Organisms can be classified into groups based on the features they share. This is a useful method for biologists because:

• Simplifies the study of biodiversity: The vast number of species can be organized into manageable groups, making it easier to study them.
• Reveals evolutionary relationships: Groups with more shared features are likely to be more closely related evolutionarily. This allows us to understand how species have evolved over time.
• Predictive power: If we know the characteristics of an organism's group, we can make educated guesses about its other traits.
• Communication: Using a standardized classification system allows biologists worldwide to communicate clearly about different organisms.

A taxonomic key is a tool used to identify organisms by comparing their characteristics to a series of paired statements. It's a practical tool for identifying organisms in the field. It typically presents a series of choices, leading the user to a specific identification. For example: "Does the organism have spines? If yes, go to statement 2. If no, go to statement 3."

A cladogram, on the other hand, is a diagrammatic representation of evolutionary relationships. It shows the branching pattern of a phylogeny, indicating how different groups of organisms are related to each other. The branches represent evolutionary lineages, and the nodes represent common ancestors. Cladograms are based on shared derived characteristics (synapomorphies).

Both taxonomic keys and cladograms are used in classification systems, but for different purposes. A taxonomic key helps to assign an individual organism to a specific taxonomic group based on its observable characteristics. A cladogram helps to understand the evolutionary relationships between different groups of organisms and to construct a classification system that reflects those relationships. Modern classification systems increasingly rely on cladograms to ensure that groups are based on evolutionary history, not just superficial similarities. A taxonomic key might be *derived from* a cladogram, using the characteristics identified in the cladogram to create the key's statements.

1. Evolutionary Relationship: A 95% similarity in DNA sequence between *Species A* and *Species B* strongly suggests that these two species share a relatively recent common ancestor. The higher the percentage of shared sequence, the more closely related the species are considered to be. 95% similarity indicates that the species diverged from their common ancestor relatively recently in evolutionary time.

2. Reasons for Similar DNA Sequences:

• Shared Genetic Code: All living organisms use the same genetic code (A, G, C, T) and the same mechanisms for DNA replication and protein synthesis. This fundamental similarity is inherited from a common ancestor.
• Conserved Genes: Many genes are highly conserved across different species, meaning they have remained relatively unchanged over evolutionary time. These genes often code for essential cellular functions, and changes to them would be detrimental to survival. Therefore, these genes retain high sequence similarity.