among mice, fruit flies, and humans without causing members of those species to develop eyes that are atypical for their species
(D) regulate specific aspects of eye evolution in certain species, suggesting that those species need not be assumed to stem from a common ancestor
(E) activate eye formation in species including humans and mice, and this helps explain how evolutionary convergence occurs



5. Proponents of the multiple origin theory would likely regard the relationship between the development of human eyes and the development of the eyes of the nautilus mollusk as most analogous to which one of the following?

(A) Traditional Italian pizza is very similar in form and ingredients to a traditional Southern French food, which, like Italian pizza, has its origins in earlier Mediterranean cultures.
(B) Mexican traditional cuisine uses round, flat tortillas made from corn or wheat, and traditional cuisines of India use tortilla-like flat, round bread made from wheat flour, even though the Mexican and Indian cultures have no traditional connection with each other.
(C) Yogurt is superficially unlike cheese, although both are made almost entirely of milk and both are traditional ingredients in Middle Eastern and European cuisines.
(D) Mozzarella cheese is traditionally made from cow’s milk in the United States, even though Italian mozzarella, which is the original model for the US version and has almost the same flavor, color, and texture, is traditionally made from buffalo’s milk.
(E) The culinary use of corn—a vegetable that was originally available only to various Native American cultures—has spread to many cultures throughout the world, some of which are very distant and different from one another.