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Biol 3400: Vascular Plant Taxonomy: Introduction

What do we mean by the variation in the plant world?

The most obvious variation is the morphological variation, that is variation in size, shape, and color. But plants also vary in the chemical compounds that they produce (think how different our plant foods taste), their biosynthetic pathways, and reactions to stimuli. There is also variation in how their embryos develop, how they undergo meiosis, how they protect their leaves from sunburn, what triggers flowering, etc., etc., etc. Taxonomists would like to include all aspects of a plants variation but, in practice, most concentrate on a particular set of characters. Those concerned with fieldwork and identification are apt to stress the variation in morphological and ecological characters; those concerned most with phylogeny currently focus on variation in gene sequences. That is because most of the preferred methods of numerical phylogenetic analysis are not well suited to examination of morphological characters.

What are phylogenetic relationships?

In essence, the ancestor-descendant relationships of groups of organisms. Phylogenetic relationships are similar in concept to genealogical relationships, in that they are the relationships that exist among organisms as a consequence of their ancestor-descendant relationships. In practice, however, we use the term phylogenetic relationships when we are considering relationships among groups of organisms, and genealogical relationships when we are thinking of individuals, particularly human individuals.

There are, of course, a few small problems in trying to determine phylogenetic relationships among plants. For instance, plants do not keep genealogical records. We have to infer their genealogy from the variation we observe. We need to appreciate, however, that many plants have amazing reproductive versatility. Many can reproduce both asexually and sexually; the seeds developing in a single fruit may have more than one paternal parent; hybridization is not uncommon; polyploidy is normal in some groups; asexually reproducers can acquire strange genetic material and, eventually incorporate it into their own chromosome structure. Then, in a rare sexual event, pass it on to others of its kind.

An additional complication of trying to assess the phylogenetic relationships among groups of organisms, such as species or genera is that one must first be confident that one knows what entities belong to the group and that all members of one's group are descended from the same ancestor. If you think that you are working with one species (or one genus) when, in reality, you are working with two, your analyses will either yield very confusing results (possibly the safest kind of results because they will make you look further), suggest the wrong answer, or suggest the right answer - purely by chance.

At this point, let me introduce a new term: Taxon, the plural of which is taxa. A taxon is a taxonomic group [to be defined soon], rank unspecified. It can be used when you do not know the taxonomic rank involved or when the statements being made refer to all taxonomic ranks. It takes much less time to write, and say, 'taxon' than 'taxonomic group' or 'species, genera, and other ranks' so, from here on in, I shall use it.

Some botanists argue that taxonomy is about circumscribing the groups to be used whereas systematics is the study of relationships among groups. I consider this is a false dichotomy and consider both activities as falling within the purview of taxonomy, but I am in the minority so will defer to the majority. Keep in mind, however, that a systematic study cannot be better than the quality of taxonomy used. In this course we shall be concerned primarily with learning to recognize taxa and the processes usually used to determine what constitutes a good plant taxon rather than with the methods used in attempting to elucidate the phylogenetic relationships among taxa. Please note the restriction to plant taxa; the methods used vary with the organisms being studied.

What are criteria can be used to determine whether a plant group is a good TAXON?

A good taxon has predictive value. This means that, if you know to which taxon a plant belongs, you should be able to predict many of its characteristics, including characteristics that were not considered when the taxon was originally described.

Most of the taxa that we work with today were formed by looking at morphological characteristics. In other words, by considering how the plants looked. Plants that looked more like each other than other plants were placed in the same taxon. We still usually take this as a starting point, largely because we are visually oriented organisms. We can quickly assess, and describe, things that we can see. Dogs might place greater emphasis on smell.

This approach sometimes lead to arguments about whether it is more important to be alike in leaf shape or number of stamens. That is when a brand new character, for instance, the ability to produce a particular kind of chemical, may be considered particularly important. If the chemical is common in individuals with a particular leaf shape, but absent from those with different leaf shapes, this would reinforce the notion that plants with the particular leaf shape and chemical constitute a good taxon even if they differ in the number of stamens that they have. It is not proof that these plants form a good taxon, just additional evidence that they do so.

Notice that I introduced a non-morphological character in the above paragraph. Taxonomists often consider non-morphological characters, but most of our work is with morphological characters because, as was stated above, we are visual organisms. It is easy, relatively quick, and inexpensive (often an important consideration) to assess morphological similarity.

