In this lecture, I will discuss the concept of genetic drift which is another important evolutionary mechanism in addition to natural selection that explains descent with modification. I will discuss how genetic drift and natural selection can result in speciation and I will finish with the overarching idea of common descent. So natural selection results in adaptive traits that make an organism well equipped to survive and reproduce in that environment. But it is not true that every trait in an organism must be advantageous and disadaptive. In fact, we observe that many mutations and resulting genetic variations are neutral, and have no impact on an organism's survival and reproduction. This is partly due to the fact that a genetic code is redundant, this means that not all changes at the level of the DNA do cause changes in the amino acid coating of proteins. Also, not all phenotypic traits appear to affect individual fitness. For example, variation in body hair in humans may have little effect on survival and reproduction of that individual. Although some of you may want to disagree and do not want to date men with much hair. In any case, such neutral or almost neutral variations accumulate naturally over successive generations by a mechanism that is called genetic drift. The strength and effects of genetic drifts are far more noticeable in smaller populations. So how does genetic drift work? Genetic drift is a random process, it is a chance process and has several phases. And the first phase of genetic drift is related to breeding numbers. We need to realize that not all individuals in a population reproduce. Or because parents produce one or only a few offspring. So if parents produce only one or a few offspring by chance, not all their different alleles will be passed on. We often use the analogy of a bottle that is filled with equal numbers of blue and red marbles. Suppose these marbles represent gametes of both parents. If these parents can only produce one offspring, you're only allowed to draw two marbles to make up the genotype of that offspring. Then there is a 50% chance that you will have two marbles of the same color and hence lose the other color for the future. If you allow these parents to produce more than one offspring, there's more chance that there is, that both colors will be present in the next generation. You can use the same logic if only few individuals of a population reproduce. The smaller the breeding population, the higher the chances are that, that population will lose genetic variation. This is one of the major concerns in conservation biology. Because small populations do harbor little genetic variation that natural selection can act upon when the environment changes. Such populations are said to have little evolutionary potential, and cannot easily adapt to a changing environment. The second phase of genetic drift is related to the unpredictability of the environment. This resembles more or less the reliability of our weather forecast. Regardless how fit or how well adaptive you are in your normal environment. Your population can be hit by natural disasters such as flooding, volcanic eruptions, winter storms which are totally unpredictable. And these processes can cut the life or many lives short irrespective of their traits and only sheer luck will allow individuals to survive. This can also cause major shifts in heritable trait variation that have little to do with natural selection. So both aspects of genetic drift can counteract the effects of natural selection, and especially where natural selection is weak and populations are small. Two more comments that I want to make about traits before I move on to the concept of dissent with modification and common dissent. The first one concerns the adaptive trait. So when it comes to adaptive traits, there is no one-size-fits-all. For example, bulk body size is important in the male walrus to dominate its male rival,s but it would be disadvantageous in males of tree swinging spider monkeys. It is no accident that a walrus is bulky and a spider monkey has a slender body. These traits have helped them to operate in their respective environments and ways of life to survive and compete for reproduction. The second remark is about neutral traits. It is not to say that all traits, or all neutral variation in traits, will be neutral indefinitely. If the environment changes, the neutral variant traits can become important for individual fitness and thus becomes subject to natural selection. Now, I want to turn to the second observation we can make on life on Earth, and that is the overwhelming diversity in life forms and species witnessed on Earth. We have discussed the Drenim genetic mutations generates essential traits variation among individuals. That natural selection and genetic drift act upon and causes the species to change of evolutionary time. So this is the concept of the census modification. You observe effect between two parents have offspring. The offspring will look and behave slightly different from their parents. And also slightly different from each other. Their descent from the parents was modification, and the differences among the offspring are partly due to these random genetic mutation and partly due to recombination, which I have not discussed. But this explains the changes in traits that we observe in individuals that belong to the same species. The question is, can it also explain the species richness? And this leads to the logical question, how new species arise? This question is our fascination also because it is difficult to observe the beginning and the end of speciation event in a human life. The theory of evolution provides an explanation for the process of speciation and provides answer to the question, why there are so many species? The principle, the process of speciation can be very simple. If members of a species become geographically isolated, each group may have to respond to different environments and different predators, and adapt in a different way to gathering food. And genetic variants will no longer be able to breed with the whole population, but only with members within each group. In this way, genetic drift and natural selection can lead to distinct populations which after given a certain amount of time will be no longer closely related enough to interbreed. And we now call these two groups different species. Finally, I want to discuss another evolutionary concept, the overarching idea of common descent. A common descent is the idea that all living things on Earth are related. They descended from a common ancestor through the process of descent with modification of many, many generation. The common descent of living organism on Earth is not direct observable effect. We have no way of going back in time. Instead, common descent is conclusion based on a massive collection effect. These effects are found independently in a study of fossils that show ancestors and transitions of traits over long time series. It is found into genetic of organisms. Almost all organism use DNA as the carrier of the information of life from one generation to next generation, it is found in comparative anatomy. It shows for example, that morphological differences have a shared internal anatomy. Again, evidence for common descent. So let's recap what we have discussed in this lecture. Genetic drift is a second evolutionary process that is responsible for the accumulation of small changes over many generations. Resulting in changes in changes in traits in populations. Genetic drift can also cause random changes in adaptive traits. And thereby oppose the effects of natural selection. Descent with modification is the accumulation of all differences of many generations from ancestral to modern life forms. And there's overwhelming evidence that all life on Earth is related and have common ancestors. So we do not know what the first life form was or exactly how it became to be. But the simple processes of natural selection and genetic drift acting on random genetic mutations and the resulting variation inheritable traits of 4.5 billion of years looks to be responsible for the diversity of life that we see today on Earth.
