Biodiversity and natural selection

(slow music) - [Instructor] Where does biodiversity come from? The simple answer to that question is evolution and we could leave it at that. We could all go home because we're done, but that's not going to make much of a video. What we really wanna ask ourselves is what is that? What is evolution and how does it result in biodiversity? I like to think of the study of evolution as following two fairly simply pathways. These paths are pattern and process. Both of these are not only fascinating areas of study, but are crucial in expanding our knowledge of how life originated and how it continues to evolve. The pattern pathway studies the shape of evolution itself by looking at relationships, relationships among organism over time. And to do that, you need to create a diagram or a structure that links these organisms in time showing a branching sequence of relationships much like a family tree or genealogy. These evolutionary trees record not only the relationships among the organisms, but the events that occurred over time that indicate why we think these different organisms are related. Patterns depicted by genealogical trees really are a subject all on their own called phylogenetic systematics. But let's set that aside for a moment. The process path is maybe a slightly better way to start. We want to talk about the mechanisms of evolution. How it actually happens. These are the drivers of the diversity along the multitude of lineages that spring out branching and branching up the tree, up the limbs of the tree of life. Darwin and even some of his predecessors understood this. They could see that things could change. That the pattern of life, this tree, existed, that evolution happened and that the relationships among organisms could be traced by looking at features of those organisms and how they change depending on where they were in the tree. They could see, for example, that the wings of birds, the front legs of mammals and reptiles and, in fact, all the four limbed animals indicated that there was some common relationship there. There was common lineage. But at the same time, you could have change among the branches within those lineages. You could get a change in the front leg to a wing or to a grasping arm. General patterns were evident in everything. But at the time, there wasn't a good understanding of the mechanisms. The processes that could explain how these obviously changing, yet related forms could come about. Darwin and his contemporaries read a lot of stuff about variation, which was visible all around them. It could all be seen. They realized that not all the individuals in a species or even in a population were exact duplicates of each other. This was a surprise to some people, but the evidence was everywhere, even in things as simple as the speed of race horses. If you didn't have variation in how fast horses could run, the races would be pretty boring. Races actually demonstrate how horses were chosen for variations in speed. Humans bred fast horses with each other to get even faster horses and these horses were selected for being the fastest and that's the key word, selection. Darwin thought, hey what if nature worked that way. What if nature selected organisms some how. He noticed that the form and the physiology and the behavior of plants and animals varied within natural populations just as much as they did in domesticated populations of things like horses. Darwin realized that what we're really talking about here are the beginnings of the understanding of the evolutionary mechanism behind evolution, natural selection. Natural selection means that some natural variance, some individuals with different form or physiology or behavior, might be better at getting through life than others. Better that is of gathering food, staying away from predators, turning sunlight into usable energy, resisting wind, having good root systems. In other words, fitting the circumstances of the environment and surviving. What Darwin was really saying is that fitness of an individual meant being better able to produce offspring that had traits like the parent. Traits that would help the offspring be better suited to the conditions of their environment. This has been referred to as survival of the fittest. Actually, I prefer the phrase survival of the fitter because fittest implies that there's an end point, that there's a goal, but there isn't. It's all relative because there are so many compromises and trade offs and being well suited to a place as complex as the natural world that organisms can never reach that perfect match in all respects. This process of the environment selecting variants that are better suited to that environment, no matter how complex is called natural selection. And those traits that make the selected variance better able to survive and reproduce and pass on those traits to future generations are known as adaptations. For example, a wild population of redwood trees might have some individuals that attain greater heights than others and this results in better exposure to sunlight on foggy days enhancing their ability to make food by photosynthesis when a change in the environment such as the fog rolling in, challenges the survival of shorter trees. This in turn not only increases their individual chances for survival, but it also makes available more energy to the taller redwoods to produce more seeds that carry this tallness trait into future generations. So you get natural selection for a tallness trait and an adaptation to an environment that can present changes. Of course, as I mentioned, these simplistic examples kind of skim over the fact that there's always a series of trade offs in nature. We have to consider, for example, that taller trees might have more trouble getting moisture from the roots all the way up to the tips of those highest branches, or that they could be more exposed to storms that could knock them down, or maybe there's some other physiological cause that we might not even have thought of. All these factors are part of a complicated balance that optimizes life to a given environmental situation or set of competing selective factors. Stuff happens. Life is never simple. To me, all these aspects come together to represent the great beauties of life, this constant interplay of processes that results in the complexity of biodiversity, what Darwin called grandeur in this view of life. The flip side of this selection coin is that individuals in a population can also be selected against, because they're less well adapted, sometimes because of susceptibility to diseases or simply by not being good at avoiding being eaten. Something that keeps those individuals from being reproductively successful. You might have noticed by now that there's an important element to this story of variation, selection, and adaptation that's missing here. Darwin noticed it too. He was a very smart guy and he fully recognized that there had to be some way by which organisms could pass on those selected traits, those adaptations to their offspring. It wouldn't work otherwise. There had to be a way that the offspring of individuals that had been selected for could inherit the traits of their successful parents and ancestors. In Darwin's day, there wasn't a good understanding of a mechanism for that. It was only much later that scientists discovered how information is stored in genetic material and passed on to offspring. Today, our detailed understanding of evolutionary processes is built on the discoveries of both Darwin and geneticists. Stepping back now to put it all together, we can see that for all this to work, several different things have to be going on. You have to have variation in nature among the members of a population. You have to have natural forces that can select for or against the enhanced reproduction of individuals who possess certain variations. And you have to have a mechanism by which those selected variations get passed on, inherited by offspring and their future generations. These simple concepts are essentially all you really need for evolution to happen and from these basic principles, we get all the complicated interweavings and interactions among all the factors that become the underlying drivers of Earth's biodiversity. (slow music)