hello and welcome back to a-level biology help today i'm going to take you through the evolution and speciation section for hoa a-level biology near the end of the video i'll be going through some exam questions and explain their mark schemes and as always there will be timestamps in the comments section so you can skip to the different sections of the video if you do not wish to watch the whole thing right so this is a quick overview of the content that we are going to go over today so we are going to go over variation in organisms the effect of selection on all frequencies allopatric speciation sympatric speciation and then genetic drift and after that we'll go on to some exam questions right so let's get started so variation so there is phenotypic variation within every population which is due to a combination of genetic and environmental factors to phenotypic variation is variation that then makes organisms look different so for example if a cat has lung or short fur which can be due to combinations of genetic and environmental factors the primary source of genetic variation is due to mutation which is which are changes in base sequence of dna if you are not confident with high mutations work go check out my video about meiosis and gene mutations mutations produce different alleles so therefore and the phenotype will vary another source of genetic variation is random fertilization of gametes independent segregation and crossing over which are the steps in meiosis which creates genetic variation these processes produce what we call recombinant alleles so alleles recombine to create new alleles causes of environmental variation can be disease predation and competition in competition the organism that has less well-adapted features is less likely to survive and reproduce we will get onto competition in more detail one on the chapter of populations and ecosystems and also predation so don't worry about those too much now so selection or natural selection has an effect on allele frequencies this is because not all individuals in a population can survive and reproduce due to factors such as disease predation and competition like i mentioned earlier organisms with phenotype providing selective advantage for example giraffes of the larger neck because they can reach the um leaves on higher trees which might be more nutrient dense are more likely to produce more offspring and pass on their favorable alleles to next generation and obviously with these um advantaged organisms the allele frequencies of them will increase over time so a change in allele frequencies over time is called evolution so there are three types of natural selection that you need to know about at a2 at as you only needed to know about two types so i'm just going to go over the first two quickly as you already might know them from a s so the first one is directional selection which is when selection favors one extreme phenotype so as i said earlier a giraffe neck being long which would be an extreme phenotype and it would have this effect on a normal distribution curve so on the x-axis we have the variation in the trait and on the y-axis we have frequency of individuals so if i just get my pen tool out the original population here is denoted by the dotted graph so the um organisms with um the long necks so these will be um about here because the variation is larger and obviously before selection takes place the frequency of these individuals is quite low as this might be due to a newly formed allele by mutation or any of the myotic events that we have mentioned obviously these organisms have an advantage so their allele frequencies increase over time which causes a normal distribution curve to shift in the direction of the favored trait which is normally to the right so results in the increase of the favored allele over time the next type that you have already covered at as is called stabilizing selection which is then selection favors the average phenotype or allele and as i said in my as video about this an example is birth weight of babies so obviously babies which have an average birth weight in the middle are more likely to survive because they will they are less likely to be susceptible to complications and the effect on the normal distribution curve is that the um curve becomes more narrow as um the highest frequency will belong to the average trait so the next type of selection that will be new to you is called disruptive selection disruptive selection is when selection favors both extreme phenotypes or alleles but at different times due to different environmental factors which might be fluctuating and it has this effect on a normal distribution curve so the dotted line here denotes the original population selection favors the both extreme phenotypes so around here and around here so the ideal frequencies of these um phenotypes will increase over time so the evolved popular population would produce this kind of a an m shaped curve because the frequency of individuals of the average phenotype is decreasing and the frequency of individuals with both extreme phenotypes increases over time an example that i like to use to explain disruptive selection is white gray and black rabbits so if you have an environment with both black and white rocks a white rabbit would camouflage against the white rocks so they will survive so the allele frequency for the white colour increases obviously with a gray rabbit they will die to due to predation as they can't camouflage between the black and white rocks because they will be easily seen by predators so this means that they are less likely to survive and reproduce so their allele frequency decreases over time and the effect of the black rabbits is the same as the white rabbits they are camouflaged against the black rocks so they survive so their allele frequency increases so in this case the grey rabbit is the average phenotype so the um evolution of this population would produce the m-shaped distribution curve as i showed you earlier however i'm going to introduce you this idea of speciation now speciation is the evolution of new species from existing ones and these new species arise when genetic differences due to selection lead to an inability of members of populations to interbreed and produce fertile offspring so genetic differences become more more different over time so much so that eventually members of the population they can't interbreed but often they produce sterile