Chapter 4: Speciation and Phylogeny

What is a species?

The term species is defined as a group of organisms which interbreeds under natural circumstances, producing viable, fertile offspring, and which is reproductively isolated from other groups. This definition is known as the Biological Species Concept. One way of evaluating this concept is to consider that a species will experience gene flow, which will tend to maintain genetic compatibility between members. Conversely, organisms that do not exchange genetic information - either through geographic or behavioral isolation - will experience genetic drift, and tend to become increasingly different over time.

Speciation

How are species formed? Geographic isolation probably is the most obvious explanation, but other possible modes include:

• Allopatric speciation: geographic isolation which impedes gene flow between two groups in a population (e.g. a mountain range, river, etc.)

• Parapatric speciation: partial geographic isolation, coupled with selective pressures, that maintains species boundaries even with some gene flow.

• Sympatric speciation: high selection pressures that create species boundaries without geographic boundaries.

The Ecological Species Concept emphasizes the role of selection in maintaining species boundaries, rather than purely abiding by the strict rules of allopatricity (total geographic isolation).

Phylogeny

Taxonomy, the classification of biological species, is a system used to organize all of the forms of life found on the planet of which we are aware. Biological taxonomic classification is based upon a hierarchical system created by Carolus Linnaeus in the 18th century. However, taxonomies can be arranged according to different criteria. This may be particularly problematic in the realm of biological classification, since arbitrary bases for classification would impede communication between researchers using different systems. For this reason, biological classification has been based on phylogeny, or relationship by common ancestry. Because any given group of organisms can only share a single common ancestor, this system provides an adequately discriminating lingua franca for biologists to use throughout the world.

The question of how phylogenies can be accurately determined, however, presents a difficult problem. This field - determining the method and criteria used in classifications - is referred to as systematics. There are several different schools of thought that take different approaches to addressing these challenges.

• phenetics: This approach emphasizes overall morphological similarity between organisms by using large numbers of traits to assess and comparing these statistical levels with other groups. One branch of this is referred to as numerical taxonomy

• cladistics: This approach emphasizes relationships based solely on common descent.

• evolutionary systematics: This school bases taxonomies on both overall similarity and common descent.

Reconstructing Phylogenies

Character states (see below) are the core component of phylogenetic reconstructions; more specifically, it is character states relative to other groups with known phylogenies that make phylogeny construction possible. This method differs from phenetics in that it is not quantity of characteristics which leads to validity of analysis. Rather, the most important issue is resolving variable traits into three classes, of which only one type is phyogenetically informative.

• symplesiomorphy: shared ancestral trait that is not present in all descendant groups. An example of this is egg-laying, which is present in chickens and monotremes (duck-billed platypus), but absent in humans and primates. These characters do not provide useful information for reconstructing phylogenies.

• autapomorphy: uniquely derived trait not shared with any other groups. An example of this is the enlarged third digit of the aye-aye, Daubentonia madagascariensis, which is not possessed by any other living primates. These characters do not provide useful information for determining phylogenies either, but are good for identifying particular species.

• synapomorphy: shared, derived character shared by all descendants, to the exclusion of a common ancestor. An example of this is live birth and lactation, which characterizes all placental mammals, to the exclusion of other groups. These are the only characters which can be used to reconstruct phylogenies.

As stated above, problems arise from the convergence of characters, which may be incorrectly designated as synapomorphies. Familiar examples of this is the identification of aquatic and aerial mammals on the basis of homologous features of their physiology. Pictured at the left are the dugong (top), the mole (middle), and the bat (bottom). The dugong has forelimbs designed to assist paddling in the sea, the mole has forelimbs specialized to dig in the ground, and the bat has forelimbs designed for aerial maneuvers. However, common descent is based on recognizing that each bone in the forelimb is derived from a common ancestral structure for all three animals.

So how do researchers decide which characters should be classified as synapomorphies? This question hinges upon the issue of character polarity - which trait state is ancestral versus which is derived. To accomplish this, a taxonomic group that is equally distant to all of the groups in question must be incorporated into the analysis. This group is called the outgroup. In the picture shown at the right, the group consisting of siamangs and gibbons (Hylobatidae) could be used as an outgroup for comparing character states within the great apes. The character state of the gibbons would be assumed to be more representative of the ancestral condition. However, this technique is not entirely sound, for there is no clear reason why we should assume the gibbons would have stopped evolving since their divergence with the great apes. For this reason, character polarity is also augmented by information gleaned from studies of ontogeny (embryology and development) and fossil material, when available. Even with this repertoire of tools by which to create phylogenetic trees, controversies abound; systematics remains one of the most contentious fields in the biological sciences.

An excellent explanation of cladistic methods and terminology is at the University of California Museum of Paleontology. Also be sure to check out the UCMP glossary of phylogenetic terms for some quick definitions. After you've become more familiar with how to classify based on phylogeny, try your hand at a systematics exercise for PC or Macintosh computers from the University of Illinois.