Chapter 2. SYSTEMATICS

Learning Objectives

Pre-Lab
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General Purpose

Conceptual

  • Be able to differentiate between different classification systems.
  • Be able to describe the role systematics plays in modern biology.
  • Be able to use different methods to develop classifications and phylogenetic trees.

Procedural

  • Gain experience deciding what counts as a trait and why traits are chosen as grouping criterion and for building trees.
  • Gain proficiency in building and reading phylogenetic trees.
  • Be prepared to build phylogenetic trees to aid in learning the phyla (one level of taxonomic organization) and their characteristics presented in the diversity section of the course.

General Purpose

Life consists of an amazing diversity of different species and forms. There are approximately one and a half million species, which have been scientifically described and as many as 10 to 100 million species in the world (see Figure 8-1).

Figure 8-1. A classification diagram showing the number of known and estimates of the number of possible living species.

Although there appears to be great differences between some species, an amoeba and a hummingbird for example, species have many more similarities than they have differences.

Equally surprising is the fact that these species naturally form a categorical hierarchy. Gorillas share many similarities with the other apes, which share similarities with monkeys, which share similarities with other primates, which share similarities with other mammals, vertebrates, chordates, animals, and eukaryotes. At each successive level of the hierarchy the similarities become less clear. The organisms within a group are more similar to one another than they are to organisms outside that group. This process of grouping or organizing organisms makes the study of organisms easier and more logical. It can also be useful in understanding the breadth of diversity and the relationships that exist between organisms.

Systematics and Classification

Humans have organized life forms into groups for thousands of years. Aristotle and other early scientists classified organisms according to particular essences shared by all members of each group and not shared with other groups. This method permitted groups to be divided into hierarchies by divisions or branches. Animals were divided into vertebrates, which have backbones, and invertebrates, which lack them. Continued subdivision leads to groups that are progressively more specific in the criteria that characterize them and eventually to groups that could not be subdivided. These latter groups are designated as species.

Categorical classifications can be created to classify anything, using division: for instance, buildings can be divided into one story buildings and those with more than one story; these can be divided into those with flat roofs and those with pitched roofs. Such a classification would be arbitrary since the form of the classification would change depending on which characteristic was measured first.

During the past 100 years, taxonomists have used systems (the practice of systematics), instead of classifications, for the grouping of organisms. The difference between the two methods brings into play the processes which shape the organisms. A classification groups organisms together based on shared characteristics whereas systematics groups organisms based on the processes that resulted in the evolution of the characteristic of the organisms. The difference is important because a classification contains no information about organisms except for which characteristics were used to generate the classification. Systematics lead to generalizations about the organisms far beyond the specific features that led to their inclusion in a particular group (e.g., why or how a certain type of fin is on the fish instead of just the fact it has that type of fin).

A systematic classification, one based on a system, is not influenced by the ordering of the features. Any ordering of features will give the same groups if the system is correct. For instance, animals can be grouped into those with exoskeletons and those without, and into those with camera eyes and those without camera eyes. No matter which feature you choose first, the same two groups (i.e., dividing the arthropods from the other animals) will be chosen. This reflects the fact that the arthropods arose from a common ancestor with exoskeletons and without camera eyes, and that these traits correlate within a lineage. If you know that an organism has an exoskeleton, then you know that it is unlikely to have camera eyes, even if camera eyes did not enter into the original classification.

Phylogeny

In the evolution section of this course, you learned evolution is the change in genetic makeup of a population over generational time. If there is enough genetic change between two groups so they cannot successfully interbreed, these groups are considered different species. Current evolutionary theory suggests that the species extant (alive) today were formed by descent from ancestral species with modifications: consequently, phylogeny [verb], the process by which species arise, provides a system for grouping organisms. A phylogeny [noun] is a taxonomic system based on descent or evolutionary history. The fact that organisms form a categorical hierarchy is evidence for evolution: each speciation event creates two daughter species equally related to the ancestral species. The daughter species still retain many traits in common, in addition to the genetic modifications. Each of the daughter species can now continue to evolve and can give rise to even more species (see Figure 8-2).

