Januari 18, 2017
congratulations! this is our lastepisode of our section on evolution and genetics,which puts us at the halfway mark of crashcourse biology. so far we've learned about dna,genetics, natural selection, how cells multiply, populations,speciation, replication, respiration, andphotosynthesitation. i'm so proud of you. but i couldn't let this sectionend without discussing the iscussion that everybody can'thelp but discuss these days:
evolution. it's a thing. it's not a debate. evolution is whatmakes life possible. it allows organisms to adapt tothe environment as it changes. it's responsible for the enormousdiversity and complexity of life on earth, which not only providesorganisms with sources of food and some healthy competition. it also gives us some trulyawesome stuff to marvel at.
and even though evolution makesliving things different from one another, it also showsus how we're all the same. all of life, every single thingthat's alive on the earth today, can claim the same shared heritage,having descended from the very first microorganism when lifeoriginated on this planet 3.8 billion years ago. there are people who will saythat this is all random- it's not. and that this clumsy process couldnot be possible for the majestic
beauty of our world. to them, i say, well at least weagree that our world is beautiful but, well you're probablynot going to enjoy the rest of this video. to me, there are two sortsof people in the world, those who are excited aboutthe power and beauty and simplicity of theprocess of evolution, and those who don't understand it. and somehow, i live in a countrywhere only 40% of the population
believes that evolution is a thing. the only possible reason for thatthat i can accept is that they just don't understand it. it's time to get real, people. first, let's understand what wemean when we talk about the theory of evolution. evolution is just the idea thatgene distribution changes over time, which is an indisputablefact which we observe all the time in the natural world.
but the theory of evolution is alarge set of ideas that integrates and explains a huge mass ofobservations from different disciplines including embryology,paleontology, botany, biochemistry, anatomy and geophysics. in every day language, theword "theory" means "hunch" or even "hypothesis." but in science, a theory is anidea that explains several phenomena at once. thus, the theory of evolution isa bunch of ideas that explain
many things that we, as humans,have observed for thousands of years. it's the theory that meticulouslyand precisely explains the facts, and the facts are indisputable. so let's spend some time goingthrough the facts, and how evolution explains themall so well. first, fossils: the fossil record shows thatorganisms that lived long ago were different fromthose that we see today.
sounds obvious, but twohundred years ago it seemed a little bit crazy. when scientists firststarted studying dinosaur fossils in the 1820s,they thought that all dinosaurs were basically giant iguanas. that's why the first fossildinosaur was named iguanodon. it wasn't until the fossils oftwo-legged dinosaurs started showing up in the 1850s thatscientists had to grapple with the idea that organisms of the pastwere somewhat similar to ones today
like, dinosaurs were reptiles, butmany of them took on a diversity that's barely recognizable to us. and of all those ancientnot-really-iguanas were all extinct, either dying outcompletely or evolving into organisms that survivetoday, like birds. fossils make it clear that onlyevolution can explain the origin of these new kinds of organisms. for instance, fossils taught usthat whales used to walk. whales are cetaceans, a group ofmammals that includes porpoises
and dolphins, and biologists longsuspected that whales descended from land mammals. partly because some modern whalesstill have the vestigial remnants of a pelvis and hind-limb bones. but it wasn't until recently,the 1990s and 2000s, that the pieces really came together. first, paleontologists discoveredfossils of dor-oo-dons, cetaceans that had differentskulls from modern whales but still had the samevestigial leg bones.
then they found even older fossilremains of another cetacean that actually had hindlegs and a pelvis. the pelvis wasn't fused tothe backbone like ours is, so it did swim like a whale,but more importantly, it still had ankle bones and they were ankle bones thatare unique to the order that includes bison, pigs,hippos and deer. so by following these clues leftbehind in fossilized bones, paleontologists were able to trackthe origin of whales back to the
same origin as bison and pigs. this leads us to another series offacts that evolution explains: not how animals were different, buthow they are incredibly similar. last week we talked aboutcarl linnaeus and how he classified organisms by theirstructural similarities. well he didn't know anythingabout evolution or genetics, but when he began groupingthings in this way, he hit upon one of evolution'smost prominent clues: homologous structures.