offspring said the fertile offspring now there are two types of speciation allopatric and sympatric so first i'm going to take you through allopatric speciation so the first step is that the population becomes geographically separated for example from the formation of a river or a mountain range so as you can see by the diagram down here we have the original population which then becomes split by a geographical barrier such as a river or a mountain range these separated populations are unable to reproduce together or interbreed together due to the physical barrier so they physically cannot reach each other to interbreed the separate populations have different environments or selection pressures a selection pressure is basically something that can influence selection so for example the predators available so this means that two um separated populations will accumulate different beneficial mutations or different beneficial phenotypes are alleles over time to help them survive this leads the separated populations have differential reproductive success so this means that they have different changes in allele frequency to each other so the final step is that the two populations eventually become so genetically different as a result from their different changes in allele frequency so that they can't reproduce to produce fertile offspring offspring and this is called reproductive isolation this term is also key so they are now classed as two different species so now i'm going to talk about sympatric speciation sympatric speciation is when populations become reproductively isolated so they are not geographically geographically isolated like in allopatric speciation which could be due to differences in behavior so a random mutation can influence reproductive behavior for example individuals are fertile at different times of the year due to mutations or different courtship behaviors so um obviously this will mean that there are massive genetic differences because which will lead to the them being fertile different times of the year so obviously the um i love frequencies fluctuate between different phenotypes because of this so this means that particular times of the year individuals will not reproduce together and over time their dna becomes so different so individuals are no longer able to reproduce to produce fertile offspring so our clusters are two different species so sympathetic speciation is when they are just reproductively isolated due to differences in behavior at different times of year caused by um mutations and allopatric is when the populations become geographically isolated which then leads on to reproductive isolation so the last idea that you need to be familiar with is genetic drift genetic drift is changing all your frequency within a population between generations so obviously this will be by chance so by chance a certain allele may be passed on to the offspring just by chance so it's not due to selection pressures so a continual genetic difference to evolution which are changes in allele frequency and therefore phenotypes and genotypes over time now the most crucial point to remember about genetic drift at a level is that it has a larger effect in small populations as changes in allele frequency will not have to take place for as long in small populations as it doesn't take as many generations for the allele frequency to take over the whole population so genetic drift has a larger effect in small populations as allele frequency changes have a larger impact as less generations are required so evolution occurs more rapidly in small populations so here we have an example of genetic drift so one kind of parent and three offspring so as you can see and there is a random annual changes from generation to generation which is not caused by selection pressures so in this offspring you just have these red squares and this one we have a mix and then this one will just have blue right so that is it for the content and now i'm going to get onto some exam style questions so let's look at this first one on islands in the caribbean there are almost 150 species of lizards belonging to the genus anolis scientists believe that these species evolved from two species found on mainland usa explain how the caribbean species could have evolved the first thing to note here is that this is an explained question so you need to justify why you're writing something not just what happens so this question is a big question as it is six marks and it is basically asking you to explain allopatric speciation so it's kind of just like fact recall but applying it to a an unfamiliar situation so this is what i've written the two original species were geographically isolated this led to the inability to interbreed due to a physical barrier variation due to mutation and different selection pressures occurred which led to differential reproductive success so there were changes in allele frequency that eventually led to the inability of the two populations to reproduce to produce fertile offspring so basically here i've just described and explained how allopatric speciation works so let's look at the mark scheme the first marking point is geographical isolation we put that so we'll get the mark notice how the word geographical is underlined here so you must refer to the word geographical to get the mark the second marking point says there are separate gene pools slash no interbreeding slash gene flow between populations we put that there is no interbreedings we would get the mark also it accepts reproductive isolation so you can put this to get the mark as well however it says here this mark should only be awarded in the context of during the process of speciation do not credit if context is after speciation has occurred so if you put the term reproductive isolation after you have written two species arise then you do not get the mark right so the third marking point is variation due to mutation which we put and marking point forces different selection pressures slash different antibiotics biotic conditions slash environment habitats we put different selection pressures so we would get the mark so you need to refer to the effect of the mutations on the populations to get the fourth walking point here it says neutral different conditions slash climates if not qualified so as it says neutral it's technically right but the examiner doesn't really want you to write it as it isn't scientific enough and also it accepts named