Sytematics is commonly defined as the study of biological diversity and the relationships among organisms. Taxonomy, that component of systematics specifically focusing on the theory and practice of classification, is not clearly separable and both are frequently used interchangeably by biologists. This chapter will first briefly review the importance of systematics to the pest management enterprise and then summarize the use of biological characteristics in systematic research. Entomophagous insects will be stressed due to their significant role in pest management and because the systematics of these groups have frequently employed biological characteristics. A distinction is often made in systematics between higher classification and species classification. It will also be useful here to discuss the role of biological characters at these two levels.
Systematics plays a central role in biology by providing the means for characterizing the organisms that we study. Through the production of classifications that reflect evolutionary relationships it also allows predictions and testable hypotheses. By allowing taxa (taxonomic groups) to be correctly identified classifications provide a key to the literature and a means for organizing information (Danks 1988). Our ability to predict biological characteristics is a particularly valuable product of classifications that often is taken for granted. For example, it is only because of robust classifications that we can assume with reasonable assurance that any wasp of the families Trichogrammatidae or Scelionidae is an insect egg parasite, that a species of Pipunculidae (Diptera) is a parasite of Homoptera, that a wasp identified as a Signiphoridae is likely to be a hyperparasite via another species of chalcidoid, and that an adult blister beetle of the genus Epicauta is phytophagous but has a larval stage which feeds on grasshopper eggs. The importance of sound systematics in pest management is obvious. Pest species and their natural enemies must be correctly identified before adequate control measures can be contemplated. Armed with an identification we automatically know something about the biology and distribution of the pest organism.
Characters and Their Use in Systematics
Taxonomic characters have been variously defined, but for our purpose we can consider them as attributes of a taxon that allow its differentiation or potential differentiation from others. Characters or traits used in taxonomy are hypothesized as being under genetic control although this is rarely tested directly. Characters are used to construct classifications and to identify the taxa which classifications recognize. A character useful for identification is not necessarily useful for constructing a classification, and vice-versa. Taxonomic characters can be conveniently categorized as morphological, physiological, molecular, ecological, reproductive and behavioral. For our purposes “biological characters” will specifically refer to the last three categories. Reproductive characters are herein restricted to mode of reproduction and reproductive compatibility.
The vast majority of classifications and keys for identification are based on morphological characters. This is not because they are inherently better for systematics but because they are more easily observed and evaluated for variation. The other kinds of characters often require expensive equipment, live material and they are more difficult to voucher. Estimating variation in particular is not a trivial matter with non-morphological characters. Given reasonably complete collections in museums, it is relatively straightforward to determine if a morphological character occurs throughout the range of a species, in all or only a subset of the known species of a genus, in all or only a subset of the known genera in a family, etc.
Use of biological characters in higher classification
Biological traits have frequently been used at all levels in higher classification, i.e. for hypothesizing phylogenetic relationships among recognized species and groups of species. Behavioral and ecological characters have been particularly popular. For obvious reasons, reproductive characters are most commonly employed in the formulation of species concepts.
In many cases, taxa are grouped informally on the basis of biological traits simply for convenience or heuristic reasons. For example, dividing the parasitic Hymenoptera into idiobionts (host killed immediately or soon after oviposition) and koinobionts (host allowed to develop a period of time after oviposition) are useful artificial groupings not intended to imply relationship. In other cases, however, biological traits have inappropriately been considered of special importance and weighted as indicators of natural groupings without attention to contradictory evidence from other characters. Thus the parasitoid families Rhipiphoridae and Meloidae (Coleoptera), and in some cases the Strepsiptera as well, have been placed together primarily because of behavioral and developmental similarities (all have phoretic first instar larvae and hypermetamorphic development). These groupings cannot be justified once other features are considered and the similarities must be attributed to convergent evolution rather than homology (similarities derived from a common ancestor).
Use of biological characters at the species level
The taxonomic use of biological characters is more common at the species level. Rather than being used to show relationship, the search at this level is for species-specific traits that allow separation or identification. Such characters are employed quite frequently in entomophagous groups of insects because (1) such data often accumulate in the process of biological control studies, and (2) many taxa are morphologically conservative, requiring other character sources for optimal characterization and identification. Most modern hypotheses of species in bisexual animal groups follow the biological species concept and carry the assumption of reproductive isolation from closely related species. Species that are reproductively isolated but morphologically identical or nearly so are referred to as cryptic species. Biological characters most frequently used to help distinguish cryptic species of entomophagous insects are ecological, behavioral and reproductive.
