Every species model has limitations, so no single definition works for all organisms.Biological species concept
• It cannot be applied to organisms that reproduce asexually, such as bacteria and some plants, because they do not interbreed.
• It cannot be applied to fossils: extinct organisms cannot be tested for interbreeding.
• Some organisms classed as different species can produce fertile hybrids. Many plants hybridise, and wolves and coyotes can produce fertile offspring.
• It is often impractical to test whether wild populations that live far apart could interbreed.Morphological species concept
• Sexual dimorphism and different life cycle stages can make one species look like several.
• Cryptic species look identical, so several species may be grouped as one.
• Convergent evolution can make unrelated organisms look similar.
• It is subjective: different taxonomists may judge the same features differently and disagree about how different two organisms must be.Phylogenetic species concept
• There is no agreed amount of DNA difference that separates two species, so the boundary is arbitrary.
• It can split one species into many small species, which affects conservation decisions.
• It needs DNA sequencing, which is costly and cannot be used on most fossils.Because of these limitations, species are best identified using several lines of evidence together, such as morphology, breeding, behaviour, ecology and DNA.
• It cannot be applied to organisms that reproduce asexually, such as bacteria and some plants, because they do not interbreed.
• It cannot be applied to fossils: extinct organisms cannot be tested for interbreeding.
• Some organisms classed as different species can produce fertile hybrids. Many plants hybridise, and wolves and coyotes can produce fertile offspring.
• It is often impractical to test whether wild populations that live far apart could interbreed.Morphological species concept
• Sexual dimorphism and different life cycle stages can make one species look like several.
• Cryptic species look identical, so several species may be grouped as one.
• Convergent evolution can make unrelated organisms look similar.
• It is subjective: different taxonomists may judge the same features differently and disagree about how different two organisms must be.Phylogenetic species concept
• There is no agreed amount of DNA difference that separates two species, so the boundary is arbitrary.
• It can split one species into many small species, which affects conservation decisions.
• It needs DNA sequencing, which is costly and cannot be used on most fossils.Because of these limitations, species are best identified using several lines of evidence together, such as morphology, breeding, behaviour, ecology and DNA.