Skip to 0 minutes and 11 secondsToday we are going to deal with the way how evolution works. The idea here is to understand first the facto of evolution, but mainly the mechanisms that are underlying the process of evolution of living beings. The idea is very simple. First, we have to see there are thousands of proofs that evolution exists; here we have a few of them. Maybe it’s not worth going one by one, but there are thousands of nice proofs. On the way we understand evolution. We see for example that living beings are not independent experimental events but they have a commonality.
Skip to 1 minute and 5 secondsThis is maybe the idea of communality of life, which is one of the most interesting ideas, in the sense that we may see the different species as a variation on a single thing. If we go onto the molecular roots, in the molecular composition of human beings we can see that we share lots of things. When we see the newspaper that a new genome has been done, always the headlines are “Ooh, this new genome, these species have 90 percent similarity with humans” Which is normal, because life is nothing but variations on a single thing.
Skip to 1 minute and 52 secondsThe idea is that evolution is ongoing and we are using it; for example artificial selection is one of the nicest ways in which we see how humans have proceed in making a fast experiment of evolution for example the case of dogs. Dogs have breeds, most of them produced in the last 200 years, and a very strong process of artificial selection. The most interesting and the most powerful idea here is to understand that evolution must imply a change in the genetic material.
Skip to 2 minutes and 33 secondsAnd here came, historically, how evolution and heredity, the two ideas met in what is called the synthesis of the theory of evolution or the synthetic theory of evolution, in which the ideas of Darwin and the ideas of Mendel merged to have a general idea of the change that the species have, but also the change that the underlying genetic material suffers. This idea of looking at the same time at the species and the DNA, looking at this idea of change in the genetic composition is what has been more powerful in the last years.
Skip to 3 minutes and 31 secondsIn fact, in the 20th century we saw this confluence of the genetics, understanding the dynamics of the genome, under what we call population genetics and the fantastic work of Fisher, Haldane, Wright, etc. in which the ideas of evolution, the ideas of heredity merge to have some laws, some theoretical model in which they built a general model of no change, and then we could how the change is understood in the genetic material. There is a very nice book, “Gene”, that is a historical view on the history of genetics. In the 1940s it came what is called the modern synthesis, in which there are three main works, three main books that came out.
Skip to 4 minutes and 32 secondsErnst Mayr as a naturalist, in which the zoology he saw the concept of species is not the concept traditionally held of pure entities but much more dynamics. Paleontology, George Simpson saw the gradualism in the changes. And Dobzhansky, that built population genetics to see how the genetic changes happen. With all these three points, Julian Huxley wrote a book called “The Modern Synthesis” which was the idea of putting together all this knowledge; and finally saw the much quoted phrase of Theodosius Dobzhansky in which he said “nothing in biology makes sense but in the light of evolution”. Because in biology evolution explains why things are the way they are. So we may have a description, or we may have an explanation.
Skip to 5 minutes and 47 secondsThe explanation will always come in terms of evolution. To understand evolution, the idea is very simple. Let’s go into the basics. And the basics is no evolution, no change.
Skip to 6 minutes and 1 secondLet-s say: we could build up a model in which we have a genetic composition and this genetic composition does not change through the generations. Under this model, then, we can see what may happen that may produce the change, and these are the mechanisms of evolution. This is the law of Hardy-Weinberg in which they say “under some conditions” – that we describe in the extended version of this video - “under which conditions there are no changes in the genetic composition but change could come” and this are the forces of evolution. How could evolution proceed? How could change happen? There are many forces.
Skip to 6 minutes and 53 secondsOne, mutation: new alleles, new variants are produced.
Skip to 7 minutes and 2 secondsSecond, natural selection: in which the probabilities of surviving and reproducing depends on the genes.
Skip to 7 minutes and 11 secondsThird, genetic drift: in which we have changes in the genetic composition in small populations. We have migration, people from one place go to the other with different genetic composition. With the model of no change we assume random mating; we assume that we are blind when mating and maybe this is not the case, and also no inbreeding avoidance of mating between related individuals. These are basic mechanisms of change in which we can see how evolution may proceed. Hardy -Weinberg law means no change. So we have to understand how the law doesn’t hold, how genomes may change. So, the interesting point is the study will be feasible from the basic elements of the genome. And this is population genetics.
Skip to 8 minutes and 21 secondsSo the point is; let’s look at the genome, let’s see the changes it has; and then there is always one point, important, is that we have to make the relationship between genotype and phenotype, which is not a simple issue, but we are going to deal with examples in which we know the relationship between genotype and phenotype. Let’s see some of these processes. First, mutation. Mutation is any change in DNA to one generation to the next. Meaning that there are some changes in the DNA in the germ cells. Sperm, eggs, but always in these cells, because the next generation will have the change.
Skip to 9 minutes and 15 secondsIn general we see mutation as a very negative force in the sense in that many times it produces disease, it produces suffering. And this is true. This is true that most of the changes on a very complex system, which is a living being, a human being will produce problems in the mechanism. And this is the reason why most of the mutations are deleterious, produce disease, produce malfunctioning of the system. But at the same time we have to see that mutation is the raw matter for evolution. Everything new, every novelty at its origin appears as a mutation. So all genetic novelties have been produced by the mutation process.
