Final answer:
Closely related species may differ in their developmental pathways primarily due to variations in their environment, which affects gene expression and subsequently the phenotype.
Step-by-step explanation:
Closely related species with the same set of developments may take very different developmental pathways due to differences in environment. While genes provide the basic blueprint for an organism's development, it is ultimately the interaction between genes and the environment that shapes the developmental process. For instance, environmental factors such as temperature, nutrition, and exposure to sunlight can lead to variations in phenotypes, even between closely related species or individuals. This is seen in the case of temperature-dependent sex determination in turtles and the impact of famine on fetal development.
Different environmental conditions can also cause variations within a species by affecting gene expression. The same genes may be expressed differently in response to varying environmental cues, leading to tangible differences in individual development and phenotype. Understanding the dynamic interplay between genetic information and environmental influence helps explain the diversity observed in the natural world.