Evolutionary Differences: Macroevolution versus Microevolution
An ongoing debate in evolution is whether there is a distinction between macroevolution and microevolution, and if so, how to define that difference. Since the advent of molecular biology, with its insight into the molecular details of mutation, there has been a surge of thought that there is no fundamental distinction: evolution is evolution. Understanding what is usually meant by macroevolution and microevolution is important, if only to better follow the debate.
The big picture: Macroevolution
Macroevolution is concerned with evolution on the grand scale, with the branching out of new species and larger groups, like families and phyla. The question is whether there are distinct events and phenomena that control evolution on this scale. The big idea behind macroevolution is that of common descent, the proposition that all life descends from a single ancestor.
Some of the questions addressed by evolutionary biologists concerned with macroevolution are
- Does evolution always occur gradually, or are there periods of intense evolutionary change, alternating with periods of relative stability? The former hypothesis is called gradualism, and the latter is called punctuated equilibrium.
- What were the causes and the evolutionary effects of the mass extinctions that are evident in the history of life from the fossil record?
- What is the relationship between evolution and biodiversity; how does each affect the other over long periods of evolutionary time?
Clearly, these are interrelated types of questions. Macroevolutionists ask big picture questions that cover a lot of ground, seeking big picture answers that address a lot of issues. In any event, the concerns of macroevolution typically don’t descend to levels below that of whole species.
The details: Microevolution
Microevolution is concerned with changes on the small scale, to include changes in the frequency of individual alleles within species and populations. The pioneering inheritance studies of Gregor Mendel were microevolutionary in scope.
Microevolution focuses on smaller groups of organisms living within well-defined habitats and attempts to correlate changes in the habitat with changes in the genetic makeup of the organisms living within it.
The sources of microevolutionary change include
- Mutation: Random alterations to the DNA sequences that make up genes and which are passed from one generation to the next.
- Natural selection: The process by which traits that are favorable in a given environment become more common in subsequent generations, and traits that are less favorable become less common.
- Gene flow: The movement of alleles between different populations of organisms. Immigration and emigration are major sources of gene flow.
- Genetic drift: Changes in the frequency of alleles within a population due to neither natural selection nor gene flow, but from random events.
- Nonrandom mating: A condition which can alter the frequency of alleles within a population due to the fact that mates select their partners based on specific criteria, such as strength or aggressiveness.
Population genetics is a branch of microevolution that attempts to explain and predict changes in allele frequencies by using these statistical models. The results of population genetics can provide useful insights into evolution on the small scale, over shorter periods of time.