Microevolution and Macroevolution

The simplest definition of evolution is genetic change within populations. Every population undergoes change in the frequencies of gene variants known as alleles (think ‘short’ vs. “tall” pea plants from Mendel’s experiments that you studied in high school biology). The frequencies can change due to several well known, observed factors:

1. Natural Selection—if an allele enables individuals that possess it to have more offspring on average than individuals possessing other variants of the same gene, it will become more common in the population, at the expense of the other variants.

2. Mutation—mutations feed new alleles into the population all of the time.

3. Chance (also known as ‘genetic drift’)—some alleles never make it into sperm or eggs by chance. Some (actually MOST) sperm never fertilize eggs. Some individuals carrying certain alleles never find mates, or die young by accident. These chance elements can cause allele frequencies to fluctuate over time, especially in very small populations.

4. Emigration/Immigration—individuals entering or leaving a population take/bring their alleles with them, changing the total frequencies of the populations involved.

This kind of change is what most people consider “microevolution”, because it generates relatively little overall phenotypic change in the population, and is limited to the level of the species. The kinds of change reflected in differences between organisms in higher taxonomic categories such as genera (plural for “genus”), families, orders, etc., is usually referred to as “macroevolution”, or evolution above the species level. This distinction has led many to erroneously assume microevolution and macroevolution are two completely separate processes. In reality, microevolutionary processes are behind the origination of the higher taxonomic categories.

Consider a population of a species of organisms, say fruit flies. As long as the flies within that population can mate easily with any other fly, the individuals will remain more or less alike genetically. Mutations or other kinds of variation will circulate throughout the population freely. But what happens if a subgroup is prevented from mating with flies in the main group? For example, what if the subgroup was carried away by a severe storm and deposited on an island? Now if any new mutations or variations arise in the subgroup, they can no longer be shared with the main group, and vice versa. The inevitable result will be that the two populations will diverge genetically—their gene pools will become progressively more and more different. They may even become so different that they cannot interbreed, at which point the subgroup may now be considered a new species. The isolation and differentiation of subgroups from parent populations is part of an ongoing process known as speciation, or the creation of new species. As speciation continues, the two species become founders of new “lineages”, with more new species branching off of each. If one graphs out this speciation process, one sees what Darwin sketched in the only illustration in The Origin of Species: a structure resembling a tree with many branches (lineages) and twigs (species) on the branches. Branches/lineages near the top of the tree are more recent than ones on branches at the bottom. In addition (and this is important), clusters of twigs/species on a particular branch are more closely related to each other than to species on other branches—they will also tend to resemble each other more, and branches that are close together on the trunk are more closely related than those further up or down the tree.

All of the above is the result of purely microevolutionary processes, genetic change (after reproductive isolation) within the populations. However, looking at the tree after a considerable amount of time, and without pruning any branches and twigs, one can begin to see some “macroevolutionary” patterns: lineages that are far apart on the tree will look less and less alike than those closer together. Since distance represents time, one can say the longer lineages have been apart, the more different from each other they will be. The combination of reproductive isolation from other lineages and the accumulation of genetic differences over time can be expected to produce dramatic differences. But here is an interesting thing: if no branches and twigs are pruned, it can be very difficult to decide how to classify the types of organisms—there will be many gradations (‘transitionals’), a sort of fuzzy continuum. A biologist trying to classify all of the organisms would have a very hard time. A question then arises: why does life on earth today appears to have so many clear discontinuities, enabling scientists like Linnaeus to construct a classification system without going (too) insane? Does that mean microevolution cannot explain the macroevolutionary discontinuities, since we do not see all of these theoretical transitional organisms? The answer is ‘no’, because in nature the tree gets pruned by the process of natural selection and extinction. Darwin’s theory states that natural selection will eliminate species which cannot adapt to the environment. In fact, the vast majority of species which have ever existed are extinct: what Linnaeus was looking at are those lineages which are very recent, or older lineages that have proved very successful over long periods of time. As a result, we see a patchy and discontinuous array of organisms that can be classified into higher taxonomic groups because the pesky transitionals which would have made the process almost impossible are extinct. In other words, microevolutionary processes, coupled with natural selection, can explain macroevolutionary differences among organisms on Earth.