Final answer:
Anatomical and molecular features typically present homologous structures due to divergent evolution from a common ancestor. However, convergent evolution can result in analogous structures, which are similar traits in unrelated species due to similar environmental pressures.
Step-by-step explanation:
Anatomical and molecular features often fit a similar nested pattern because of homologous structures; these are similarities in organisms that are due to inherited traits from a common ancestor. The pattern is a result of the evolutionary process known as divergent evolution where species with a common ancestor evolve and adapt to different environments, leading to the diversity of life forms we see today. This pattern can be altered by the process of gene flow, which introduces new genetic material into a population, potentially disrupting the established nested patterns of evolution.
However, sometimes anatomical and molecular features don't fit this pattern because of convergent evolution, where unrelated species evolve similar traits due to similar environmental pressures, resulting in analogous structures. Structures that are superficially similar but not derived from a common ancestor can lead to confusion when trying to establish phylogenetic relationships.