What is the difference between Microevolution and Macroevolution?

Summary

 

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Usage

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:

• 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).

Another would be the changes in beak sizes of some species of Galapagos finches, apparently in response to environmental pressure. [1]

• 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.

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). This means that the number of defined species is far higher than it would be if all individuals capable of interbreeding were considered to be one species.

Significant in this context is that the segregation into and definitions of many species are relative and based to a large extent on the subjective view of the classifier, and often take into account the number and variety of the (group(s) of) individuals to be classified rather than any fundamental basis or other objective criteria. (Note that I’m not criticising this approach, it probably makes taxonomic sense; but this practice does mean that the threshold for macroevolution is relative rather than absolute.)

So, given the various – and varying – criteria for defining species, if the definition of macroevolution is based on the formation of species (whether by speciation or anagenesis) 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.

Surely it would be preferable for ‘macroevolution’ to have a more consistent and substantial meaning than this?

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 [3] and persisted up to the scientific revolution. Buffon (in the 18th century) illustrated this way of thinking when he wrote:

… if it was once proved … that there was a single instance of one species having been produced by the degeneration of another ... there would be no bounds to the power of nature, and, we might with equal reason suppose, that from one single individual being, in the course of time, she might have produced all the organized bodies which are now spread over the universe. [4]

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 change, even if small, 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. And – crucially – that there is a limit to the morphological changes that can be achieved in this way, and that to go further requires new genes.

Richard Dawkins recognises this:

If anything, selective breeders experience difficulty after a number of generations of successful selective breeding. This is because after some generations of selective breeding the available genetic variation runs out, and we have to wait for new mutations. [5]

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); i.e. that 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 notional 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.

It makes the term 'macroevolution' of little, if any, explanatory value

In his essay on Macroevolution in the Encyclopedia of Evolution Stephen Jay Gould tried to bring some clarity to the discussion by noting that, quite apart from relating macroevolution to any particular taxonomic level (such as species), the term can be used in two substantially different ways: [6]

1. Simply as a descriptive term, i.e. without inference about causes or mechanisms.
   For Gould that meant any evolution above the level of species (he excluded anagenesis), but the principle could apply to higher taxonomic levels.
2. In a causal sense, i.e. to refer to evolution that requires mechanisms other than natural selection acting on variations (or genetic drift) – which he calls Darwinian evolution.

Gould favoured (a), and at first sight it may seem appropriate to opt for a simple descriptive meaning, because it implies an open-mindedness or lack of presumption which is commendable in a scientific context.

However, in fact it makes the term macroevolution redundant and of no explanatory value. What is the point in having a term that means nothing more or less than supra-species? Surely it would be much more useful for macroevolution to be used to describe evolution that requires more than can occur solely by shuffling and selecting subsets of genes.

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

It does not deal consistently with mutations

Which leads to perhaps the most important failing, and I think a key issue – 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 to me 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 (discussed below).

Migration

This lack of clear understanding is illustrated – and fostered – by the way in which evolutionary textbooks treat migration and mutation as equivalent (as sources of genetic diversity) when in fact they are fundamentally different. [7]

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 diversity.

Because mutations alter genes – even if only by corrupting them – they, too, increase genetic diversity. (See here for example(s) of morphological variability introduced by corrupting genes.)

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 (perhaps re-introduce genes that had previously been lost by selection).

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 that either this distinction 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. [8] 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 arising by mutation are so rare that evolutionists struggle to find examples of them. Although mutations are reasonably frequent (typically 1 in a billion per nucleotide per generation), the production of useful /constructive new genes (not merely corrupted ones) is exceedingly rare.

This fact alone should be sufficient to justify using the production of new genes as being a much more meaningful threshold for ‘macroevolution’ than its usual current usage. And all I’m advocating here is that we should reserve ‘macroevolution’ for evolution that has involved the production of new genes. Not only would this make the term more meaningful, it would also promote better scientific scrutiny and understanding of evolutionary processes.

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).

