What is the difference between Microevolution and Macroevolution?

Although the usage of ‘microevolution’ and ‘macroevolution’ are not entirely consistent, in terms of how they are most commonly used the difference between them is straightforward:[1]

• Microevolution is evolutionary change within a species.
  A well-known example of this is the spread of industrial melanism within the peppered moth during the 19th century (and the reversal of this change since the mid 20th century).[2]
  Another would be the changes in beak sizes of some species of Galapagos finches, apparently in response to environmental pressure.[3]
• Macroevolution is evolutionary change above the level of species,
  i.e. when a species changes sufficiently to be called another species (called anagenesis), or one species diversifies into two (or more) which is called speciation.
  An uncontentious example of this would be the diversification of an ancestral finch (from the S. American mainland) as it dispersed among the Galapagos Islands and adapted to various habitats to produce the present range of Galapagos finches.
  A more controversial example would be the presumed evolution of birds from dinosaurs.

Unfortunately, behind this clear façade lies some unclear thinking and wayward reasoning.

First, there are weaknesses to using species as the demarcation between microevolution and macroevolution:

• The definitions of species are subjective, even arbitrary.
• It overrates the significance of species.

Second, there are substantial pitfalls to having such a low threshold for macroevolution, because the same word is also used for evolution on the largest scale.

• At the very least, this loose usage militates against clear thinking about the processes taking place in any particular instance of evolution.
• More seriously, the use of a common word to describe evolution that does not require mutation misleads some into thinking that such evolution also substantiates evolution that does require mutation.

I shall first expand on these points, and then suggest that it would make more sense if ‘macroevolution’ were based not on degree of morphological change but occurrence of new genes.

Failings of linking ‘macroevolution’ to speciation

It makes the meaning of macroevolution subjective and inconsistent, even arbitrary

The definition of species is imprecise or even variable. Although it is commonly said that a species is based on the ability to interbreed, i.e. that a species includes all individuals capable of interbreeding (and producing fertile offspring), in practice most defined species are characterised on the basis of being morphologically distinct i.e. even if they can interbreed with other defined species, especially if the morphologically different populations are also separated geographically (perhaps so that normally they do not interbreed, even though they could).

Segregation into and the definitions of many species are relative, taking into account the number and variety of the group(s) of individuals to be classified, and based to some extent on the subjective view of the classifier, rather than any fundamental basis or other objective criteria.[4,5]

So, given the various - and varying - criteria for defining species, if the definition of macroevolution is based on the formation of species then what this term means will vary depending on the context; i.e. what (e.g. degree of morphological difference) would be considered to constitute macroevolution in some contexts would not in others.

It is anachronistic

Indeed, defining macroevolution in terms of speciation is anachronistic. It stems from the classical idea of the ‘fixity of species’ which arose from Plato’s idealistic view of the world[6] and persisted up to the scientific revolution. Buffon (in the 18th century) illustrated this way of thinking when he said that

… if it were proved … that a single species was ever produced by the degeneration of another ... no bounds could be fixed on the power of Nature.[7]

In other words, so strong had been the hold of the doctrine of the fixity of species in the minds of the natural philosophers that they had perceived each species being demarcated by a ‘species barrier’; and when they identified a significant, even if small, change in the morphology of a species, by breaking through this perceived barrier they felt that this meant there would be no limit to the degree of change that might occur.

And Darwin followed that, believing that the changes occurring through domestic breeding, even though it was known at the time that they were limited, could be extrapolated without limit in nature.

But Buffon, Darwin and their contemporaries were completely unaware of the innate genetic diversity of species which enables substantial changes in morphology to be achieved simply by shuffling and selecting subsets of the available genes i.e. without mutations to provide new genes.

So we need to take on board our modern knowledge and understanding of genetics: that not only significant morphological change but also speciation (as it is generally used) can arise solely through segregating and selecting different subsets of genes from an initial gene pool (e.g. the Galapagos finches). That is, the perceived ‘species barrier’ is not as significant as the early biologists thought; in fact there is no barrier at all.

