Clear Boundaries
A donkey is a donkey, and a horse is a horse. When the two father offspring together, man has helped. Because in the animal world, according to the widespread opinion, natural hybrids are rather rare. Plants, on the other hand, are much more interested in infidelities with relatives, which is also a common impression. Both are wrong.
Fourth semester, Monday afternoon, exercise "Plant Sociology". We are standing in a nice little grove with various identification books under our arms, with which we are trying to track down the willow species. A few weeks later the same game, only this time we have blackberries in front of our noses in a forest clearing. After an hour and a half of frustration, despite explicit special literature and help, at least one thing is clear to many of us: We prefer to leave it to the experts and those in the group who want to become it to identify willows and blackberries at species level.
In addition to the debate about leaf shape and color, the old discussion often flared up as to how justified the species status is for some willows and blackberries if they produce common, possibly self-fertile offspring with other species - and thus clearly exceeds these limits of biological species definition. Can it still be considered a separate species?
Some researchers go even further: In their eyes, the concept of biological species does not work at all in the realm of botanists. Not only does hybridization seem to be far more widespread in the plant world than in animals. Even asexually reproducing specimens or representatives that can hardly be distinguished externally, but are clearly distinguishable genetically through duplication of the genome, also do not obey the rules of the reproductive community that is isolated from others with a multiple generation guarantee.
But appearances are deceptive, say Loren Rieseberg, Troy Wood and Eric Baack from Indiana University in Bloomington. The scientists approached the problem from the statistical side and first consulted the literature on hybridization in plants. They only chose works by numerical taxonomists who try to determine species by comparing as many features as possible, especially external ones: What matches belongs in one pot, what differs in another.
From the 218 studies tracked down, the scientists extracted all possible information on lifestyle and forms of reproduction as well as systematic classification. They determined the extent to which there are groups within different plant genera - which consist of several species - that do not differ in appearance. According to numerical taxonomy, such clusters would then have to be regarded as independent species.
In fact, they found such clusters in more than eighty percent of the described plant genera-but only about half of these groups had species rank. The numbers were similar for the animals: the researchers found corresponding clusters for almost ninety percent of the genera, but only half of the clusters were found as species.
Responsible for most of the misjudgments was apparently the urge of overzealous taxonomists to assign a species status to small differences rather than allowing a certain variation within a species, conclude Rieseberg, Wood and Baack from their data analysis. In addition, asexuality and polyploidy played a crucial role in the mismatch between physical similarity or difference and species status. Curiously, however, the propensity to hybridize did not affect taxonomic problems.
To avoid accusations that the amount of data was too small, the scientists repeated their analysis with almost 900 animal and plant species for which they had information on both their external appearance and hybridisation. And they found that those who do not reproduce together also differ significantly in appearance in three quarters of the cases - in plants. The authors are of the opinion that the assessment of the species, which is based purely on its external appearance, is on a solid footing.
Botanists are overly influenced by some 'botanical horror stories' like dandelions, blackberries or oak trees
(Rieseberg et al.) Existing plant species turned out to be more clearly independent, reproductively isolated units than animals – namely, not even forty percent of the groups examined sealed themselves off from related species. Ferns showed the lowest tendency to hybridize, while birds still mixed with each other the most and in turn produced fertile offspring. Contrary to previous views, hybridization would be more of a zoological than a botanical problem. 'Botanists are overly influenced by some 'botanical horror stories' like dandelions, blackberries or oak trees,' the scientists conclude. And they didn't even make up one percent of the whole.
However, it remains to be seen whether these results will stand up to some critical scrutiny. The scientists themselves point out that although numerical taxonomy was very popular a few years ago, it has long since been replaced by more modern methods. It should also not be forgotten that the investigative approach of demonstrating reproductive independence through a lack of hybrids does not take into account the question of isolation mechanisms that have an effect in advance. In addition, the researchers could not use this method to analyze the problem of whether there is sufficient gene exchange between potential plant mating partners growing at a distance. And of course the question still remains unsolved as to why a species should be en titled to this rank if it can produce fertile offspring with other species - anyone who consistently applies the biological species concept here would actually have to deny it this independence.
But at least the impression that everyone in the plant world willingly produces fertile offspring at any time is probably too distorted by individual examples and therefore wrong: blackberries, willows and co are not the rule, but the exception. But the lecturer comforted us with that years ago.
