Biological evolution is defined as any genetic change in a population that is inherited over several generations. These changes may be small or large, noticeable or not so noticeable. In order for an event to be considered an instance of evolution, changes have to occur on the genetic level of a population and be passed on from one generation to the next. This means that the genes, or more specifically, the alleles in the population change and are passed on. These changes are noticed in the phenotypes (expressed physical traits that can be seen) of the population. A change on the genetic level of a population is defined as a small-scale change and is called microevolution. Biological evolution also includes the idea that all of life is connected and can be traced back to one common ancestor. This is called macroevolution. Biological evolution is not defined as simply change over time. Many organisms experience changes over time, such as weight loss or gain. These changes are not considered instances of evolution because they are not genetic changes that can be passed on to the next generation. Evolution is a scientific theory that was proposed by Charles Darwin. A scientific theory gives explanations and predictions for naturally occurring phenomena based on observations and experimentations. This type of theory attempts to explain how events seen in the natural world work. The definition of a scientific theory differs from the common meaning of theory, which is defined as a guess or a supposition about a particular process. In contrast, a good scientific theory must be testable, falsifiable, and substantiated by factual evidence. When it comes to a scientific theory, there is no absolute proof. It’s more a case of confirming the reasonability of accepting a theory as a viable explanation for a particular event.