Atavisms: 4 Lost Traits That Returned

[Music] there's this idea in evolutionary theory often referred to as dolos law that once a complex trait disappears it can't come back which makes sense everything about a living creature can be traced back to the activity or expression of its genes once a gene stops being actively used tiny mutations start to build up so over time it deteriorates and soon enough it is too far gone to function the same way again I mean sure a species could evolve something similar to what it once had but it cannot go back take all the whales and dolphins in the mammalian order cetacean for instance like all other mammals they evolved from a land dwelling animal that itself descended from a water dwelling fish and nowadays a cetaceans do have a few fish like traits like tails that propel them through the water but they didn't get those by reverting back to the tails of their ancient fish ancestors new tweaks and adjustments to their genes turned the base of their spine into a totally new kind of tail which is why those tails are turned 90 degrees relative to the ones that fish have and if for some reason they were to be pushed back towards living on land they wouldn't be able to just dig around in their genomes to find the genes for legs and switch them on except actually that does happen every once in a while a dolphin or a whale is born with surprisingly complete hind limbs such reappearances of traits thought to be lost forever are known as atavisms these apparent defiance's of dolos law can teach us all sorts of things like how limbs actually grow but perhaps most importantly they're helping us understand how evolution happens by revealing what genetic information is truly lost and what is simply tucked away for a rainy millennium the ancestor of all cetaceans a four-legged hippopotamus like creature entered the water around 50 million years ago and for about 34 million of those years it's descendants have lacked hindlimbs which is a really long time long enough that you would think the random accumulation of genetic changes would prevent those limbs from ever coming back but every once in a while people find a dolphin or whale with rear appendages and that's probably because the genes for hind limbs never went away see for about 15 million years after that hippo ish creature took to the water its descendants held on to their hind limbs so these legs got smaller they kept the same structure complete with toes they just shrunk and that was probably due to a phenomenon called heterochromia a change in the timing of gene expression you see the embryos of whales and dolphins still develop hind limb buds those are structures that grow into limbs and in all vertebrates that growth is largely overseen by a gene called hand 2 which essentially gives the okay for other genes to do their jobs so researchers think the gradual shrinking of hind limbs was probably caused by a bunch of little genetic changes that affected the duration and intensity of hand 2 expression in those rear limb buds especially later on in fetal development then around 34 million years ago cetacean hind legs abruptly disappeared and that's probably because their hand too got switched totally off so instead of hetero chrony it's an example of heterotroph ii a change in the location of gene expression this was a considerable improvement as even those mini legs created drag that slowed them down so getting rid of them entirely made the animals sleeker faster swimmers and all of this explains how this atavism can happen now even though it's been forever since these animals had real hind limbs cetaceans still use hand to to direct the growth of their arms into flippers which help them steer and stop and that means that to revolve legs could be as simple as a mutation that switches this gene on in the back to know exactly how hand 2 gets reactivated to make mini legs is still a bit of a mystery studying this particular atavism has given researchers a better understanding of the changes that whales have gone through to become the gentle giants of the sea and there are lots of similar examples of one-off atavisms like this from extra toes in horses to extra nipples and people but sometimes atavisms changed the course of evolution leading to the re-emergence of traits in an entire species limpets are snails but they sure don't look like them instead of having long coiled shells they have short squat little ones so they sort of look like they're carrying around teeny shields on their backs these non coiled shells are especially useful for squeezing into tight spaces or hunkering down in the pounding waves of shoreline environments and scientists estimate that limpets lost their coiling at least 20 million years ago so it was thought that their ability to curl up their shells was lost for good some limpets in the colibri family do have coiled shells though out of the 200 or so species 10 to 12 have coiled shells as adults and at first researchers thought those species just hadn't gotten around to losing their ancestral coils but in 2003 scientists realized they'd actually re-evolved them and when they looked a little closer at one genus in particular they found out how that could be see the snails in the genus crapola all have coiled shells as babies but grow up to be uncoiled adults this means that the genes for coiling are still kicking around and expressing at some point during development so all a limpet has to do to revolve coiling is expressed those genes for a longer period of time and that seems to be exactly what's happened in those recoiled limpet species the timing of this gene expression has changed leading to a coiled shell all the way into adulthood so it's another example of hetero chrony except this one stands out because it is the only atavism we know of so far that arises solely from a change in developmental timing and it reveals just how powerful small tweaks to timing can be so if you look super closely at the dirt in your garden or a nearby forest you'd probably find what appear to be teeny-tiny beetles but they're actually soil mites and the odds are pretty high that the ones you find are female because long ago entire lineages of these mites lost their ability to have sex they reproduce through parthenogenesis instead