Macro-evolution and micro-evolution are terms that are sometimes used in a scientific context, but are more often thrown around by creationists to get around some of the instances in which evolution has actually been observed. The basic idea is:
- Macro-evolution is evolution on a large scale.
- Micro-evolution is evolution on a small scale.
Unfortunately, there is no clear dividing line between the "large" and "small" scales being referred to.
A problem of definition
A common creationist claim is that micro-evolution has been observed, but macro-evolution never has. In fact, if any type of evolution is actually observed, it is likely to be called micro-evolution simply because of that fact. Thus, in this sense, the terms have little use beyond anti-evolution apologetics.
In science, to the extent that the terms are used at all:
- Micro-evolution generally means change within a species.
- Macro-evolution generally means change between species.
The word species typically refers to a group of individuals that can interbreed with each other. So, for example, the huge variety of dog breeds that have resulted from hundreds of years of human influence would be an example of micro-evolution, since different breeds of dog can still interbreed (though it may be physically awkward in some cases). However, since dogs and foxes, for example, cannot interbreed, the split of the two species from a common ancestor was an example of macro-evolution.
Does this mean that speciation (a split of one species into two) cannot be directly observed? Certainly not. Speciation has been observed under laboratory conditions: scientists have successfully induced the evolution of new species that cannot breed with the original strains.
Because speciation is an observed phenomenon, creationists like to redefine terms by talking about macro-evolution not in terms of species but in terms of "kinds." But a kind is not a scientific notion, and creationists are notoriously vague when it comes to defining what it means:
- Is a chihuahua the same "kind" as a Saint Bernard?
- How about a chihuahua and a fox?
- How about a wolf and a house cat? (They're both mammals, after all — and even in the same family, carnivora.)
To some extent, the same difficulty arises in biological taxonomy above the level of species (i.e. genus, family, order, class, phylum and kingdom, to use the classical Linnaean system). However, biologists have spent hundreds of years fine-tuning the system so that new species can be fit into the "tree of life" on the appropriate branch to a great degree of precision. The same cannot be said for sort of "Biblical taxonomy" suggested by the word kind.
Creationists do not agree with one another about exactly what "kind" means, and have never proposed a rigorous definition. They only seem to agree on the ad hoc sense that whenever it can be agreed that evolution is solidly proved between two species, they must not be the same kind. This is an example of moving the goalposts.