abiotic biotic conditions so the fifth marking point says differential reproductive success or selected organisms survive and reproduce we wrote about differential reproductive success so we'll get the mark also it accepts person allele such genes to next generation as the equivalent to reproduce so you can put this instead of reproduce to get this fifth marking point the sixth and final marking point says which leads to a change or an increase in allele frequency note noting how the word allele here is underlined so you must refer to allele frequency to get a mark we wrote about increases in allele frequency so we would get the mark so we would get all six marks for this question also note that here it says accept increase in proportion slash percentage of equivalent to frequency right so let's move on to the next question so schizophrenia is a mental illness doctors investigated the relative effects of genetic and environmental factors on development development of schizophrenia they used sets of identical twins and non-identical twins in their investigation at least one twin in each set had developed schizophrenia so here it says identical twins are genetically identical and non-identical twins are not genetically identical the members of each twin pair were raised together so you're probably already thinking this means that all types of twins have the same environment so the table shows a percentage of cases where both twins had developed schizophrenia so in this table we have the type of twin so identical and non-identical and here we have the percentage of cases where both twins had developed schizophrenia so an identical is 50 percent and non-identical at 15 so the first question asks us explain why both types of twins are using this investigation so obviously you need to refer to the fact that identical twins are identical so they have the same genetic factors however non-identical twins have not got the same genetic factors however they have the same environments as well as identical twins so this is what i've suggested so identical twins so genetic influence as they have the same genetic factors and non-identical twins just show the environmental influence so they can give a bigger indication as to what of an effect environmental factors have on schizophrenia so let's look at the mark scheme the first marking point identical twins show genetic influence slash differences between them show environmental influence so you put either one of these to get the mark it says neutral allows a comparison so it's technically right but isn't really specific enough to the question and also it says it must be clear which set of twins is being referred to so you can't just put twins in your answer you need to refer to identical or non-identical twins so that the examiner is clear on what you have written second marking point says non-identical twins also show an environmental or non-genetic influence we wrote about environmental so we'd get all two marks for this question it says also here must be clear which set of twins is being referred to again and do not credit repetition of bullet points in the stem so if you put in your answer the exact same wordings as these bullet points here you do not get any credit so you don't get any marks so let's move on so the next part the question asks what do these data suggest about the relative effects of genetic and environmental factors on the development of schizophrenia so as you can see by the table in identical twins there is a much larger percentage where both twins have developed schizophrenia which suggests a wood that genes play a greater role than environment as non-identical twins have a lower percentage and non-identical twins just share their environment whilst identical twins show their gen share their genetics as well so jeans must play a greater role so if we look at the mark scheme it says genes play a greater role all the environment plays a lesser role notice how these suffix er is underlined here so this means that the question must be comparative as the mark scheme says here so if you just put that genius play a great role your environment plays less wrong you do not get the mark you need to use comparative terms so with the suffix er and also it says neutral genes are involved and involves genes and the environment so these are technically right but they're not they are not scientific enough or specific enough to the question so you might not get a mark if you put those points so let's move on to this question so suggest two factors that the scientists should have taken into account when selecting the twins used in a study so as this is a suggest question it just requires your own ideas or your own application of knowledge so it just says suggesting two factors that the scientists need to take into account so you don't need to provide any explanation so the factors that i've suggested are age as this might affect um cases of schizophrenia due to um different experiences experienced at different age just and the second point is ethnicity as ethnicity can sometimes play a role in your susceptibility to schizophrenia so if we look at the mark scheme it says here any suitable suggestions for a maximum of two marks so here are a range of points that you could have written so age we put this so we will get one mark sex of non-identical twins it says non-identical twins as obviously identical twins will be of the same sex as they are genetically identical family or medical history of mental illness illness as obviously if um someone in their family has schizophrenia then they might be more susceptible to it no use of recreational drugs and this may um lead to a higher risk of schizophrenia and also ethnic origins so ethnicity so we would get all too much of the question as the question says two max if you put more than two points in your answer you do not get more than two marks you can only get a maximum of two in the market scheme also it says here neutral environment as in question stem and neutral uncle unqualified ideas such as health or lifestyle so these are still technically right however they are not specific enough and obviously it says neutral environment as the person wants you to suggest your own ideas right that is all i want to say for this video thank you very much for watching if you have any questions big or small please leave them in the comments i'll be more than happy to answer them and i'll see you in the next video