Relative to common chimpanzees, bonobos live in a more restricted geographic range and are less divided than chimpanzees, who are divided into at least three well recognized subspecies. The area of suitable habitat available to bonobos may always have been smaller than that of their congeners to the north, especially during periodic contractions of the tropical forests of equatorial Africa, when much of the forest cover south of the Congo River was substantially reduced.
Perhaps during these times, the bonobo populations passed through, bottle necks, resulting in the smaller effective population size (Nₑ). There could be little, if any social or reproductive factors that could give chimpanzees a larger Nₑ than bonobos: bonobos have a more equal adult sex ratio, and chimpanzee females are not more promiscuous than bonobo females.
In contrast, there are many social and ecological differences between chimpanzees and gorillas that may result in lower Nₑ of gorillas. Gorillas live in smaller social groups, which contain one or two adult males who monopolize access to several females in the group, whereas chimpanzee females mate promiscuously within the social group. Species with strong polygyny, will have reduced male Nₑ, and therefore a lower overall Nₑ.
In addition, compared to chimpanzees, gorillas live in a more restricted habitat. Thus, gorillas have more limited ecological tolerance than chimpanzees: and past environmental changes have sent them through several bottle necks, resulting in lower Nₑ.
Bonobos passed through genetic drift because:
Correct Answer: A. of the periodic reduction of their habitat
Solution:Bonobos passed through genetic drift primarily because of the periodic reduction of their habitat. The contraction of the tropical forests of equatorial Africa, particularly during environmental changes, reduced the habitat available to bonobos south of the Congo River.This reduction likely forced bonobo populations into smaller, isolated groups, leading to genetic bottlenecks. These bottlenecks, in turn, decreased the genetic diversity within the population, causing drift in genetic characteristics unrelated to reproductive advantages or natural selection.