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problems for darwin : lecture two

convergent evolution

Homologous evolution refers to the different shapes of, or uses of, structures which were possessed by a common ancestor, in species descended from that common ancestors

Convergent evolution is something that has troubled Darwinians. It may be defined as the appearance of similar traits in different lineages.

For example, different types of animals have similar forms (body plans) in different parts of the world and these are not the body plans of their common ancestor. Jerry A Coyne in Why Evolution is True points out that a number of Australian marsupial mammals closely resemble placental mammals found elsewhere in the world.

Other examples of convergent evolution are the development of similar structures, such as eyes, in different species whose common ancestor did not have that structure. To be clear, eyes seem to have arisen independently in a number of species whose common ancestor, which lived hundreds of millions of years previously, did not have eyes.

Bats, insects, and birds all have wings, but the structure of the wing is quite different. Their common ancestor did not have wings.

What is troubling is that evolution by natural selection is undirected. Yet, if it is undirected how come it tends to finish in the same place?

There is a naïve argument used by unsophisticated Darwinians to explain convergence. It runs “what works in one part of the world works everywhere else”.

This is true, but it misses the point. The argument boils down to “eyes would be useful here, therefore they have evolved here”.

This is called a teleological argument. It conflates purposes and causes. A purpose can not be a cause.

According to the theory of evolution by natural selection natural selection works on the multitude of small, undirected, variations that arise in a population.

The problem that convergence poses for the theory is that eyes, for example, are accepted as improbable events. The challenge for Darwinians has always been to get over the hurdle of irreducible complexity and explain how eyes could have evolved at all, on even one occasion. It really seems that a quite different explanation is needed to explain how eyes evolved independently on a number of occasions.

Consider two species which both evolve eyes. Their common ancestor lived 100 million years previous and did not have eyes. Over the 100 million year interval during which the two species have been reproductively isolated their common genetic heritage from the ancestor has been undergoing variation, at different rates and in different places in each species. At some time in the past both have been in environments where eyes would be an advantage (as would, presumably other adaptations). The problem is that “eyes” are not a one step event. Starting from a genetic base with overlap but tens of millions of years of accumulated difference, the Darwinian has to construct a series of variations, probably different for each species, where each member of the series has a reproductive advantage, and where each series finishes up as an “eye”. The eyes need not be anatomically identical but they need to perform comparable functions.

This is a formidable problem for Darwin’s theory of evolution by natural selection.

A partial explanation is the concept of protein plasticity.

Proteins are objects, and like other objects they have a shape in three dimensions. Protein plasticity refers to two quite different things.

On the one hand, some proteins can only take one shape. In a sense they are quite rigid. Others are plastic in the sense that depending on the local chemical environment (particularly how much water is around and where it is) they can take a variety of shapes.

On the other hand, looking at protein function, sometimes this is very dependent on an exact amino acid sequence (i.e there is very little tolerance for variation in the coding gene sequence). At other times there can be a number of amino acid substitutions with little effect on function.

Plasticity provides some wriggle room within which convergence can occur.

intermediate forms

Darwin’s theory of evolution by natural selection faces the problem of “intermediate forms”. The phrase covers two quite distinct issues.

• The concept of descent with modification from a common ancestor is supported, in general terms, by the fossil record. However the fossil evidence for natural selection as the mechanism for evolution is less clear. The essence of evolution by natural selection is the accumulation of small changes yet the fossil record seems to show species coming into existence fully formed and remaining that way until they disappear from the fossil record. That is, the fossil evidence for “intermediate forms” is weak
• Life today seems locked into a relatively small number of body plans, when a much wider range is expected. Stephen Jay Gould refers to this as the problem of disparity. Channeling is another term used. Within each body plan there is a great deal of diversity, but there are too few body plans. This is the same concept sometimes dealt with as an example of 'convergence' (see above)

These two issues can be described as the problem of stasis and the problem of patchy occupation of morphospace.

The problem of evolutionary stasis is the flip side of the problem of rapid speciation. Together these are known as punctuated equilibrium.

Stasis should not be observed according to Darwin’s theory of evolution by natural selection. Darwinians often try to get around the problem by arguing either that stasis represents “optimal adaptation” or that stasis is to be expected during periods of environmental stability.

The geological time periods during which stasis is observed are not times of environmental constancy. They often span tens of millions of years.

Even if the environmental stability argument were accepted, the Darwinian still faces the problem that Darwin’s theory requires variation to provide continual small changes. There should be evidence of this variation in the fossil record, and it should be seen as a gradual smearing out of the range of morphological features as small variations accumulate.

The reason for this is the Darwinians’ belief that most of the small changes proposed by variation are neutral in terms of their reproductive advantage. Therefore they should accumulate rather than being disposed of by natural selection.

The ‘optimal adaptation’ argument is also flawed. It requires that the small changes proposed by variation are all disposed of by natural selection as they represent deviations from ‘optimality’. However these ‘optimally adapted’ organisms all become extinct, even if later rather than sooner, and often that extinction is through the appearance of a better adapted species.

Darwinians deal with this objection by saying that “optimal adaptation” is something that applies to the best of a particular set of small changes. Another set of small changes means another ‘optimally adapted’ species. So two species with different adaptations can both be “optimally adapted” and one can out-compete the other, driving it to extinction.

Darwin’s theory of evolution by natural selection expects the fossil record appearance of forms intermediate between, say, dinosaurs and birds (also known as transitional fossils) to be rare.

In a Darwinian framework evolution can reasonably be expected to occur at a greatly accelerated rate as new environments are invaded (with many modifications conferring a reproductive advantage) with a consequent narrow window for the appearance of transitional forms in the fossil record. The problem of irreducible complexity remains though.

Much has been made, by opponents of Darwin’s theory of evolution by natural selection, of gaps in the fossil record. Loosely speaking the presence of gaps in combination with stasis suggests Creationism.

A problem with pointing to a gap in the fossil record as evidence against Darwin’s theory of evolution by natural selection is that, with further looking, the missing fossil may be found (as has occurred on multiple occasions) and it is hardly reasonable to respond to the discovery of a transitional fossil by declaring that there are now twice as many gaps!

The problem of the patchy occupation of morphospace is quite different from the problem of stasis.

Morphospace, or the space of possible physical forms, is an abstract concept.

A striking feature of the body plans, and even of the adaptations and enhancements, that occur in nature is their clumping.

Insects have a body form quite distinct form that of spiders, and from vertebrates. Even within the vertebrates there are a very limited number of body plans. There are no intermediate forms between these body plans.

Darwin’s theory of evolution by natural selection has difficulty explaining what appears to be the evolutionary channeling of life into a small set of all the available forms. The basis of the difficulty is that the theory assumes non-directionality of the variability upon which natural selection acts.

Once again, the idea of optimal adaptations is used by Darwinians. According to this application of the concept, morphospace is clumped because the in-between life forms produce fewer offspring, on average, than the life-forms that we observe.

This is the answer that the theory must provide, but is it true? When one looks at the range of environments available to many species is it really true that the body form adopted is the best in each and every part of that range?

One interesting example of clumping is the order Perissodactyla which contained fourteen families in the tertiary period. However, only three of these families – the tapir, the rhinceros, and the equidae (horses, donkeys, and zebra) survive today.

Darwin’s theory of evolution by natural selection provides a nice (but untested) explanation for this loss of disparity, but at the expense of conceding that natural selection takes place at levels higher than that of the organism (so called macroevolution or competition between species, families and other higher levels of the animal kingdom)

The explanation is that natural selection has led to the extinction of the missing species.

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