the fact that so many organismsshare so many finely detailed structures shows usthat we're related. let's go back to the whale. like my dog, lemon, and me,the whale has two limbs at the front of its body,its front flippers. and so does this bat, its wings. inside our limbs we all have thevery same structure: one longish bone on top, connected to two thinbones at the joint, followed by a cluster of small bones calledthe carpals, and then our fingers,
or digits. we each use our forelimbs fortotally different purposes: the bat flies, the whale swims,lemon walks and i... you know, jazz hands! building limbs like this isn't themost efficient way to swim or fly or walk. our limbs have the same structurebecause we descended from the same animal, something likethis more-gan-uh-cah-don here,
which, yeah, has thesame forelimb structure. in the first stage ofour existence, every vertebrate looks almostexactly the same. why? because we're all descended fromthe same initial vertebrates. so our structures are thesame as other mammals and other vertebrates, sure, but italso turns out that our molecules are the same as, like, everything. in fact, if we were ever tofind life on mars or something,
the sure fire way of knowingwhether it's really extra-terrestrial is to check andsee if it has rna in it. all living things on our planet usedna and/or rna to encode the information that makesthem what they are. the fact that we all use the samemolecule itself suggests that we are all related,even if very distantly. but what's more, by sequencingthe dna of any given creature, we can see preciselyhow alike we are. the more closely relatedspecies are, the more of
the same dna sequences they have. so the human genome is98.6% identical to that of the chimpanzee, our closestevolutionary relative, and fellow primate. but it's also 85%the same as a mouse. and i wonder how you're going tofeel about this, about half of our genes are the same as in fruitflies, which are animals, at least. so, just as your dna proves thatyou descended from your parents, your dna also shows that youdescended from other organisms
and ultimately, from that oneprokaryotic microorganism 3.8 billion years ago thatis the grandparent of us all. now when it comes to speciesthat are very similar, like say, marsupials, theirdistribution around the world or their biogeography, is alsoexplained extraordinarily well by the theory of evolution. animals that are the most similar,and are the most closely related, tend to be found in the sameregions, because evolutionary change is driven in partby geographical change.
as we talked about inour speciation episode, when organisms become isolatedby physical barriers, like oceans or mountains, they take theirown evolutionary courses. but in the time scaleswe're talking about, the geographicalbarriers are much older, and are often even theresult of continental drift. so, marsupials. you know about marsupials. they can be found in many places,but they aren't evenly distributed
around the world. by far the highest concentrationof them is in australia. even the majority of mammal fossilsin australia are marsupials. so why is australia rife withkangaroos, koalas and wombats while north americajust has, opossums? fossils show us that one ofmarsupials' earliest ancestors found its way to australia beforecontinental drift turned it into an island 30 million years ago. more importantly, after australiabroke away, placental mammals like
us evolved on the main landmass andquickly outcompeted most of the marsupials left behind,in what would become north and south america. so, very few marsupialsremain in the americas, while australia has been driftingaround like some kind of marsupial love boat. darwin's finches are anotherexample of biogeographical evidence as he wrote in the origin ofspecies, darwin observed that different species of finches onseparate galapagos islands were
not only similar to each other butwere also similar to a species on the south american mainland. he hypothesized that the islandfinches were all descendants of the mainland finch and changedover time to be more fit for their environments,a hypothesis that genetic testing has since confirmed. now, you'll remember, i hope,a few weeks ago, when i told you about peter and rosemary grant, theevolutionary biologists/lovebirds who have studied galapagosfinches since the 1970s.
one of their greatest contributionscame in 2009 when studying finches on the island of daphne major. they discovered that the offspringof an immigrant finch from another island and a daphne major finchhad become a new species in less than 30 years. this is just the latest exampleof our fourth body of evolutionary evidence:direct observation of evolution. the fact is, we have seen evolutiontake place in our own lifetimes. one of the fastest and most commonchanges we observe is the growing
resistance to drugsand other chemicals. in 1959, a study of mosquitos in avillage in india found that ddt killed 95% of the mosquitoson the first application. those that survived reproduced andpassed on their genetic resistance to the insecticide. within a year, ddt was killingonly 49% of the mosquitos, and it continued to drop. the genetic makeup of the mosquitopopulation changed because of the selective pressures causedby the use of ddt.
but it's not just tinychanges in tiny animals, we've also observed largeranimals undergoing some pretty striking changes. in 1971, for instance, biologiststransplanted ten italian wall lizards from one island offthe coast of croatia to another. thirty years later, the immigrantlizards' descendants had undergone some amazing, fundamental changeslike, even though the original lizards were mainly insect eaters,their digestive systems had changed to help them exploit the island'smost abundant food source: plants.
they actually developed musclesbetween their large and small intestine that effectively createdfermenting chambers, which allowed them to digest vegetation. plus, their heads became wider andlonger to allow them to better bite and chew the grasses and leaves. these are all great examples ofmicroevolution, allele frequency changes that happens ratherquickly and in small populations. macroevolution is just thatmicroevolution on a much longer time scale.
the sort of thing that turnshippos into whales is a lot harder to observe for a species that,200 years ago, thought dinosaurs were big iguanas, but part of thepower of the human mind is being able to see far beyond itself andthe time scales that our own individual lives are limited to. and i for one, ampretty proud of that. let's all at least agreethat the world is a beautiful and wonderful place. and life is worth studyingand knowing more about,
and that's what biology is. if you want to go back and watchparts of this video again please click on the annotations in thelittle table of contents over there. if you have questions for us,please leave them on facebook or twitter or in theyoutube comments below. thanks to everybody whohelped put this together. and we'll see you next time.
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