1. Physical characteristics: This is the most obvious factor, and it includes things like:
* Anatomy: Do they have a backbone (vertebrate vs. invertebrate)? What kind of limbs do they have? What internal organs do they possess?
* Morphology: What is their external shape and structure? How do they move? What are their sensory organs like?
* Physiology: How do they function? What are their metabolic processes like? How do they reproduce?
2. Evolutionary relationships: Biologists look for shared ancestry and common traits to understand how different species are related. This involves studying:
* Fossil evidence: What extinct organisms resemble the current species?
* DNA and genetic analysis: How similar are their genes?
* Comparative anatomy: How do their structures compare to other species?
3. Behavioral characteristics: This includes how the animal interacts with its environment and other animals. It may involve observing:
* Feeding habits: What do they eat? How do they hunt or forage?
* Social behavior: Do they live in groups? How do they communicate?
* Reproduction: How do they find mates? How do they raise their young?
These three factors are interconnected and constantly being refined as new data emerges. They help create a complex and ever-evolving system of classification, allowing scientists to understand the vast diversity of life on Earth.