The point is this: now that we know that the old idea of the fixity of species was wrong - that there is no species barrier, and no particular significance attaches to speciation - we should not fall into the same trap as Buffon and Darwin and think that because a species can change and/or divide into two or more different species that this demonstrates that any amount of evolutionary change is possible.

Unfortunately it suits supporters of evolution to keep the idea that speciation is a big deal - as if it did prove the case for evolution as a whole. And using ‘macroevolution’ for any evolution above the species level suits their purpose.

Failings of such a wide range of meaning for ‘macroevolution’

It does not deal consistently with mutations

Perhaps the most important failing of such a wide meaning for ‘macroevolution’ is that it does not deal consistently or satisfactorily with mutations.

On one hand, by using ‘macroevolution’ to refer to any evolution above the species level there is no doubt that this includes evolution that is due entirely to differential segregation of genes present in an initial population (e.g. Galapagos finches). That is, it clearly includes evolution that does not involve any mutations.

But on the other, ‘macroevolution’ is used universally to refer to the evolution of organisms with substantial new structures - such as eyes, limbs, flowers, and body plans - that would necessarily require (many) new constructive genes (which are presumed to arise by mutation).

Hence, by using macroevolution so broadly it does not distinguish between at least two fundamentally different degrees of evolution.

It obfuscates

Not only does it not distinguish, it militates against clear thinking about what processes are occurring. Indeed, it seems that:

Either many biologists do not actually perceive the substantial difference between (a) evolution that is due solely to the segregation of genes and (b) evolution that requires new genes, and hence do not appreciate the loose way in which they are using macroevolution.

Or they do see it, but are deliberately using ‘macroevolution’ loosely in order to provide at least semantic support - because biological evidence is lacking - for the view that macroevolution is nothing but accumulated microevolution.[8]

Migration

This lack of clear understanding is illustrated - and fostered - by the way evolutionary textbooks treat migration and mutation as equivalent (as sources of genetic variability) when in fact they are fundamentally different.[9]

In this context, migration refers to the situation where one population of a species receives an influx from a group of individuals of the same or similar species. The group of migrating individuals has a somewhat different genetic constitution from the receiving population, so the combined population has greater genetic variability.

Because mutations alter genes - even if only by corrupting them - they, too, increase genetic variability.

So both mutation and migration increase the genetic diversity of a population, and it is in this sense that they are treated as equivalent.

However, there is a clear and fundamental difference between them:

In one case migrants introduce functional genes into the receiving population.

But in the other it cannot be assumed that mutation introduces functional genes. In fact the evidence is quite the opposite: the vast majority of mutations are known to be deleterious, or at best neutral, with very little if any evidence for mutations giving rise to new useful genes.

But it seems this distinction either is not perceived, or mutation is deliberately conflated with migration to try to provide support for the view that mutations do produce new genes.

It misleads

Not only does the use of macroevolution in this broad or loose way militate against clear thinking, it is also likely to mislead. That is, using ‘macroevolution’ to include evolution that involves only the segregation of existing genes, for which there is ample evidence, all too readily gives (or even is deliberately used to give) the misleading impression that there is evidential support for evolution that would require new genes. Indeed some biologists argue that the whole of evolution of life on earth is merely an extrapolation of changes such as in the moths or finches.[10] But this is completely false, needs to be recognised as such, and challenged.

A more meaningful definition of macroevolution?

It will be clear from the above comments that I think a significant distinction exists and should be recognised between evolution that is due solely through the selection and segregation of pre-existing genes, and evolution that involves new genes.

Some will argue that the distinction is of little or no significance - because both are part and parcel of the overall evolutionary process(es). However, for this to be valid would require that new genes are produced at a rate that is comparable with the production of new variations by mixing/segregating existing genes.

But it is evident that there is a huge disparity: whereas new gene combinations (and corresponding morphological variations) are produced (in abundance) every generation, new genes (not merely corrupted ones) arising by mutation are so rare that evolutionists struggle to find examples of them.

In other words it would make much more sense if ‘macroevolution’ were based not so much on degree of morphological change, but on genetic change, specifically the occurrence of useful new genetic material (e.g. new functional genes).