Two other characters that are frequently given a lot of weight, particularly by field-oriented taxonomists, are ecology and geographic distribution. If what is thought to be one taxon is found growing in two distinct habitats, for instance beside streams and on dry mountain slopes, one might suspect that two different taxa are involved. Again, it is not proof that there are two different taxa involved, but it should stimulate further study. Similarly, if the same taxon is identified as growing in two widely separated locations (the two sides of North America or in North and South America), it is worth examining more closely whether other evidence supports their inclusion in a single taxon. Disjuncts exist, but it is also possible that some differences are being overlooked.

Why is it possible to construct groups that have predictive value?

In a word, inheritance. Taxa inherit their characteristics from their ancestors. There are some mutations, both gain and loss, and chromosomal re-arrangements that can lead to changes in morphology, biochemistry, physiological (and hence ecological) abilities, but these changes occur in a pre-existing genetic make-up. So, just as children are generally more like their parents and close relations more like each other than distant relations, so species are usually more like their ancestral and closely related species than distantly related species. Yes, this is a simplistic statement, but it is a good place to start.

CHARACTERS

I have used the term 'character' fairly frequently in the above paragraphs. It is worth spending some time considering what a character is, and what it is not. Scientists do tend to use it in different ways. If you are aware of the different meanings that it may have, you will find it easier to determine how a particular author is using it.

A character is a feature that can be measured, counted, described, or otherwise expressed. It is an abstract entity. Petal color is a character. Plant height is a character. Position 33 from the end of a gene is a character. Red is not a character; it is a character state.

Characters have states. Red could be a character state for the character petal color; 3 cm could be a state for the character plant height 3 cm; cytosine is a possible character state for position 33 on a gene.

In taxonomy, some characters are more equal than other characters, but which characters these are varies from group to group. First of all, one needs to decide if a character is a good taxonomic character for the plants that one is studying. What makes a character a good taxonomic character? This means a character that is useful in determining to which group a plant belongs. Davis and Heywood (1973) suggested four criteria:

1. The character varies less within putative groups than between them. If this is not the case, either the character is not useful or the groups are bad.

2. The character is genetically determined but does not have a high intrinsic genetic variability. For instance, if the offspring of the same pair of parents can have different states for the character, the character is not a taxonomically useful.

3. The expression of the character is not significantly modified by the environment.

4. The pattern of variation in the character being examined correlates with the pattern of variation of other characters.

Davis and Heywood point out, circularity in taxonomy; it is inherent in criteria 1 and 4. But spiral staircases are circular; they are still an effective means (though somewhat giddymaking) of moving from one floor to another. Circularity in taxonomy is of the spiral staircase kind.

Good for what?

The above discussion refers to taxonomic characters. Characters are also used for other purposes than circumscribing taxa, for instance for identification, diagnosis, and description. What qualities would you look for in characters used for these purposes?

RANK

Everyone realizes that some plants are more alike than others. I suspect that most of you can distinguish a pine from other conifers. This statement implies that you can recognize two levels of grouping - one that consists of different kinds of pine and then a more inclusive group, conifers, that includes pines plus many other kinds of conifer. One way to recognize such differences in degrees of similarity is to recognize groups as having different ranks. Taxa at the lowest ranks consist of many plants that are very alike; taxa at the higher ranks include more plants but these plants do not have as much in common with each other as the plants within taxa at the lower ranks.

The highest rank recognized nowadays is the Domain. Plants belong to the Eukaryote Domain, as do humans. Below this comes the kingdom. Plants belong to the kingdom of plants - surprise! Humans belong to the kingdom of animals. The kingdom of plants is now restricted to such things as flowering plants, gymnosperms, mosses, liverworts, and green algae. It used to include fungi and all the algae. There are then additional ranks until one gets to the lowest formal rank, one that is rarely ever used but is theoretically possible, a subform.

The rank to which a taxon belongs determines how its name is structured. For more on this, see botanical nomenclature.

AN IMPORTANT POINT

Taxonomy as we know it is a human endeavor. It represents our attempt to make sense of the biological diversity around us. Over the centuries it has been found that certain approaches to achieving this understanding are more productive than others. These approaches require an understanding of biological principles, but they also help us acquire a better understanding of biology. It is in this sense that taxonomy can be considered a science. It is, however, a science in which two individuals faced with the same information may draw different conclusions as to how much variation should be included in the same taxon. Taxonomists have to accept diversity of opinion as a fact of life.

You must also recognize that no one gave plants a set of instructions as to how they should behave, how they should restrict their reproductive endeavors, and how variable they could be. During the semester, you will become aware that plants are remarkable for their versatility and plasticity. This frequently frustrates the efforts of humans to place them in neat and tidy boxes. It is, however, these abilities that enables plants to grow in so many different habitats and to survive extremes of climate even though they cannot run away and hide in a cave or climate-controlled construction (aka a house).