 Think about the amazing diversity of living things on earth ranging from single-celled organisms like bacteria to complex, multicellular organisms like ferns, moths, birds, and humans. Even among a given family or single species, there are widely varying features like color and size. Would it surprise you to learn that all these different groups of organisms are thought to have arisen from a single common ancestor? How exactly did the stunning diversity of life stem from one organism? The answer is evolution. Evolution refers to the change in a population that is inherited over generations. To understand that, we can start with the depiction of the tree of life, also called the phylogenetic tree, which is a way to visualize how different species of organisms are evolutionarily related. The tree of life was constructed based on evidence from fossils, the genetic makeup of different organisms, and a wide range of scientific experiments. On the tree stemming from the common ancestor, different lines represent the organisms, and nodes shall wear two or more groups diverged from each other, forming multiple distinct species. A species refers to a group of organisms that can interbreed with each other. Evolution that happens at the population level, meaning within an individual species, is referred to as microevolution. In contrast, macroevolution is used to describe evolution at or above the individual species level. Let's zoom in to talk about microevolution. Every population contains inherent variation, and there are multiple factors that can influence how frequently a given variant is found in that population. These factors include migration, also known as gene flow, natural selection, mutation, and genetic drift, and they drive microevolution within a population. Let's use the example of how evolution shaped the finches on the Galapagos Islands, which was studied by Charles Darwin back in the mid-1800s, a single population of finches that originally landed on the islands and started living and breeding there. Evolution then, a fraction of those birds flew over to different nearby islands. This migration event led to different frequencies of certain genes on the different islands, which can then persist if the birds on each island breed with each other. Another way evolution can come about is natural selection. The finches on different islands had different beak sizes and shapes. In addition, the islands on the Galapagos have different types of food sources for the birds. Seeds, leaves, and insects. Birds with one size and shape of beak were best able to eat certain foods, so those birds survived best on islands that had that type of food available. This is referred to as having a fitness advantage. Over time, the finches with the beak shape better for eating the corresponding food source bred with each other and thus passed on that trait on through the generations. A clear example of evolution. Another factor that influences the process of evolution is mutation, which refers to changes in the DNA sequence of genes that encode for different traits. If a mutation occurs that results in a new trait and that trait is advantageous to the organism, it will generally provide a fitness advantage. Therefore, that trait is more likely to be passed on to subsequent generations. If the trait is detrimental to the survival or breeding of an individual within a species, it is unlikely to be passed on. For example, a mutation that led to a brilliant red pigment on the birds might be disadvantageous because it would make those finches more visible to predators. And lastly, evolution can occur through random genetic drift, which refers to change in frequency of a gene variant in the population over time by chance, meaning that genetic drift does not take into account whether the gene variant is advantageous or disadvantageous. While these examples we've talked about so far refer to random events driving evolution, there are examples of how human behavior can drive microevolution. For example, in response to pesticide use, some insects have developed resistance to those chemicals, allowing them to survive when exposed to those pesticides. As you've seen, evolution occurs from one generation to the next and is influenced by multiple factors, migration, natural selection, mutation, and genetic drift. Studying evolution allows us to understand the history of life on earth and the remarkable diversity that comes with it, as well as providing a framework for thinking about evolution in future generations.