If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

Main content
Current time:0:00Total duration:6:06
AP.BIO:
EVO‑1 (EU)
,
EVO‑1.O (LO)
,
EVO‑1.O.1 (EK)
,
IST‑4 (EU)
,
IST‑4.A (LO)
,
IST‑4.A.1 (EK)
,
IST‑4.B (LO)
,
IST‑4.B.1 (EK)

Video transcript

in many videos when we've discussed evolution and natural selection we've talked about how variation in a population can fuel natural selection and evolution so if you have a population of circles obviously a very simple model here maybe some of these circles are that off-white color or maybe some of them are blue and maybe some of them are this salmon color for certain traits your environment might make certain of them better for reproduction better for survival of hating predators better for finding food and let's say these circles for whatever reason they're an environment where maybe being blue makes it a little bit easier to evade predators and a little bit easier to reproduce and find food well then in the next generation in the next generation because the blues more likely to be able to get to reproduction because they weren't eaten you're likely to have more blues so let me draw a few more blues and maybe a few a little bit less of the other ones because they're also competing for resources amongst each other at least in this model that I'm doing and so over time if this blue phenotype remember Friant phenotype is the expressed trait that's actually observable versus the genotype which is the underlying genetics which is sometimes observable and sometimes not but as you can see if in this can this environment blue seems to carry some advantage even if it's a slight probabilistic advantage over many generations blue will start to dominate and so you start to see that evolution of this population to being more blue as a species so you have these blue circles so one way to think about it is you have variation in a species is really what natural selection is based off of certain variants might be more favorable than others so that is what's really necessary for natural selection to fuel evolution to fuel evolution now a key question is where does this variation in a population come from and to think about that we just have to remind ourselves where our phenotypes come from how do these Express traits get expressed well in all the living organisms we're way aware of we have DNA as human beings we have 23 pair of chromosomes and each chromosome you could view as just a very very very very long strand of DNA and sections of that DNA code for various traits and each of those sections that code for say a certain protein or a part of an enzyme we call those things genes we call those things genes so chroma we have multiple chromosomes with xx as human beings different species have different number but as human beings we have 23 pair of chromosomes each chromosome you view as a long strand of DNA parts of that DNA code for specific genes and then if you were to zoom in if you were to zoom in on those genes you would see these nucleotide sequences and this is all a review we've seen this in other videos where you see your adenine or guanine your cytosine or thymine in orders in order that carries the information that will eventually be coded into mRNA which then gets coded into protein now there's two primary sources of variation one source of a variation is sexual reproduction sexual reproduction now not all organisms reproduce sexually but many of the ones that we know including human beings do we're a male member of the species and a female member of the species each contribute a random half of their chromosomes to the next organism so one way to think about sexual reproduction is it keeps shuffling the different versions of the genes that you have in the population into different combinations of those versions of genes and so that generates variation but sexual reproduction by itself will not create new versions of genes which we call alleles or new genes entirely and so the primary way that that happen is through mutations and you might have guessed that we're going to talk about that because I had this title up here so another source of variation and you could almost view this as a more fundamental one because this would happen even in organisms that aren't reproducing sexually is that over time there could be just random mistakes there could be edits to these genes and it could it could be a random maybe this G gets turned into a C randomly or maybe this T and a gets cut out during the DNA replication process these mutations which are all about genotype and let me make this very clear so when we're looking at the sequence we're thinking about genotype differences in genotype are not always obvious from expressed traits so sometimes they do change phenotype or their observable in phenotype sometimes they're not but when they are observable in phenotype as I just mentioned many times it could be a negative change in phenotype where it makes it less viable for that organism or it's harder for them to survive and reproduce but every now and then it could result in a variation in phenotype that is maybe neutral or even confers some type of advantage so it might have been a random mutation that somehow turned one of these white circles into a blue circle and there might have been another mutation that turned a white circle into a square and that this wasn't even viable as a as an organism but the blue circles happen to be in the environment they're in happened to be a favorable variation
Biology is brought to you with support from the Amgen Foundation