Biogeography is a very interesting subject, and something I want to talk about a bit. When you compare animal life on different continents, you’ll probably mainly see differences. There are no tigers in Africa but they do have giraffes, no hedgehogs in South America but do have agouti running around. There are plenty of reasons why a certain animal or plant occurs only in one specific spot, or sometimes why they are very widespread. Different places have different circumstances, all of which give a certain direction to evolution. There’s not much use for an animal to gain climbing skills in savannah areas, so why would there be a sloth loafing around there, right? Evolution also has its boundaries, usually defined by evolutionary history. There are no big leaps in traits. Evolution can be compared to your base house, a good groundwork already there. Evolution adds another shack, a window or a door. But it doesn’t change the whole house, it only reworks it a bit here and there. There won’t suddenly be a mammal with six legs or an insect with mammalian lungs. It’s of course possible that insects develop a new set of breathing apparatus, but that won’t happen unless the circumstances require it, and it had the time for it to develop (let’s say, a few million years). It’s all about survival of the fittest. That doesn’t mean it’s the best overall, it means it’s the best in the current environment. The mighty dinosaurs got extinct because they couldn’t cope with the effects of the meteor impact, but mammals could survive. Being adaptable to different environments is of course a great trait to have, and is why humans are so successful all across the globe.
The main message is, it’s all about environmental influence. The organism adapts itself the climate, to the local temperatures, water supply, sun intensity, … And that obviously partly depends on where that locality is on the world. Closer to the equator you’ll have warmer weather with little seasonal changes, different soils mean different plants to grow on it, mountains and other geological phenomena give different challenges to overcome. All that gives rise to the different biomes like deserts, savanna, rainforest, temperate forest, oceans … Comparing one rainforest to another on a different continent, you’ll find that a lot of the parameters can be very alike (they did both lead to a rainforest after all), but animal life in it will not, because of the different evolutionary background. However, the same circumstances will lead to the same adaptational needs in an organism. So you see the same traits pop up in different animals that live in the same or similar biome. Think of a shark and a dolphin, one is a fish, the other a mammal. But they both need to be able to swim well in the water. So they both developed fins. The same goes for birds and bats, all got their wings to conquer the sky and catch their flying prey. Or what about the prehensile tails in monkeys, pangolins and chameleons to help in climbing trees and the long tongues of anteaters, armadillos and pangolins to scoop up ants. This is called convergent evolution, where unrelated (or distantly related) organism develops similar adaptations, appendages, organs and general body forms, because the environment demands it. It’s convergent evolution that can make you think whales are fish or that the bat is a bird or that every small, scurrying thing is a mouse.
There is one family of mammals that has taken the notion of convergent evolution. Members of this family have evolved traits that can be likened to that of hedgehogs, shrews, opossums, mice and otters. I’m talking about the tenrec, which can be found on Madagascar and some parts of the African continent itself. They’re a rather separate branch of mammals, their closest living relatives being the elephant shrews and golden moles. They have taken in the niches of their more known counterparts in these areas. They can be found on and under the ground, in the water and up the trees.
Most tenrecs are shrew tenrecs (genus Microgale), and are like our regular shrews, insectivorous with no other extraordinary or notable adaptations. Some do look more like mice and some have the prehensile tail that warrants the comparison to opossums. But then you have two species of otter shrew, and the web-footed shrew, that are adapted to life in water where they prey on aquatic insects, crayfish and even little fish, just like an otter in other rivers far, far away. The giant otter shrew (which IS a tenrec) even looks more like an otter than a shrew. The mole-like rice tenrec is a tenrec that pretty much lives its whole live underground. You know, like a mole. Then you have your lesser and greater hedgehog tenrec which look like… Pauze for dramatic effect… Hedgehogs! Spines and everything. There’s also two species of streaked tenrec that have less spines, but more colours and look more like a Pokémon than anything else. Most of the tenrecs are hard to find, let alone taking a good photo of one of the more elusive species. So here’s some photos of varying quality:
It’s this diversity in shapes that doesn’t make it immediately clear all these tenrecs are part of one big family. It’s exactly this that makes convergent evolution one of the bigger problems in systematics. Put a shrew, a shrew tenrec, a hedgehog and a hedgehog tenrec next to each other and most people would group them wrong, if they only use the general looks of the animals. It’s an obstacle that’s not extremely hard to overcome with some thoroughness, and with the rise of genetics it’s no issue anymore at all. In fact, genetics has overhauled a good amount of pre-existing branches of the tree of life that were solely based on comparing phenotypic (observable) characteristics. It’s the genotype (the collection of genes) that have since then proven that these comparisons were wrong. This is true on both a micro- (genus level) and macro-scale (family and up), but the bigger changes were on the lower levels. No one would ever consider that a spider would be closely related to an earthworm of course. You don’t even need genetics for that. It gets a bit trickier if you consider for example the coyote, eastern, red and gray wolf (not even mentioning all their subspecies).
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