Humans are naming animals, maybe better known as Homo nominatur than Homo sapiens. Our language works by dividing the world into smaller groups of things to play with. To start in biology species were defined by their morphology, which can be a problem since often superficially identical organisms are actually sets of cryptic species, e.g. green vegetable bugs with different populations using different mating songs and pheromones. Species can also appear to be unchanged morphologically over millions of years while undergoing continual evolution of less obvious traits like their immune systems. More commonly a species has been defined as a population of interbreeding organisms, with different species separated by reproductive barriers, or more recently that the population has a common evolutionary origin. Both approaches miss fact that interbreeding between what are usually accepted as species is itself a common way for new species to be created. The existence of porous barriers between species is well documented in plants, but also known in insects, vertebrates, and even mammals. The barriers between bacteria and viruses are so porous that traditional classification of species has been more or less abandoned.
Many domesticated crops are of hybrid origin. Potato and maize are derived from several wild species, apple is a four way hybrid and wheat is a combination of three wild species from two different genera. Three wild brassica species (B. rapa (turnip), B. nigra (mustard) and B. oleracea (kale/cabbage/broccoli/cauliflower)) hybridised in all possible combinations to make three new crop species- B. juncea (Asian green leaf mustard), B. napus (canola) and B. carinata (Ethiopian kale)). Hybrids in animals are less commonly documented but can still result in fertile offspring, unlike the often cited example of mules formed from crossing horses and donkeys. Even mules produce offspring on rare occasions. Most bears, small cats and large cats form fertile hybrids, as do many dog species. Many of Darwin’s finch species on the Galapagos are now believed to be derived from hybridisation, with a new species being formed recently as researchers tracked the offspring of the original cross. Domestic chickens are derived from a hybrid of at least two wild species, and chickens are cross fertile with pheasants, guineafowl and peafowl, classified into different genera and families. Duck species and geese species also are widely inter-fertile.
Even modern humans are now known to be the result of hybridising with earlier hominin species a mere 50 000 years ago (agriculture started about 10 000 years ago by comparison). All non-African populations contain considerable amounts of Neanderthal DNA, with the even more ancient Denisovan people contributing genes to people in Asia, with the highest contributions to people in Papua New Guinea. More tentative evidence suggests an even more ancient hominin species crossed with modern humans in Africa. Even though individual Europeans only have 1-5 % Neanderthal DNA, collectively they have preserved about 40 % of the whole Neanderthal genome. Neanderthal genes helped humans diversify their immune systems while Denisovan genes contributed to the adaptation of modern Tibetans to high altitude.
Merging the genetic diversity of two populations creates a bewildering range of possible combinations, from which a new population/species can be selected to suit a new ecological niche. Species which are the best at hybridising to form new species (e.g. orchids) tend to be hyper-diverse and rapidly adapt to small and narrow ecological niches. Many invasive species are the result of hybridisation events. For example the weedy lantana found in Australia is the result of hybridisation of original new world species grown as ornamentals. Given it doesn’t grow anywhere else in the world, is it an Australian native? Mother of millions (Kalanchoe) in Australia and introduced tumbleweed in the USA are also recent hybrids. There is often a prolonged lag phase when an exotic species is introduced when the plant behaves itself, only to race across the landscape at a later date. Crossing of distinct introductions of the same species is a potential source of the extra genetic diversity required to allow the population of a plant to rapidly adapt to a new ecosystem. Small populations of a species tend to accumulate damaging mutations over time, which can be filtered out during a hybridisation event to restore vigour.
The established species concept is also founded on the notion of divergence and phylogenetic trees, commonly constructed by analysing variations in common genes between different species. Repeating the analysis on a group of species using different genes often produces a different pattern of ancestral relationships, hints of ancient hybridisation events that scrambled and crossed the branches of the tree. More befuddling than this is the phenomenon of lateral gene transfer, where DNA is moved between unrelated species, usually via bacteria and viruses. Plants and insects regularly swap DNA with their bacterial symbionts and vertebrates are suspected to do so as well. The evolution of the placenta, leading to the emergence of mammals, has been linked to a viral gene that was transferred into the vertebrate host. Advances in human directed genetic engineering is likely to make this mechanism of breaking down species barriers even more significant in the future.
The final consideration to ruminate upon is that species do not emerge or exist in isolation of each other. Most complex multicellular organisms rely on a complex microbiome of multiple species of viruses, bacteria, fungi and more complex creatures to function and evolve. Those complex cells themselves were derived from the combination of an archea bacterial mother cell, colonised by a virus that became the nucleus, energised by an oxygen eating bacteria that became the mitochondria and powered by a cyanobacteria that became chloroplasts. These four way franken-cells then learned to clump together in the trillions to make plants. Symbiosis between multicellular organisms is also responsible for massive increases in species diversity, such as the relationship between pollinating insects and plants that transformed relatively simple non-flowering plant communities to the wide array of flowering plants we know today. Humans have the potential to become the universal symbiont, with a more powerful impact promoting evolution and biodiversity than any other species.
A botanist who often gives me inspiration (Tony Santore at the wonderful podcast/youtube channel “Crime Pays But Botany Doesn’t”- https://joeblowe.podbean.com/) often expresses his dismay at clueless people who ask “what is it good for?” when shown some rare plant, reducing it to some resource to be exploited or failing that, ignored. Tony is of the school that sees abstract and intrinsic beauty and immeasurable value in such species. While I can empathise with that outlook, I answer the question this way: species are good for making other species, either on their own, or in hybridisation with related species, or through symbiosis and/or gene exchange with unrelated species. Extinction is a zero-sum game on a finite planet on the time-scales that matter because niches never stay empty for long. And humans have the potential to be the accelerant on the bonfire of life, wielding creation just as exuberantly as we have historically wielded destruction.
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