An introductory/summary video will cover all aspects of the upper-level topic of evolution - this video forms the second part. There is no doubt that natural selection is in many cases the central evolutionary mechanism that causes evolutionary change - but even Darwin recognized that it is not the only process that drives evolution. In addition to selection, the evolutionary mechanisms (forces that lead to a change in the genetic structure of a population) mutation, gene flow, genetic drift and nonrandom mating affect the genetic composition of populations. Remember that we defined evolution as a process in which the frequency of gene variants changes. Before evolutionary factors can change the frequency of gene variants, gene variants must first arise. In this context, mutations can play a crucial role: they create genetic variability in a population. These random changes in the DNA sequence form the raw material of evolution - because variations in the gene pool of a population also cause its phenotypes to vary and natural selection can act on them. In the few cases in which a mutation has a beneficial effect on its carrier, the frequency of this mutation increases. But even previously neutral or disadvantageous alleles can suddenly have a beneficial effect when environmental conditions change. Viruses and bacteria have particularly high mutation rates - so it is no wonder that evolution is already effective here in short periods of time. If selection favors different characteristics, this can lead to two completely different lines emerging from the same ancestor and ultimately new species emerging. A species can be defined as a group of populations that form a reproductive community and are reproductively isolated from other species. They can therefore only crossbreed or reproduce with each other, but not with other species. It is therefore clear for all speciation processes that the result is at least two species that are reproductively isolated from each other. The path to this point differs significantly, however - a distinction must be made between allopatric, sympatric and parapatric speciation. Allopatric speciation occurs when a population is split up or separated by a physical barrier. After a population has been split up by a geographical barrier, the subpopulations develop differently for various reasons. However, it is usually different environmental conditions and thus different selection pressures to which the subpopulations have adapted in the course of evolution. The emergence of a new species occurs gradually. Subspecies initially evolve through mutations, i.e. changes in the genetic material. Although they already differ, e.g. morphologically (in their external appearance), they can still reproduce with each other. Further separate development ultimately also causes genetic isolation. If the barrier is removed, the populations can repopulate the area in between and form mixed populations, but can no longer interbreed. In contrast, sympatric speciation occurs without physical barriers. However, speciation without physical isolation only occurs under special circumstances - and is rarely observed in nature. One example of how sympatric speciation can occur is through disruptive selection. Here, the common forms of a population are exposed to selection pressure, whereas individuals with extreme characteristics have advantages. Parapatric speciation is seen as a middle ground between allopatric and sympatric speciation. Two subpopulations populate distribution areas that are geographically adjacent to one another. In contrast to allopatric speciation, the process of speciation does not take place in two spatially completely separate areas, but also not in the same area as in sympatric speciation. Here, too, disruptive, splitting selection is at work due to different environmental conditions, as is explained in more detail in the corresponding video. You can also find this table with the most important differences between the various forms of speciation in this video. Millions of different species have emerged from a single ancestor through speciation events over the course of evolution. Family trees are a graphical representation that provides a tool for showing evolutionary relationships. In the video about phylogeny, we will look at how to analyze such family trees. All the best and feel free to subscribe :p