The hypothesis of ecological restrictions regarding the latitudinal gradient in species richness.
According to the biological species idea, a species is any collection of people that coexists in one or more populations and has the potential to interbreed in order to give birth to healthy, fertile offspring.
According to the morphospecies theory, a species can be recognized by its morphological characteristics and distinguished from other species by its physical characteristics. The morphological species notion is what is meant by this.
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