Phylogenetics is one type of systematics. The value of constructing a phylogenetic classification goes far beyond our scientific interest in the evolutionary process. Shared derived traits are often of practical interest for approaching a wide variety of scientific processes. Using a phylogenetic approach taxonomists can:

  1. Determine which species are most closely related.
  2. Allow generalization about behavior patterns thought to be similar to those of our ancestors. If we know whether humans are more closely related to gorillas or chimpanzees, we can evaluate observations of the two groups accordingly when trying to infer the origins of traits such as language and tool making.
  3. Help us look for the cures and causes of some human diseases, and to anticipate outbreaks of others. Diseases typically spread faster within a phylogenetic group than between groups, and then become more virulent in the new hosts. Some diseases, such as the retrovirus HIV, are thought to have arisen as a consequence of the evolution of the vertebrate immune system.
  4. Aid in the search for biomolecular solutions for pest and disease vector control.
Figure 8-2. A diagram of new daughter species evolving from a single lineage and then continuing to evolve further into more daughter species.

Naming Organisms

Taxonomy, the branch of systematics devoted to the naming of organisms, uses a system of hierarchical classification to name organisms. The taxonomy used today dates back to Linnaeus in the 1700s. Each species was given a two part name (binomial name). Homo sapiens is the binomial for humans. The first word in the name represents the genus and the second word is the species. The species is the subcategory within the genus category. Each taxonomic category is increasingly inclusive. Genus is the category which contains one or more species groups. Family contains a related group of genera (the plural of genus). Order is the group which includes related families. This continues through Class, Phylum (or Division), Kingdom, and ends with Domain being the most inclusive category. There are currently three domains: Bacteria, Archaea and Eukarya (see Figure 8-3).

Figure 8-3. The three domains showing their phylogenetic relationship. Note that as part of the evolution of Eukarya, prokaryotic cells from the Bacteria lineage were incorporated by endosymbiosis to become mitochondria and chloroplasts.

Monophyletic vs. Polyphyletic

In the 1930s and ’40s evolutionary taxonomy made the main goal of systematics the identification of monophyletic taxa. Monophyletic taxa are defined as groups of organisms that share a common ancestor and include all descendants. The term monophyletic is reserved for clades (Figure 8-4).

Figure 8-4. Monophyletic, paraphyletic, and polyphyletic groups.

A clade is a group that includes all organisms back to their common ancestor. Clades can be smaller, such as each of the yellow circles, or more inclusive, each of the blue circles. In both cases, all organisms derived from the common ancestor along with the common ancestor are included and the clade is monophyletic. Organisms in clades share many traits. If a scientist searching for a model system for disease study, biomolecules for disease or infection treatment, or organisms with similar behavior, morphology or biochemistry, the scientist would first look within the monophyletic group.

Incomplete groups, ones that do not include all the descendants of the most common ancestor, are called paraphyletic (Figure 8-4, each of the green circles is paraphyletic). If the grouping contains members of multiple clades without the most common ancestor, this is polyphyletic (Figure 8-4, purple circle). One of the goals of systematics is to maximize monophyletic groups and eliminate paraphyletic and polyphyletic groups when producing phylogenies, as this creates a system with the most useful information.

2.1 Homologous and Analogous Characteristics

One key to early evolutionary taxonomy was the distinction between analogous and homologous characters. Analogous characters perform the same or similar functions but are structurally different. Your legs and an insect’s legs perform the same function (i.e., allow the organism to walk) but are structurally very different. Homologous characters have the same or similar structure but perform either the same function or different functions. Your arms and the front flipper of a whale contain very similar bones and are derived from the same type of embryonic tissue, yet they perform very different functions. Likewise, a chimpanzee’s arm and a human arm are homologous because they are both derived from the same type of embryonic tissue.

Analogous characters are evidence of convergent evolution. Convergent evolution results when different taxa evolve from environmental pressure in similar ways. The front fins of fish and the front flippers of whales are an example of convergent evolution. Homologous characters can be evidence of divergent evolution, in which an ancestral characteristic becomes adapted to new roles. For example, your arms and the wings of birds, show homology. The common ancestor to both humans and birds had forelimbs. In humans these evolved into arms with hands for grasping. In birds the same structure evolved into wings for flight.

For more information on systematics and related material please view the following video:

Pre-Lab Quiz

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