Have you ever wondered why you need a new flu shot every year? The viruses that cause the flu constantly change and evolve. Viruses killed by a vaccine one year will be back the next year with different proteins on their surfaces. The newly evolved viruses are immune to the old vaccine, which is why a new flu shot must be administered. The evolution of viruses is an example of microevolution, which occurs at or below the species level. A species is a group of organisms that share similar characteristics and are capable of producing offspring. Populations and communities are examples of categories below the species level.
According to microevolutionary theory, differences in genotype can cause dramatic changes within a species over time. Small differences between parents and offspring can become increasingly more common in successive generations. Over time, dramatic differences between members of the same species can occur. For example, Chihuahuas and Great Danes are both part of the same species, Canis lupus. However, hundreds of years of breeding for selective genotypes have created significant differences between them.
Have you ever seen pictures of a Tyrannosaurus rex? They look a little like large lizards, but dinosaurs are actually more closely related to birds. It seems unbelievable that the robin in your backyard is a descendent of the mighty T. rex. How did these incredibly large, ferocious animals evolve into small, feathery birds? Macroevolution explores the evolution of organisms above the species level. It is evolution on a grand scale that analyzes the history of life on earth. Unlike microevolution, macroevolution cannot be studied directly. It is possible for scientists to track the changes in a population over a period of several years, but it is not possible to go back in time and see how dinosaurs evolved. Macroevolution is based on fossils, geology, and comparisons to living organisms.
Macroevolutionists study continuous lines of descent called lineages. For example, the evolution of the modern day horse from its earliest known ancestor, the eohippus, is a lineage. There are four main patterns in macroevolution—stasis, character change, speciation, and extinction. Stasis occurs when a lineage remains the same for a long period of time. Organisms do not evolve during stasis. Horseshoe crabs, for example, have almost identical structures to fossils of horseshoe crabs from 250 million years ago. These organisms are showing stasis. Character change occurs when lineages evolve. For example, as the eohippus evolved into the modern day horse, it showed character change.