Skip to 10 minutes and 17 secondsWhen you have a change in the DNA sometimes it will produce this change, sometimes it will be totally neutral in the sense that there are many places in the genome – a huge part of the genome – in which mutations do not matter in the sense that are not going to produce any phenotypic change. These are neutral, and the other ones are the ones in which we are going to see how selection may act on them.
Skip to 10 minutes and 45 secondsThe mutation in the DNA sometimes only impacts in a single nucleotide and produces one variant of a single nucleotide; this is what we call a snip single nucleotide polymorphism, and in other cases it's more important, it may be a deletion, an insertion, an inversion of a given fragment and sometimes is much more complex, there are mutations that are really much more complex. Here we have the examples of substitutions, in which we can see the four elementary bases of the DNA, how they may change one by the other. So for example some changes are much more easy to be produced than others, if the two bases are both purines or both pyrimidines, because chemically they are much more similar.
Skip to 11 minutes and 41 secondsSometimes the change of a single base will produce the same protein, sometimes it will change... so this is synonymous, meaning at the end the product is the same, and nonsynonymous when there is some nucleotide change that produces an amino acid change that, in many cases, will produce interesting phenotypic changes. Here we have the example of hemoglobin. Just to see one nice example in which we have the gene that produces the protein called globin, which is the protein that fills up the erythrocytes, the red cells in our blood, in which we have just a single change of one nucleotide that produces a change of a single amino acid. Glutamic changes to Valine.
Skip to 12 minutes and 37 secondsIn this case, what happens is that the properties of hemoglobin changes quite a lot; and in this case people that have this variant that we call “hemoglobin s” will have a disease if they have the two genes in turn. They are homozygous for hemoglobin s they will have a disease produced by this abnormal hemoglobin. But we are going to see later that this same change has also interesting properties because in the heterozygotes they will have resistance to malaria. But this is another issue that we are not going to talk right now. Hemoglobine S is a very nice example in which we see these different properties
Skip to 13 minutes and 35 secondsand the question is: which is the fate of a mutation? Well, it will depend on the phenotypic effect and also on chance, because it could be that buying luck this specific change of a given individual may not be transmitted to the descendance or it may be fully transmitted. So both the phenotypic effect and chance are two very important factors that tell us to which extent the change will expand on Earth in the following generations. The important point is that the production of the mutation is totally random, it does not depend on the phenotypic effect. Selection will come after but the production is totally random. Second, natural selection. Natural selection will come after that.
Skip to 14 minutes and 35 secondsIn this case, what we have is a variation, and this variation produces changes in the chances that individuals have to survive and to reproduce. Here what we have is that natural selection may have a general view, the view we have seen in many TV shows, in which you can see the struggle for survival. We are not going to see natural selection this way, but looking at the variation at the molecular level, which are the variants that are going to be? To give properties that are going to increase with time, and which are the ones that are going to be deleted.
Skip to 15 minutes and 28 secondsSo, in this case we have different DNA sequences that give different properties and in this case, some cases will have the negative or purifying selection, in which the selection are going to produce a decrease or elimination of this allele; or positive, in which we are going to have an increase because it's something that is going to be adaptive. Another mechanism is drift. Drift is just flipping coins, this is chance. When populations are reduced the composition of a generation and next generation may vary just by random it's just a chance process; a very important one when the number of individuals is more and may produce changes.
Skip to 16 minutes and 16 secondsThese changes may be very nicely seen in the simulations, in which we have random changes that we can see in the three slides as being produced with very small, intermediate and large population sizes. We see in the left part, in the first part, that the chances are really... Oscillations are very, very strong, and in this case, in some cases the allele frequency reaches 1 or 0, meaning that the variation is lost. And this is really important because there is no way back, they have to wait that by chance mutation produces an invariant. Drift is a very important mechanism because each population will produce its own process of differentiation in relation to the rest.
Skip to 17 minutes and 9 secondsIn humans, in prehistoric humans, it has been a major factor of differentiation among populations. Migration is another one, in which we have changes in the genetic composition when there is a migration of one population into the other, if the two populations are different and will depend on the proportion of individuals. The more and more different, the more impact migration is going to have. So overall, we can see that this is a general process, and this general processes, we said at the beginning, they show us the commonality.
Skip to 17 minutes and 53 secondsThey show what was already called many years ago as the Tree of Life; meaning that all living forms have a single origin, and that the diversity of lives are just variation on this single thing. All living forms have a common ancestor. Darwin saw that, but molecular data has been very nice and very clear in showing us not only that this is true, but being able to reconstruct this tree of life.
Skip to 18 minutes and 34 secondsSo this tree of life, in which we can see that there is a huge amount of variation, but most of the variation in life is in bacteria and archaea, so meaning that these are in prokaryotic forms of life and in all the cases we can trace back a common ancestor for all forms of life existing now. Overall, in this chapter we have seen how evolution works, how the change has to be seen not only in the shape and physiology of species, but also on the genome. Because of the genome we can model and we can understand the dynamics of the process. Evolution explains why things are the way they are.
Skip to 19 minutes and 30 secondsEvolution -and thanks to the molecular review- now we are able in most cases, to be able to understand the tempo and mode in evolution. The time when things happen and how, which changes, happen in evolution.
How evolution works
The synthesis between evolution and heredity explains the mechanisms: mutation, natural selection, and drift. They lead us to a common origin and the tree of life. Which is the most relevant proof for the existence of evolution?
© Universitat Pompeu Fabra