Since 1905 to the present; focusing on plants

Nov 22, 2006 15:05 GMT  ·  By

Many anti-evolutionists claim that only "micro-evolution" has been factually observed - i.e. only the differentiation within a certain species. In other words, they claim that only the appearance of new "races" has been observed (think of dogs) and not also of new species. However, this is simply false.

The first alleged observation of speciation - the appearance of a new species - actually dates back to 1905 when Hugo de Vries observed an apparent large mutation within a population of evening primroses. Although de Vries' new species is questionable for various reasons (and his theory that evolution works via occasional large mutations is false), other observations and experiments have followed. For a precise genetic definition of species click here.

Speciation via hybrids

The easiest way to produce a new species experimentally, especially in case of plants, is to crossbreed two different species for a sufficient number of times until the resulting hybrid, which is often infertile, suffers a fortunate mutation and turns into another species - different from both original ones.

The first such example dates back to 1912 and involves two species of primroses, Primula verticillata and Primula floribunda. When L. Digby crossed them he usually obtained sterile hybrids. However, sometimes he also obtained a fertile plant, which had double the number of chromosomes of its sterile counterparts. This new species has since become a common garden plant called the Kew primrose, Primula kewensis.

Another similar example involves the hemp nettle. This flower occurs naturally (the scientific name of the species is Galeopsis tetrahit). In 1932 Muntzing speculated that it resulted naturally via the same process by which the Kew primrose was created. More specifically, he claimed that it arose from the crossing of Galeopsis pubescens and Galeopsis speciosa.

To test this idea they have crossed these two plants experimentally and, sure enough, there appeared hybrids that resembled both the visible features and the chromosome morphology of the hemp nettle! The guess has been correct.

Encouraged by these results Frandsen took a longer shot in the 1940s and speculated that Brassica napus may be the result of the crossing of other two types of cabbage: B. oleracea and B. campestris. Once again the experiments supported the idea.

Frandsen also experimented with other species of cabbage and found that B. carinata results from the crossing of B. nigra and B. oleracea, while B. juncea can be recreated by hybridizing B. nigra and B. campestris.

In 1950 Owenby demonstrated that Tragopogon mirus, a flower that lives near Pullman, Washington, was produced by the hybridization of T. dubius and T. porrifolius. In 1989, DNA studies have shown that this type of hybridization has actually appeared in eastern Washington and western Idaho independently at least three times.

Similar experiments have also been performed with ferns but let's try something else.

Speciation via separation

But how about the direct appearance of new species without hybridization? The classic Darwinian mechanism of speciation involves the separation of a population in two groups because of certain factors (the geographical barriers being the most obvious ones). In time, the two groups diverge into different species. By "diverge into different species" it is meant that after a number of generations the individuals in the two groups cannot (and will not) interbreed anymore - if they are brought together again their offspring are sterile. According to this theory, the appearance of "races" is the preliminary step before speciation. So, can this theory be tested experimentally?

In 1969 Pasterniani has taken two varieties ("races") of maize (corn) which are both part of the same species, Zea mays. One variety has white seeds while the other one has yellow seeds. Pasterniani planted both varieties on the same field but at each generation he kept only the seeds from the plants that showed the lowest amount of "hybridization" between the white and yellow varieties. (This isn't really hybridization as it is happening within the same species, but, for the lack of a better word, I use the quotes.) He determined this amount of "hybridization" genetically not just by looking at the plants.

After only five years of such selection, the differences grew dramatically. Pasterniani distributed the two strains homogenously among each other so, if the had interbred at random, they would have done it in 50 percent of cases. At the beginning of the experiment, the white strain naturally interbred with the yellow strain in proportion of 35.8% and the yellow strain in proportion of 46.7%.

After only five years, the naturally occurring interbreeding dropped at 4.9% and 3.4% respectively. In other words, the plants had become so different that they rejected each other. This mimics what happens in nature when two varieties are separated and then, when they rejoin, they are so different that they no longer interbreed - they have become different species.

Natural factors such as geographical separation play the role Pasterniani has played here. But what Pasterniani did was to create a much more powerful kind of selection, one that acted much faster than the natural selection. Why? The effects of his selection showed so quickly, quickly enough for the empirical test of speciation, because he looked directly at genes, while natural selection "looks" at the phenotype. In other words, natural selection selects the genes indirectly, based on the effects they have on the phenotype, while Pasterniani selection worked directly.

Another example that eliminated the artificial selection all together involved the phenomenon of developing a tolerance to toxicity. Some plants within a certain species might develop, via mutation, a tolerance to some toxic factor. This difference doesn't make them different species, no more than people who are lactose tolerant are a different species from those that are lactose intolerant. However, the plants with the tolerance to the toxic factor can live in an environment which is inaccessible to the others - and this can lead to a separation and eventually to speciation. Does this sound far-fetched?

In a study in 1983 Macnair and Christie have described precisely this phenomenon. Some yellow monkey flowers have developed a tolerance to copper. As a consequence they can spread where other yellow monkey flowers cannot. If one looks at the genes of the copper tolerant plants ("Copperopolis") and compares them to the genes of the copper intolerant plants ("Cerig"), one finds very small differences. One would not expect them to be different species.

However, when Macnair and Christie tried to crossbreed the Copperopolis with the Cerig they were surprised to discover that the hybrids were even less than sterile, the hybrids couldn't even survive. Their leaves turned yellow very early in life and they died.

When the scientists looked more carefully at why the hybrids died, they found that the very same gene that offered the copper resistance to Copperopolis was causing havoc inside the hybrids and killed them. This is because a gene doesn't have only one single effect. So, what Macnair and Christie had shown was that speciation can be the outcome of only small genetic changes, involving a small number of genes. Speciation can occur much easier than one would guess.

This of course is what we also know from comparative genetic studies of various animals such chimps and humans or mice and bats. Our ape ancestor that lived a few million years ago didn't have to undergo extensive genetic changes to become a human or a chimp or a bonobo. The only difference is that Macnair and Christie have reported the phenomenon as it happened, rather than a few million years later.

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