Final answer:
Guppies increased their age at maturity in response to predation on juveniles after being moved to a different stream. Research by John Endler demonstrates the significant genetic variability within guppy populations and their adaptability to environmental changes. These findings highlight the importance of genetic diversity for the stability of ecosystems and the evolutionary trade-offs that species may face.
Step-by-step explanation:
When guppies were moved from stream A, where a fish preyed upon adult guppies, to stream B, with a fish that preyed upon juvenile guppies, their evolutionary response after 30 to 60 generations to the increased predation on juveniles was to B) increase the age at maturity. This change is an adaptive response to predators that target the juvenile life stage, as it can lead to more juveniles reaching maturity before being preyed upon and thus, can improve the chances of reproduction. The concept that predators can exert natural selection and affect the life history traits of their prey is supported by the research done by John Endler on Trinidad guppies. Endler showed that guppy populations could rapidly adapt to changes in predation pressure. This evolutionary flexibility suggests that there is significant genetic variability in guppy populations, allowing them to adapt to new environmental conditions.
The question asked also touches upon the idea of trade-offs in evolution. The selection for less conspicuous colors in the presence of predators versus the preference for more conspicuous colors due to sexual selection by female guppies exemplifies this concept. A change in female preference to larger size over brighter coloration could shift the selection pressures, again affecting the population's traits.
Understanding how genetic variation influences a species' adaptability and its role within an ecosystem can provide insights into the importance of maintaining genetic diversity for the stability of an ecosystem. Such knowledge could have implications for conservation strategies and management of natural resources, emphasizing the role of phenotypic and genetic diversity in enabling population resilience and adaptation.