The term “evolution” is commonly thought as the process that leads to diversification of species or to gradual change in certain characteristics of living organisms. However, biologists often distinguish this definition of evolution from evolution that involves genetic changes at the population level.
Macroevolution Explained
The term “macroevolution” is used to describe long-term and large-scale evolutionary changes that result in new taxa (new taxonomic groups) formed from ancestral species. These taxa reflect evolutionary processes that produce speciation (new species), adaptive radiation (diversification of species to new areas), and extinction (loss of species or groups) measured in geological time.
Macroevolution is therefore responsible for the diversity of species that we see today or in the past. Not surprisingly, its legitimacy is reliant on the fossil record which reveals major changes such as toes in horses, mass extinctions, and Cambrian explosions.
Macroevolution, however, is controversial; partly because many creationists believe that although evolutionary change may be possible within species, one species cannot possibly evolve into another species. Some biologists also dispute whether macroevolutionary processes actually exist, because genetic changes that occur within species (= microevolution) provide the same processes that lead to above-species change.
So What is Microevolution Exactly?
Specifically, microevolution refers to changes in gene frequency within a population over the generations. This action is more easily observed than macroevolution. An entire field, called population genetics, is dedicated to analyzing the causes of gene-frequency change.
A readily observed cause of microevolutionary change is selection. As an example, resistant strains of bacteria emerged in just a few generations after the application of the antibiotic, because those bacteria with the resistant gene were favoured (or “selected”).
Differences Between Macro- and Microevolution
The obvious distinction between macro- and microevolution is time and scale – macroevolution is measured in millions of years and at the grand scale of things, whereas microevolution occurs in the lifetimes of individuals in a population and involves comparatively short-term changes.
But what about the processes that govern the two evolutionary actions? Using the previous example, it may be clear that one process of microevolution goes like this: random DNA mutation produces new gene, the gene codes for a heritable trait (e.g. resistant to antibiotics) that is adapted (suited) to the environment, individuals with this trait are selected, and populations (of antibiotic-resistant individuals) evolve.
It may be argued that change in the genetic composition of populations leads to speciation, and therefore macroevolution operates on the same genetic processes as microevolution.
Although macroevolutionary processes are less clear, biologists agree that speciation usually requires an isolating mechanism to prevent the new species interbreeding with the old. The isolating factor can be biological, such as evolution of different sexual apparatus, but it can also be physical as in a mountain range that restricts exchange of individuals.
Furthermore, selection may favour those species characteristics in a lineage (a sequence of species) that are more likely to survive mass extinction events. These characteristics may be very different from those that selection operates on (speciation rather than on lineages) between events of mass extinction. This may explain how mammals survived and diversified after the mass extinction of previously dominant but more mass-extinction susceptible dinosaurs.
Recent work has also associated macroevolutionary processes with genome-wide changes and with mutations in genes that regulate development.
Relationship Between Macroevolution and Microevolution
New species appear possible only after within-species (micro)evolution has separated a population genetically. Further factors, such as isolating mechanisms, may be needed to produce speciation. Later, above-species (macro)evolution may occur; speciation itself lies at the boundary between microevolution and macroevolution.
Readings
Reznick DN & Ricklefs RE (2009) Darwin’s bridge between microevolution and macroevolution. Nature 457: 837-842.
Grantham T. (2007) Is macroevolution more than successive rounds of microevolution? Palaeontology 50: 75–85.
Ridley M. (2003) Evolution. 3rd Ed. Wiley-Blackwell.
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