[Music] hi it's Mr Anderson and this is AP biology science Practice 6 it's on scientific explanations and theories so remember in science we come up with a good question We Gather good data we analyze the data and then over time we start to develop theories which explain how the world works and so natural selection was developed by Charles Darwin and it's been tested and tested over time what is it it's a mechanism that explains how Evolution can occur in other words the finches on the galpagos had variation in their beaks and they each adapted to their specific specific food supply over time and so um how does a theory develop in other words where do theories come from well they start when a scientist comes up with an idea they then perform experiments and they look does the evidence that I'm collecting fit or support this idea if it doesn't then I go back I add a bad idea I got to get a good idea this time so I come up with a better idea perform more experiments if the evidence does does support the idea then I can create a theory and that theory is used to better understand the universe so natural selection is a great example of that what happens next then we discover new evidence and can we modify the theory a little bit so it can explain this new evidence and so natural selection had to be modified when we started thinking about sexual selection and so if we can then we improve upon the theory and so this cycle occurs over and over and over again it's a feedback loop to make sure that our Theory uh matches really what's going on in the universe if the theory doesn't match that so before natural selection we had this idea that God had simply placed all of the species on the planet perfectly evolved to where they are and so if that theory doesn't fit with the data that we're collecting that leads to a revolution and so over time science gets better and better and better explanations get better and better so again the College Board wants you to be able to look at theories and apply theories in each of the four big Ideas so in evolution not only should you understand how natural selection works but how antibiotic resistance or changes in bacteria as a resistance to our antibiotics is how that fits into natural selection or maybe you understand how feedback loops work how we regulate the amount of blood glucose but what if we have changes to the insulin receptor how would that affect our explanation or we all know that the genetic information sits in the DNA but how do we get changes that aren't related to changes in the DNA how can the environment change what genes we're going to express and that has to do with remember the histones and methylation of the histones or opening up of those histones so we can get to the DNA or if we're looking at systems how is it that the the uh the order by which molecules are added to macro molecules for example the amino acids are added to proteins how is that going to affect the overall structure of the protein itself and so I'm going to go through the five ways they want you to deal with theories and explanations in the first one they would like you to be able to justify claims with evidence and so which of the following statements most directly supports the claim that different species or organisms use different metabolic strategies to meet their energy requirements and so these are examples of questions you might find on the test and so you may want to pause the video and take a stab at each of these so you may want to read through a b c and d as I worked through this they all were good but the only one that really got to the root of changing metabolic strategies was D so the right answer is D they also want you to construct explanations based on evidence and so let me set this up we've got different plates of augur um on these ones there's no ampicillin in the augur ampicillin remember is an antibiotic on these we do have ampicillin we have wild type eoli just regular eoli and then eoli that we' transformed with a uh ampicillin resistant plasmid and so looking at this data which of the following plates have only ampicillin resistant bacteria growing the right answer answer we would say is C or four because we've transformed some of these bacteria now we probably transformed them here but they only show ampil and resistance in in plate four or let's go to the next one state the conclusions reached by mendle and his work on the inheritance of characteristics and explain how these following deviate from this conclusion so here we're trying to articulate the reasons that explanations and theories are refined or replaced and so again if I were to do uh mendal laws I would first say that law of segregation that alal are going to segregate as we form gametes and then the second one is the idea of Independent Assortment that genes are not going to affect other genes and then I could go through and look at aoma linkage and so do genes ever assort non- independently they do if they're on the same chromosome or we could look at sex linkage or polygenic inheritance isn't going to follow straight mendelian inheritance and so again it's not that mendal was wrong it's that we had to refine his theory over time so this would be like an essay type question they also want you to be able to make claims and predictions about natural phenomena so this would be another essay question explain two of the following three processes using an appropriate example for each and then for each process discuss its impact on the genetic makeup of the population so not only are we explaining the phenomena but we're saying you know for each of these what's how is that going to impact the genetic makeup genetic drift remember is when we decrease the population size an example could be a bottle NE of the northern elephant seals or the cheetah migration would be movement into or out of an area artificial selection like on our fast plants is when we are actually doing the the um choosing who mates with whom uh and then if we go to the last one this is a pretty tough genetics question you may want to take a stab at it and so here they're presenting you with a lot of data and then you come up with alternative explanations so you could pause the video and take a stab at this one they are telling you that we've got a dominant and a recessive a they they aren't giving the parental numbers here it's a cross they give you the F1 and the F2 and you have to determine the genotypes of the original parents so they're giving you the data and then you have to work backwards to kind of figure out the inheritance pattern that's going on um this is kind of how I work through this we looked at the parental cross and so they gave us a parental of a wild type male and a white-eyed female I then kind of gave my two opportunities number one since we're getting differences in the females and the males we know what's a sexlink trait and now I have to figure out is it sexlink recessive or Sex Link dominant and so I said here's my wild type male maybe the wild type is going to be dominant and the white eye eye is going to be recessive and then here's my other one where the white-eyed females maybe that's that's dominant as well and so then I did all the possible f1s and so if we did these two parents you could do this just as a punet square I could end up having a male that is different than the the male in the P generation and same thing here so that fits so we could have a white-eyed male and a wild-eyed female being created but then in the other alternative just doesn't match up um and so again you're looking at data you're applying that seeing how it fits in with a theore we have to refine that theory and so theories get refined over time so how the germ Theory didn't come about until we could actually see germs and we could test them using the scientific method so before that we thought you got sick using m theory that there was something like bad air that moved around and caused us to get sick and until pasture developed his pasture flask and did experimentation on that you know the germ Theory shouldn't have been accepted until we knew this that air could move into the pasture flask but the bacteria would start to settle out here did we know that it was germs rather than bad bad air that was being passed on and so again theories get better and better over time and on this test they want you to be able to apply those theories to new data and I hope that's helpful