What is a species? The truth is; nobody knows. Charles Darwin once said; “… I look at the term species as one arbitrarily given for the sake of convenience to a set of individuals closely resembling each other..”.
Of course, a lion clearly isn’t a zebra. But if you look at insects, the boundary line between different species is not so well defined. In many cases, evolutionary lineages of butterflies and moths have very subtly diverged in many geographical areas – resulting in differences in appearance or lifestyle. Usually, if two populations have diverged so much that they are unable to produce (fertile) offspring, they are regarded as different species. For this reason, genitalia have been an important point in the past, for Lepidoptera have complicated genitalia that fit together almost like “puzzle pieces”; species with different shaped genitalia will fail this lock/key mechanism and will be unable to pair with eachother. In some cases they can pair, but produce infertile hybrids. When two lineages do not produce fertile offspring that reproduces in the wild, most of the time we call them different species. And if they still can reproduce but are still vastly different in ecology or appearance, we call them subspecies.
Generally, if you ask a biologist ‘what is a species?‘ they will answer with “a group of living organisms consisting of similar individuals capable of exchanging genes or interbreeding”. That sounds like a broad definition, because it is! There is no clear definition of what is a species or not. Some species do not exchange genes or interbeed at all (microrganisms such as bacteria for example; and others that reproduce asexually). And some organisms appear to be the same species on the surface level but actually consists of multiple groups of individuals that reproduce in their own unqiue ways and are not compatible with eachother; such as asexually and sexually reproducing dandelions (Taxaracum) that have mutated to have diploid, triploid or tetraploid groups of individuals that are not compatible with eachother. But to us, the dandelions in our garden appear to be the same ‘thing’ while in reality, many of them are incompatible with eachother and have unique evolutionary lineages.
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