a type of asexual reproduction where embryos developed from unfertilized eggs that often leads to entire species that are 99% female and nearly 10% of all soil mite species are parthenogenesis species in the desmond Amada sub order the largest known grouping of animals that reproduces this way but weirdly this group also contains the family Croton aia t soil mites which seem to have gone from having sex to reproducing parthenogenesis again that may be because they don't really act like soil mites anymore see these mites spend their lives in tree bark instead in fact most sexually reproducing mites are found in tree barks and mosses rather than in soil there are a number of hypotheses about why sex is more prevalent in these habitats like there might be more predators and parasites there so the species need the increased genetic diversity that comes with having sex but it's extremely unusual for a species to regain the ability to have sex after being asexual for so long somehow despite not making any males for millions of years they held on to the genes needed to make them and at this point we're not totally sure how they did that but researchers are eager to learn because hanging on to inactive sex genes seems like a waste of genetic space it's possible for instance that these genes serve some other important purpose for the mite survival and if that's true then figuring out why they'd keep these genes around for so long could teach us a lot about what else they do who said ax worms live in the depths of the oceans where they eat bones for a living hence some have taken to calling them bone or zombie worms and their diet isn't their only a radical feature they also exhibit some pretty extreme sexual size dimorphism which is to say the females are a lot bigger than the males in fact male bone worms are so small that they live in harems inside the female's body and not just like 2 or 3 males scientists have counted as many as 6 in seven males living inside a single female that's right a very specific number that's how you know they did their homework except for OSA Dax Priapus that is in this one species males are up to one-third the size of females and they live their own independent lives researchers are pretty sure that the species came from ancestors that had miniature internalized males like other bone worms and it's thought in general these worms evolved such differently sized sexes because they feed off the bones of dead animals in the deep sea so food is somewhat scarce females can't get around the fact that they need to eat a lot to make baby worms and the bigger they are the more they can give to their offspring but male worms can be small and still make plenty of sperm and the smaller they are the less space and food they use up leaving more for the females they want to inseminate so it's not hard to imagine how females might have gotten bigger and bigger while males got smaller and smaller until eventually the males were small enough to fit inside their mates now their sole purpose in life is to produce sperm dwarf males don't even eat they just digest the yolk they got as eggs and then pump out sperm until that energy resource dries up the question becomes why then the males of this one species would go back to eating and living on their own most hypotheses center around the fact that the females of this species are particularly small themselves when compared with other bone worms anyway small females produce smaller yolks which don't provide the energy for males to produce noodles of sperm so the species may have evolved larger males to shirk the requirement for maternal yolk altogether also smaller females don't take up as much bone space so maybe there was simply room for the male's to go out on their own and by actually eating they could produce more sperm plus males that don't live inside a female can reach get this other females which helps mix up the genes in a population more effectively so perhaps there was something which led the species to crave more genetic diversity it's also possible that a perfect storm of all of these hypotheses ended up creating the unique evolutionary conditions which drove this species back towards larger independent males biologists will probably get a little more insight into this mystery but determining how this atavism occurred because right now they're not even sure how a worm becomes male or female let alone what genes govern things like body size and they're interested in the specifics here because as far as we know this atavism is the only example of the loss of dwarfism in an animal so already it's shaking up our understanding of what kinds of evolutionary shifts are possible tomorrow we study the living creatures around us the more we're realizing that we've made a lot of assumptions about how evolution works and that's why atavisms like the four we discussed today are always thrilling for researchers to discover whether they occur in an entire species or are a fluke appearance of a long-lost trait out of ism's give us insight into the ancestral lineages of organisms and help prove or disprove what we think we know about evolution thanks for watching this episode of scishow and a double thank you to all of our supporters like all you awesome people who have signed on as channel members the money you pledged each month as a channel member helps us keep making fun science videos like this one here on YouTube where they could be watched for free so thanks channel members and if supporting free educational science content sounds like something you might be interested in then I encourage you to click the join 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Channel: SciShow

Views: 531,997

Rating: 4.9472494 out of 5

Keywords: SciShow, science, Hank, Green, education, learn, Dollo’s Law, Atavism, avatism, Cetacea, whale, dolphin, mutation, evolutionary theory, evolution, trait, gene, evolve, land-dwelling, water-dwelling, hind limb, genetic, heterochrony, Hand2, heterotropy, Limpet, shell, coiled shell, Calyptraeid, Crepidula, gene expression, Soil mite, parthenogenesis, Crotoniidae, bone worm, zombie worm, Osedax worm, Osedax, sexual size dimorphism, Osedax priapus, ancestral lineage

Id: 0MBnFc_AYEg

Length: 12min 41sec (761 seconds)

Published: Sun Jun 14 2020