How do species like the whiptail lizards overcome the obstacles of genetic monotony and disease vulnerability if they cannot exchange genetic material? Unless an animal can recombine the DNA they already have, they will produce an offspring with an identical set of chromosomes, in which any genetic weakness would have no chance to be overridden by outside genetic material from a mate.

Research reveals that these lizards maintain genetic richness by starting the reproductive process with twice the number of chromosomes as their sexually reproducing cousins. These celibate species resulted from the hybridization of different sexual species, a process that instilled the parthenogenetic lizards with a great amount of genetic diversity at the outset. Researchers found that these species could maintain the diversity by never pairing their homologous chromosomes (as sexual species do by taking one set of chromosomes from each parent) but rather by combining their sister chromosomes instead. This discovery means that these lizards have a way of distinguishing sister from homologous chromosomes, but how they do this is something now being investigated.

Indonesian dragons can reproduce both sexually and parthenogenetically; and parthenogenesis may also occur naturally when males and females are both present. This ability probably resulted from the dragon's isolated habitat, living on islands in the Indonesian archipelago. It may have enabled the dragons to establish new colonies if females had found themselves washed up alone on neighboring shores following a storm. A case has been documented of a Komodo dragon switching back to sexual reproduction after a known parthenogenetic event. A single female could theoretically have male offspring asexually, then switch to sexual reproduction with them to maintain a higher level of genetic diversity.

Because the genetics of sex determination in Komodo Dragons uses the ZW system (where ZW is female, ZZ is male, and WW is inviable) the offspring of asexual breeding will be ZZ (male) or WW (inviable), with no ZW females being born. The wild Komodo dragon population is approximately 75 percent male.

Evidently, in the case of these Komodos, the doubling of the egg genes occurred when, in essence, another egg, rather than sperm, did the job of fertilization. Oogenesis, the biological process of making an egg cell, typically also yields a polar body--a mini ovum of sorts, containing a duplicate copy of egg DNA. Normally, this polar body shrivels up and disappears. In the case of the Komodos, though, polar bodies evidently acted as sperm and turned ova into embryos.

The main idea of a passage is the largest, most comprehensive idea that is true for the passage. The ideas expressed in all of the other answer choices are both correct and they are presented in the passage. What determines that answer B expresses the passage's main idea is the large scope of the idea, that the subject of the passage is various aspects of asexual reproduction.

The correct answer is 2.

Once again, the following quote from the end of paragraph one provides an overall explanation of why asexual reproduction evolved: "Other advantages of asexual reproduction include the ability to reproduce without a partner in situations where the population density is low, which reduces the chance of finding a mate (some desert lizards), or during colonization of isolated habitats such as oceanic islands, where a single (female) member of the species is enough to start a population (the Komodo dragon)."

The correct answer is 3.