The infraspecific level
The only infraspecific category officially recognized by zoological nomenclature is subspecies. However, there are a number of other informal categories below species that have been used in entomology. Included are biological race, host race, strain, semispecies, and biotype. The definition of most infraspecific categories is not without controversy but they are quite frequently used nonetheless, especially in biological control as investigators try to distinguish among conspecific populations that differ in attributes potentially critical to successful pest management. We can return to the walnut aphid parasite Trioxys pallidus as an example of the potential importance of intraspecific variation in pest suppression. This wasp, originally introduced from Europe to the west coast of North America successfully parasitized walnut aphid but not the filbert aphid. Additional exploration in Europe resulted in importation of an additional race which did attack the filbert aphid and led to its successful control (Unruh and Messing 1993). Although such populations do not receive formal taxonomic recognition and the appropriate category to utilize is not always clear (host race?), the variation identified is of considerable importance. Also, the taxonomic tools used to discover and document such intraspecific variation generally are the same as those employed at the species level.
Appropriate use of biological characters in systematics
As indicated above, there is no inherent difference in the value of morphological and biological characters. Both, if properly used, will express genetic differences that can be of considerable value to classifications and identification. To reiterate however, the word of caution is that intrataxon variation for biological characters is considerably more difficult to assess for logistical reasons. Consequently there is always the danger of basing taxonomic decisions on too little data. For example, simply because a small number of cultures in the laboratory are reproductively incompatible does not necessarily imply that compatibility is all or nothing in nature. We may find that intermediate linking populations occur that defy easy categorization (Pinto and Stouthamer 1994). The problem is compounded when we are dealing with differences discovered in allopatric populations since in this case simple geographic variation cannot be easily discounted without further study. Thus biological characters should be assessed in as many populations as possible before using them in systematics. Ideally, it will be possible to correlate the new biological characters with one or more morphological differences that themselves can then be more widely evaluated for variation.
1. Importance of Systematics
A. Understanding Biodiversity: Systematics helps in identifying and classifying organisms, ensuring a structured understanding of biodiversity. It allows scientists to catalog and describe species, preventing duplication and confusion.
B. Evolutionary Relationships: Helps in reconstructing the evolutionary history of organisms using phylogenetic trees. Provides insights into how species evolved from common ancestors.
C. Conservation Biology: Assists in identifying endangered species and their evolutionary significance. Helps in prioritizing conservation efforts based on genetic diversity and ecosystem roles.
D. Agricultural and Industrial Benefits: Systematics aids in identifying beneficial crops and pest species. Helps in improving plant breeding, pest control, and sustainable agriculture.
E. Medicine and Public Health: Systematic classification of microbes helps in disease diagnosis and treatment. Understanding evolutionary relationships helps in developing vaccines and antibiotics.
F. Ecological Research: Aids in studying ecosystems, food webs, and species interactions. Helps in identifying invasive species and their impact on native biodiversity.
2. Applications of Systematics
A. Taxonomy and Classification: Provides a standardized system for naming organisms. Ensures accurate identification and differentiation of species.
B. Phylogenetics and Evolutionary Studies: Helps construct phylogenetic trees to study evolutionary links between species. In comparative genomics to study genetic similarities and differences.
C. Biotechnology and Drug Discovery: Systematics helps in identifying species with medicinal properties (e.g., antibiotics from fungi). Used in genetic engineering and synthetic biology for creating beneficial products.
D. Environmental Conservation and Ecology: Identifies keystone species critical for ecosystem stability. Helps in restoration ecology by selecting appropriate species for rehabilitation.
E. Forensic Science: Helps in species identification in criminal investigations (e.g., DNA barcoding for wildlife forensics). Used in determining the origin of illegal wildlife trade specimens.
F. Paleontology and Fossil Studies: Systematics helps classify extinct species and study their relationships with modern organisms. Provides insights into past climates and evolutionary changes over time.