Evolution: Biology

Basic Ideas: Speciation

Content

Mutation and selection are basic processes of microevolution that lead to an alteration within the species. The number of species, however, is not increased by this means. But evolution does not only contain change in the course of time, but also the development of diversity, an increase of the number of species. So besides the alteration of species, speciation is a second elementary evolutionary process.

This article explains under which circumstances speciation can take place, which processes occur and at which rate this is possible.

What is speciation?

What happens during speciation?

Founder effect, genetic drift and sympatric speciation

Speed of speciation

Summary

What is speciation?

Speciation means the splitting of a species into two or more species. It requires a geographical or ecological separation of the initial species. In each part of the separated populations the evolutional factors mutation, selection and recombination can show a different effect (pic. 66). If no crossbreeding occurs after a later reoverlapping of the populated areas, the one single former species has been separated into two (see pic. 67).

First, we will examine a well proven example. In southern England new species developed on grounds that were contaminated with heavy metal from slag heaps. Picture 68 describes this process. The slag heaps are contaminated in a way that drifted seeds normally will die very soon or cannot even germinate at all (†). But very few will grow out and bear fruit: They form a new population on new habitat and their complete offspring is able to populate the contaminated slag heaps. It turned out that the slag heap plants hardly crossbreed any more with closely related forms of the surrounding, non-contaminated habitats from which they originally derived. With this genetic isolation took place, which means concerning the criterion of biospecies that new biological species developed (see article concept of species). So until today new biological species can develop under natural circumstances. Does this indicate a beginning macroevolution? The following paragraph deals with this question.

What happens during speciation?

The slag heap plants mentioned above have been examined genetically. It turned out that the plants growing on polluted ground did not acquire any new features but that within these certain species the ability of toxitolerance existed before they populated on contaminated ground. Only species that show toxitolerance on normal locations as well were able to colonize contaminated ground. Toxitolerance is part of the natural variability of these species. (The number of toxitolerant species is too high for being explainable by spontaneous mutations.)

Obviously, there was a selection of already toxitolerant forms. The exclusive (one-sided) selection leads to specialization (see pic. 69), which may be linked to a reduction of the crossbreedability with the normal form on unpolluted ground: Genetic isolation and in the end speciation take place. In this case genetic isolation can be effective for instance in a shift of flowering time.

Here the toxitolerance can probably be explained by a reduced assimilation of mineral salts out of the ground; so it does not present an evolutional improvement, but rather a reduction, that turned out to be of advantage in this specific environment of contaminated ground.

The process observed with slag heap plants shows just as similar cases that speciation cannot be seen as a beginning macroevolution, but is often connected to specialization and with this to an impoverishment of the gene pool. This is why repeated speciation rather leads to an impoverishment of the gene pool and so to a reduction of variability (see pic. 70). Although mutations can counteract the impoverishment of the gene pool, studies show that this at long sight cannot counter the trend to impoverishment. When the splitted species cannot react in a flexible way anymore to environmental changes because of their one-sided specialization, they are threatened with extinction.

We can record that the development of new biological species is a microevolutionary process and does not give any hint of macroevolution.

Founder effect, genetic drift and sympatric speciation

Concerning the classical concept of speciation a geographical separation is necessary for the splitting off of a new species. A small founding group has to be separated from the population: Founder effect. The smaller the founding group the bigger the difference between the average (=alle Allele der Gene einer Art) gene pools. Several genes appear in more than one (=Zustandsformen eines Gens) allele within the gene pool. In a small founding group just a small, non-representative part may remain. This kind of coincidental shift within the allele frequency, not caused by selection, is called genetic drift. So the founding group often starts from a different genetic starting basis than the rest of the population. This means that from the start the conditions for speciation are given. Other mechanisms can lead to further microevolutive changes up to barriers of crossbreeding.

More and more results seem to make speciation probable even without geographical separation. This process is called sympatric speciation (”in the same area”). If for instance parasites that are specialized on a certain host suddenly change this host on long term, they and their offspring are ecologically separated from the starting group and can microevolutionally develop isolated from each other.

Speed of speciation

Several studies have shown, that speciation can happen within few generations, if the output group is genetically polyvalent (various, many sided) and the founding group small and so genetically “one-sided”. The gene pool of a small founding population or the remaining population can rather easily be changed in a thoroughly and lasting way. This connection is based on numerous observations. Recalling the example of the slag heap plants that in several cases populated contaminated slag heaps as a new location, it is obvious that the development of a new biospecies took place within few generations. In this case, the majority of the population falls victim to the selection due to the enormously changed environment.

The following example shows that even morphological changes can lead to new biospecies within very short time. Clarkie shows a loss mutant called bicalyx that has its petals transformed into sepal-like structures (pic. 71; a case of a so called homeotic mutation). This mutant became stably established in the open. This basic type repeatedly shows other mutants as well, that significantly reduce the fertility between mutants and parents – a step towards speciation. In combination with the bicalyx-mutant, this could easily lead to a new biospecies that would be clearly different from the parental species.

Speciation by restriction of the modification pool. This scarcely noticed model of speciation gives further hints on the possibility of quick speciation. It depends on the assumption that the basic forms of living biospecies once had a rather large spectrum of modification (possibilities of adaptation caused by environmental stimuli), that has been restricted more or less by loss mutations or founding eventsin the course of time (pic. 72). Specialized biological species develop through restriction of the original modification pool. This can be seen for instance by the fact, that the offspring can only populate rather specialized locations while the forerunner had a broad spectrum of possible locations. This approach formulated by Lönnig (1993) is especially of interest concerning the concept of basic types in creation science and is examined more closely in the biology of basic types. Because here the speciation is based on losses, it can possibly take place in short periods.

Summary

Speciation is not to be assumed as first steps of macroevolution. The only evident processes are of microevolutional nature. Empiric results indicate, that speciation leads to an impoverishment, specialization and dead end-development (increased danger of extinction) – and are not the beginning of further reaching variation.

Translator: Sarah Aziz, 26.12.2009

Author: Reinhard Junker

 
© 2009