Final answer:
The diversity of life and the similarity among species is explained by evolutionary theory, which demonstrates that species share common ancestors and therefore exhibit hierarchical patterns of similarity. Darwin's theory of evolution provides the framework for understanding these relationships, and classification is now based on phylogeny to reflect these patterns.
Step-by-step explanation:
The diversity of life on Earth and the existence of groups of similar species can be explained through Charles Darwin's theory of evolution. Darwin wasn't the first to consider that species might be interconnected or interrelated, but he was the first to provide a comprehensive theory that put the pieces of the puzzle together. The hierarchical pattern of similarity among species is a result of how organisms are classified using taxonomic levels, from broader categories such as domains to more specific ones such as species.
Darwin observed that the fundamental similarity of groups of organisms was due to their shared evolutionary ancestry. For biologists, it's essential to classify organisms based on phylogeny to reflect these evolutionary relationships and to understand the resulting organizational structure. As new species are discovered, biologists continue to explore and refine their methods of classification to better reflect the underlying evolutionary processes.
Explanations for the diversity of life include mechanisms like speciation, where one species branches into two or more new species over time. Previously, scientists used arbitrary similarities for classification, but now they seek to find features that reflect the evolutionary past of organisms, despite the challenges posed by the incomplete fossil record.