Final answer:
The plateau in effectiveness after several generations of artificial selective breeding is known as a selection plateau. It occurs because genetic variability diminishes or further enhancement of the trait negatively impacts survival, leading to limited gains from continued selection.
Step-by-step explanation:
After several generations of artificial selective breeding, a plateau is reached where artificial selection is no longer effective. This is called a selection plateau. During the process of artificial selection, humans select individuals with desirable traits to reproduce, passing on specific genes to the offspring. Over time, the variability of the trait in question may decrease, reaching a point where further selection does not result in any significant improvement because there is limited genetic diversity left to act upon or because the trait has been optimized to the extent that further enhancement would negatively impact other important traits, such as survival.
This situation is analogous to natural selection, where traits can proceed to a point at which natural selection against a character's further enhancement prevents its further evolution. For example, overly colorful feathers in birds may make males more susceptible to predation, limiting the evolution of even brighter plumage despite it being favorable for mating purposes.
Furthermore, if artificial selection results in a loss of wild species genetic diversity, it can cause inbreeding depression, which necessitates breeding with wild populations to maintain viability. This is an issue as genetic diversity is crucial for the long-term health and adaptability of a species.