Final answer:
We are using the biological species concept when looking at our relatives in the hominid tree. An alternative species concept is the morphological species concept.
Step-by-step explanation:
When looking at our relatives in the hominid tree, we are using the biological species concept. This concept defines a species as a group of organisms that can interbreed and produce fertile offspring. In the context of the hominid tree, we classify different species based on their reproductive compatibility, such as whether they could interbreed and produce fertile offspring with other species in the tree.
An alternative species concept that could be used is the morphological species concept, which focuses on the physical characteristics of organisms. It defines a species based on their physical similarities and differences. However, this concept may not be suitable for classifying species in the hominid tree, as it may not capture the full range of biological variation and genetic relationships among different species.