It is also clear that too many biologists do not think clearly about what is happening in any particular instance of evolution, and the use of a common term ‘macroevolution’ to describe a wide range of levels of evolution fosters that unclear thinking.

Unfortunately, I wonder whether the vagueness of use of ‘macroevolution’ is also promoted by some partly in order to discourage clear thinking about what is happening – because clear thinking would highlight the problem of the origin of new genetic material, and thereby expose a fatal flaw in the theory of evolution.

Is macroevolution just accumulated microevolution?

Most biologists probably follow Darwin’s view that evolution proceeds through a long series of small changes each of which can occur readily – not through exceptional large-scale jumps; i.e. that macroevolutionary changes are effected through the successive accumulation of many small (microevolutionary) steps.

Indeed, for some, that macroevolution is reducible to accumulated microevolution is an a priori tenet of evolutionary doctrine. For example, here is Ernst Mayr, according to Gould the ‘prime architect of modern neo-Darwinism’ [9] :

The proponents of the synthetic theory [of evolution] maintain that all evolution is due to the accumulation of small genetic changes, guided by natural selection, and that transspecific evolution is nothing but an extrapolation and magnification of the events that take place within populations and species. [10]

However, we need to examine the evidence, which requires careful consideration of the different processes taking place in evolution (which all too often are conflated):

There are 3 key processes to evolution:

1. The production of genes (which provide genetic variability) – presumed to arise by essentially random (undirected) mutations.
2. The production of variations – by mixing the available genes in the course of reproduction – again, essentially random, though with limitations depending on e.g. the location of genes within the genome.
3. The selection of favourable gene combinations (those associated with favourable phenotypic variations) – which is not random, but probabilistic (statistical) based on generally enhanced survivability and reproductivity of individuals having favourable variations; i.e. this is natural selection.

The evolutionary position regarding microevolution and macroevolution can be summarised as follows:

1. Evolution is substantially true, so all 3 processes are ongoing, more-or-less simultaneously; successive instances of microevolution will accumulate and naturally lead to macroevolution, i.e. microevolution and macroevolution are a continuum.
2. It is generally thought that either a species changes gradually until it is sufficiently different (from some starting point) that it is considered a different species (anagenesis), or different populations of a species diverge gradually until they are sufficiently different from each other to be considered separate species (speciation).
   Either of these may involve segregation of existing genes and/or production of new ones – it does not matter which – so, again, macroevolution will emerge from successive accumulated microevolution.
3. There is ample evidence for (2) and (3) which are dependent on the existence of genes; so it is reckoned /presumed that (1) must also occur.
4. In addition, (1) and (2) are often conflated because both are random (and contrasted with non-random natural selection). Notably by Ernst Mayr who frequently stressed that evolution entails 2 steps:[11]
1. random generation of variations (by mutation and genetic processes, he lumped them together), and
2. non-random selection.

And, as there is ample evidence for (ii) this misleads many into thinking that there is also evidence for (i) – without considering the significant difference between (1) and (2) above.

A key assumption of this widely-accepted view of evolution – which treats evolution due to segregating genes and evolution requiring new genes as comparable – is that new genes will arise reasonably readily and frequently – at a rate that is comparable with the production of new variations by the mixing and segregation of existing genes. Yet, as mentioned above, this is patently not the case.

So it is also clear that the evolutionary dictum that macroevolution is just accumulated microevolution is advocated for essentially ideological rather than empirical reasons. And it is maintained by not looking too carefully at the scientific evidence – specifically, by presenting evolution that is due to mixing and segregating existing genes as if it were evidence for evolution through the acquisition of new genes.

Finally, whether or not the usage of 'macroevolution' changes along the lines I am suggesting, it is incumbent on biologists at least to recognise the different processes that occur in evolution, and to stop using loose usage of 'macroevolution' as a semantic argument to try to extrapolate from evolution that involves only the segregation of existing genes to justify evolution that involves the production of new genetic material.

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Page created February 2017; last revised April 2017.