homology and analogy both describes similarity but they are often confused with one another let's take a look at the difference between homology and analogy and how the presence of homology is one of the best sources of support for evolution when we look at nature different species of plants and animals are often similar to each other but why is this obviously one reason is because they have similar lifestyles like three of the four animals shown here which eat ants and other insects that's why the anteater the echidna and the aardvark all have those long noses for probing into ant and termite mounds two of these species the echidna and the porcupine have different diets but are similar in having spines that they use for defense in nature we see a variety of different species having similar traits often similar traits are similar because of obvious things like diet or protection however sometimes they're similar but not because of a similar lifestyle it turns out there are two distinct reasons why organisms are similar so we use two different technical terms to describe the similarity analogy describes similarity due to shared function or lifestyle this is when the traits of organisms are similar because they do similar things our common sense can often be used to explain why traits are analogous in the examples we just saw the noses of those species that ate ants and termites have the same shape for a pretty obvious reason poking around in crevices and holding a long tongue could poke into small tunnels the reason for the similarity of the spikes was likewise obvious sometimes these similarities don't make sense because of current utility however common sense sometimes fails to provide an explanation for the similarity in these cases the process of evolution provides an explanation for these similarities homology describes similarity due to shared history or relatedness why are the organisms or traits similar it's because they both inherited the same traits from their ancestors let's take a look at a few examples that illustrate analogy and homology first a couple of non-biological ones and then some biological examples why are there pyramids in South America and North Africa so similar why did two different cultures create buildings that have basically the same shape it wasn't because of a shared history because there's no evidence of a connection between these two cultures these buildings the same due to a shared function they're both large impressive buildings made with only basic technology if you only have manual labor and stone but you want to make a big impressive building you make a large base and taper it as you go upwards to maintain its structural stability the design of these pyramids is analogous they are similar because they're performing the same function under the same constraints another non-biological question we can ask is why our keyboards so similar why are the letters arranged the same on such a wide variety of devices the top picture here shows a telephone a tablet and a keyboard the letters are all essentially in the same place even though the devices are distinct this isn't entirely because of a functional reason because the letters could actually be in better locations the technology of touchscreens could put the letters anywhere we wanted including a single row around the edge or something like that in fact there are other arrangements of letters on keyboards that have been shown to increase the typing speed and accuracy of people using them seriously look up the Dvorak keyboard the reason why all these keyboards have the same arrangement is because of a shared history keyboards were first invented for typewriters and then later as computers replaced typewriters these new machines inherited the position of the letters and then when screens themselves took over typing duties they inherited the positions of the letters as well interestingly the positions of the keys and original typewriters were deliberately chosen to slow down typing speed the way that old typewriters used to work is that when you press the button a metal rod would swing out to contact the paper and they move back into resting position if you typed too quickly then the second rod could swing forward before the first had returned and they could get stuck and you would have to manually unstick them because this problem the keys were arranged to minimize this and make it more difficult for people to type quickly obviously computers and electronic devices don't have this mechanical problem now but they have inherited this inferior design because of their history so why do these devices have their letters in the same positions it's not because those the best positions for the individual devices but because of their shared history the arrangement of letters is homologous looking at a more biological example we can ask why swimming animals are often so similar in the shape of their body they are said to be torpedo-shaped named after an object that travels quickly through the water their bodies are not the same shape because of their shared ancestry this phylogeny depicting the evolutionary relationships of fish penguins and dolphins shows that they would not have inherited the same body shape from their ancestors this is because the ancestors of penguins and dolphins lived on land and would have looked very different in this case their body shapes are due to a shared function reducing drag for an organism that is pushing its way through water the shapes of the bodies of these organisms are analogous similar because of current function but some biological similarities don't make sense in terms of function the most famous example of homology comes from the four limbs of animals if we look at these five different species of mammals their forelimbs all have the same basic range amount of bones they have one large bone connected to a pair of large bones then a bunch of small bones and finally five or fewer fingers this similarity of bone design occurs even though these limbs are doing very different things moles dig through dirt that provides strong resistance bats flapped their wings to provide flight anteaters pot hard mounds of packed earth monkeys grasp objects and dolphins swim through the water if you were to design limbs to do these very different functions from scratch there would be a whole bunch of different arrangements of bones you could imagine that would work and some would be better for one function or another so why do all these animals use the same bones it's because if we look at their phylogeny and in the fossil record we can see that their ancestor had four limbs with the same arrangement of bones which they have all inherited the bones and the four limbs of these animals are similar not because of current function because those are very different they're similar because of shared ancestry and are therefore homologous there are a variety of other homologies and living things similarities between organisms that are better explained by shared ancestry than by the constraints of current function there are genetic similarities and sequences that have no clear function for example introns are generally excised out of the RNA during translation so the sequence doesn't matter much yet the introns of different species are often similar the blood group system doesn't have a clear function why do we share these blood groups with chimpanzees and gorillas because our common ancestors had those blood groups there are also genetic similarities and genes that are very different functions genes called crystallins in some species make up the structural component of the lens of the eye and are almost identical to genes they're used to produce digestive enzymes the sequences of those genes are so similar because they're both recently descended from what used to be a single gene that did both functions the Hox genes involved in axial and limb formation have sequences that are extremely similar to one another a major reason for this is because they're descended from ancestors tRNA is used by most organisms for transferring amino acids but when you look at it only the very distal portions have a direct function a variety of different shapes of RNA should be able to provide the same function but the fact that everything uses the same shaped tRNAs is because they inherited it from an ancient ancestor despite their very different function the anatomy of flowers indicates that they are just modified leaves produced by many of the same genes why do these two very different structures have such similarity it's because flowers evolved from leaves so their similarities reflect that history in fact if we think about anatomy in general why there's so few different body plans there are so many living things yet so few different arrangements of bones and bodies this power of ancestry prevents us from seeing some analogies we might otherwise expect we don't see any vertebrates with more than two pairs of limbs like insects have or any mammals with feathers like birds have it's not that those traits wouldn't be functional it's that the history of life prevents those adaptations in the history of biology and evolutionary theory homology has a central role in the 1800s there wasn't a good explanation for homology biologists had identified a bunch of similarities and some made sense because they were analogies but there were others like the bones in the four limbs of mammals for which they didn't have a good explanation this obvious and clear pattern demanded an answer but biology lacked a good one an influential German idea at the time was that there were a limited number of bowel plans something like biological blueprints for living things but those natural philosophers didn't have a good explanation for why there was such a limit meanwhile all creationists could say was that's just the way things are which is an unsatisfying explanation in fact while there are many different independent lines of evidence for evolution this channel has several other videos that describe these an explanation for the presence of homology was the primary reason why Darwin's process of evolution was so quickly accepted by the scientific community providing a logical explanation for something that had been mystifying biologist for decades was a triumph of evolutionary theory note that while Darwin's proposed process of evolution was quickly accepted his proposed cause of evolution natural selection was not accepted right away in fact it wasn't until decades after he was dead that it was unanimously accepted within the biological community evolutionary theory is what allows us to see that the noses of anteaters the profiles of sharks and the very genes we have in our genome are better understood when we take an evolutionary perspective on life like subscribe share and enjoy this mini video of a Tamandua and an agouti 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