Systematics

Systematics, whether based on morphological or molecular data, depends upon the study and interpretation of characters and their states. A character is any observable feature of a taxon, which may differentiate it from other taxa, and the different conditions of a character are called its states. Characters vary in the number of states recognized, and are either binary -having two states, such as presence or absence of wings - or multistate - having more than two states, such as the three states, digitiform, hooked, or rhomboid, for the shape of the epandrial process of the genitalia of Drosophila shown in Fig. 5.5. An attribute is the possession of a particular state of a character; thus a digitiform epandrial process is an attribute of D. mauri-tiana (Fig. 5.5a), whereas a hooked process is an attribute of D. similans (Fig. 5.5b), and a rhomboid one an attribute of D. melanogaster (Fig. 5.5c). The choice of characters and their states depends on their intended use. A diagnostic character state can define a taxon and distinguish it from relatives; ideally it should be unambiguous and, if possible, unique to the taxon. Character states should not be too variable within a taxon if they are to be used for the purposes of diagnosis, classification, and identification. Characters showing variation due to environmental effects are less reliable for use in systematics than those under strong genetic control. For example, in some insects, size-related features may vary depending on the nutrition available to the individual developing insect.

Characters can be classified according to their precision of measurement. A qualitative character has discrete (clearly distinguishable) states, such as the shapes of the epandrial process (described above). A quantitative character has states with values that can be counted or measured, and these can be further distinguished as meristic (countable) traits (e.g. number of antennal segments, or number of setae on a wing vein) versus continuous quantitative in which the measurements of a continuously varying trait (e.g. length or width of a structure) that can be divided into states arbitrarily or by statistical gap coding.

Although the various insect groups (taxa), especially the orders, are fairly well defined based on morphological characters, the phylogenetic relationships among insect taxa are a matter of much conjecture, even at the level of orders. For example, the order Strepsiptera is a discrete group that is recognized easily by its parasitoid life style and the adult male having the fore wings modified as balancing organs (Taxobox 23), yet the identity of its close relatives is not obvious. Stoneflies (Plecoptera) and mayflies (Ephemeroptera) somewhat resemble each other, but this resemblance is superficial and misleading as an indication of relationship. The stoneflies are more closely related to the cockroaches, termites, mantids, earwigs, grasshoppers, crickets, and their allies than to mayflies. Resemblance may not indicate evolutionary relationships. Similarity may derive from being related, but equally it can arise through homoplasy, meaning convergent or parallel evolution of structures either by chance or by selection for similar functions. Only similarity as a result of common ancestry (homology) provides information regarding phylogeny. Two criteria for homology are:

1 similarity in outward appearance, development, composition, and position of features;

2 conjunction: two homologous features (character states) cannot occur simultaneously in the same organism.

A test for homology is congruence (correspondence) with other homologies. Thus if a character state in one species is postulated to be homologous to the state of the same character in another species, then this hypothesis of homology (and shared ancestry) would be better supported if shared states of many more characters were identified in the two species.

In segmented organisms such as insects (section 2.2), features may be repeated on successive segments, for example each thoracic segment has a pair of legs, and the abdominal segments each have a pair of spiracles.

Serial homology refers to the correspondence of an identically derived feature of one segment with the feature on another segment (Chapter 2).

Traditionally, morphology (external anatomy) provided most data upon which insect taxa were described, relationships were reconstructed, and classifications were proposed. Some of the ambiguity and lack of clarity regarding insect relationships was blamed on inherent deficiencies in the phylogenetic information provided by these morphological characters. After investigations of the utility of chromosomes and then differences in electrophoretic mobility of proteins, molecular sequence data from the mitochondrial and the nuclear genomes have become standard for solving many unanswered questions, including those concerning both higher relationships among insect groups and recognition of species. However, molecular data are not foolproof; as with all data sources the signal can be obscured by homoplasy, and there are other problems (discussed below). Nevertheless, with appropriate choice of taxa and genes, molecules do help resolve certain phylogenetic questions that morphology has been unable to answer. This is particularly true for deeper relationships (family or ordinal levels) where, using morphology, it may be difficult or impossible to recognize homologous character states in the taxa being compared, due to independent evolution of unique structures within each lineage. Another source of useful data for inferring the phylogenies of some insect groups derives from the DNA of their specialist bacterial symbionts. For example, the primary endosymbionts (but not the secondary endosymbionts) of aphids, mealybugs, and psyllids co-speciate with their hosts, and bacterial relationships can be used (with caution) to estimate host relationships. Evidently, the preferred approach to estimating phylogenies is a holistic one, using data from as many sources as possible and retaining an awareness that not all similarities are equally informative in revealing phylogenetic pattern.

Beekeeping for Beginners

Beekeeping for Beginners

The information in this book is useful to anyone wanting to start beekeeping as a hobby or a business. It was written for beginners. Those who have never looked into beekeeping, may not understand the meaning of the terminology used by people in the industry. We have tried to overcome the problem by giving explanations. We want you to be able to use this book as a guide in to beekeeping.

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