[Music] thank you in the last video we talked about the biological species concept which defines organisms as the same species if they can breed together and create fertile offspring the basis of this generally requires a male and a female and the mixing and passing of genetic material to The Offspring while this certainly is the case for many organisms including humans not all organisms perfectly fit into this definition for many reasons showing that this definition has some faults asexual reproduction is a method that many other organisms use like bacteria they produce copies of themselves in a process called binary fission there are no male or female bacteria and no direct mixing of genetics from two or more bacteria when this process of reproduction takes place in addition to binary fission bacteria can also transfer plasma DNA to one another in a process called horizontal Gene transfer this is how small amounts of DNA can be mixed and matched between bacteria of the same species causing genetic variation but the process of reproduction is not required for this to happen whereas organisms like humans can only mix their DNA and pass along genes to the Next Generation through sexual reproduction the point I'm trying to make is that defining species by reproduction is difficult and while the biological species concept works for some organisms it most certainly does not work for all sticking again with the biological species concept for this next part we know that organisms of the same species must be able to reproduce and produce fertile offspring being fertile just means that you are also able to reproduce the basis of creating a fertile offspring revolves around one thing chromosome number if the chromosome number of the organisms are the same which again is a Telltale sign that they are the same species then they will generally produce offspring of the same species that are fertile take two horses a male and a female that each have 64 chromosomes in a diploid cell if they were to have sex and conceive a child assuming no genetic problems occurred then their offspring will also have 64 chromosomes everything works now let's say that a male horse with 64 chromosomes inseminates a female donkey that has 62 chromosomes this can and does happen and can produce an offspring which we commonly refer to as a mule the mule is a hybrid between the two species and ends up with 63 chromosomes in its diploid cells this odd numbered difference is the main reason why this mule is infertile it doesn't match up with the 64 chromosomes of the horse or the 62 chromosomes of the donkey and because it has an odd number of chromosomes the process of meiosis and homologous pair matching is altered odds are that if it tries to reproduce with either it will fail as happens to many hybrid species we have been talking about the great diversity between organisms over this video and the last and as you can tell with all of the different species we see on our planet the process of labeling sorting and identifying organisms can be difficult but let's say that you are strolling through your neighborhood one day and come across a plant that looks really cool and you want to know what species it is or if you are a scientist who has discovered a species you think is new and you want to begin to classify what other organisms it could be related to in either case a dichotomous key can be used the term dichotomy refers to dividing things into two parts that are completely different and in a dichotomous key we are doing that with characteristics of organisms separating them based on their differences until we get to the answer we are looking for which is usually the name of a specific species let's take a look at an example and then talk about a few important characteristics of the design let's say that you found this organism in the wild and you had no idea what it was and you found this very oversimplified dichotomous key to try and figure it out you start by reading the first number one which is separated into two sections A and B A States the organism has wings covered by an exoskeleton and B says that the wings are not covered by an X exoskeleton upon inspecting your specimen you see that there is no exoskeleton covering the wings making statement B true you follow the directions for B and it tells you to go to step three step three a states that the organism has wings pointed out from the side of the body and B states that the wings Point towards the posterior of the body again we can see that the wings are pointed towards the posterior end so we know B is true we follow the directions for B and it tells us that the organism is a house fly we are now done reading the dichotomous key for this specimen as it told us the name of the organism which is what we were seeking at the beginning now this dichotomous key was created for four organisms you can imagine that if you were to use a real dichotomous key for thousands of organisms it would be much more complex than this but with every dichotomous key the rule is there should always be one less step than total organisms to be identified so for this dichotomous key made for four organisms the most efficient and correct way to make the key e would be to have three steps if you had to make a dichotomous key for nine organisms it should only have eight total steps I suggest doing some research on your own to see how amazing and complex these dichotomous keys can get if you find an unknown organism and use a dichotomous key to identify it it might take you some time to help and even surpass this process scientists have other means of identifying organisms which are using environmental DNA and barcodes a DNA barcode describes a common section of DNA that is found within a species and used as a template to compare when sequencing DNA obtaining these DNA samples is where environmental DNA comes into play say that there is a river in your town and you want to know what some of the species are that live in that ecosystem a water sample can be collected which will contain random pieces of DNA that are floating around from dead cells that have fallen off of organisms DNA found in feces or other organisms simply dying and decomposing and release their DNA into the environment we call this Collective DNA that can be randomly found in the environment environmental DNA or e-dna once collected the DNA samples can be sorted out and Amplified through PCR to later be sequenced through sequencing you might have found some DNA barcodes within a database that match with bluegill smallmouth bass and carp telling you that it is highly likely those species can be found within that River ecosystem this technology can also be used to gauge the health of an ecosystem by identifying indicator species some species are extremely sensitive to pollutants and can die off very easily in environments where pollution levels are high while other species can survive in higher levels of pollution being more resilient examples of these organisms include a stone fly nymph which generally only survives in clean ecosystems without pollution and a tube effects worm which can generally withstand a high level of pollution so if you were to collect a sample of e-dna and you see the tube effects DNA is present and stonefly nymphs are not present it might be an indicator that the ecosystem is polluted and requires further testing one of the benefits of using e-dna and species barcoding is that you do not need to actually catch any of the organisms to sample the presence of the species though it does not tell you other important information like the population sizes of any present species every technique has its benefits and downsides so make sure to know how to compare them for the IB exam look [Music]