chapter four: Speciation and Phylogeny

Chapter Review

What Is a Species?

The term species is defined by some as a group of organisms that interbreed under natural circumstances, producing viable, fertile offspring, and that are reproductively isolated from other groups. The last chapter covered the sorts of evolutionary changes that happen in populations, and species can often be understood to be collections of populations. While microevolution produces changes in populations, macroevolutionary processes affect entire species, or even larger taxa, like genera and families. The above definition of a species is known as the biological species concept. One way of evaluating this concept is to consider that a species experiences gene flow between the populations that make it up and this tends to maintain genetic compatibility between members. Conversely, populations that do not exchange genetic information—either through geographic or behavioral isolation—experience increased isolation or genetic drift and tend to become increasingly different from other groups over time, perhaps even evolving into new species.

Speciation

One of the most vexing questions for Darwin, and for every evolutionary scientist since, is how do different species come to be? Geographic isolation is probably the most obvious explanation, but other possible modes include the following:

Allopatric speciation: geographic isolation that impedes gene flow between two groups in a population (for example, a mountain range or a river).
Parapatric speciation: partial geographic isolation coupled with selective pressures that maintain species boundaries even with some gene flow.
Sympatric speciation: high selection pressures that create species boundaries without geographic isolation.

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) as the biological species concept does. The relationship between populations and their environment is more complex than one would think. Different populations don’t segregate themselves in simple ways that can later be easily reconstructed after they have speciated. Geographic isolation does not have to be permanent, and hybrid forms are favored in certain environments and not others. These are some of the reasons for why scientists continue to debate how speciation occurs.

Phylogeny

Taxonomy, the classification of biological species, is a system used to organize all of the forms of life found on the planet. Biological taxonomic classification is still based on a hierarchical system created by Carolus Linnaeus in the 18th century (ironically, Linneaus never believed in species change over time. His strict creationist views informed his research, which he saw as the accurate description of all of God’s Creation). 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 impede communication among researchers who are using different systems. For this reason, biological classification has been based on phylogeny, relationship by common ancestry. Because any given group of organisms can share only 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 schools of thought that take different approaches to addressing these challenges.

Phenetics: emphasizes overall morphological similarity between organisms by using large numbers of traits to assess and compare statistical levels with other groups; one branch of this approach is referred to as numerical taxonomy.
Cladistics: emphasizes relationships based solely on common descent.
Evolutionary systematics: bases taxonomies on both overall similarity and common descent.

Although cladistics provides a sound theoretical basis for constructing taxonomies, determining common descent becomes increasingly difficult when applied to the natural world, due to homoplasy, or evolutionary convergence. Cladists, though, have developed a methodology that specifically addresses this issue.

Reconstructing Phylogenies

Character states are the core component of phylogenetic reconstructions. More specifically, character states relative to other groups with known phylogenies make phylogeny construction possible. This method differs from phenetics in that it is not the quantity of characteristics that 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: a shared ancestral trait that is not present in all descendant groups (for example, egg laying, which is present in chickens and monotremes—the duck-billed platypus—but absent in humans and monkeys); these characters do not provide useful information for reconstructing phylogenies.
Autapomorphy: a uniquely derived trait that is not shared with any other groups (for example, 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: a shared, derived character found in all descendants, to the exclusion of a common ancestor (for example, live birth and lactation, which characterizes all placental mammals, to the exclusion of other groups); these are the only characters that can be used to reconstruct phylogenies.

As stated earlier, problems arise when there is convergence of characters, which may be incorrectly designated as synapomorphy. Familiar examples of this are the identification of aquatic and aerial mammals on the basis of homologous features of their physiology. Figure 4-1 shows the dugong (top), mole (middle), and bat (bottom). The dugong's forelimbs are designed for paddling in the sea, the mole's forelimbs are specialized for digging in the ground, and the bat's forelimbs are designed for flying through the air. However, common descent is based on recognizing that each bone in the forelimb is derived from a common ancestral structure for all three animals.

Figure 4-1. Drawings of the forelimbs of three mammals by Richard Owen. Credit: Richard Owen, On the Nature of Limbs (1849) J. van Voorst, London.

So how do researchers decide which characters should be classified as synapomorphies? This question hinges on the issue of character polarity—that is, which trait state is ancestral and which is derived. To determine 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. For example, siamangs and gibbons (Hylobatidae) could be used as an outgroup for comparing character states within the rest of the great apes (orangutans, gorillas and chimpanzees). The character state of the gibbons is assumed to be more representative of the ancestral condition. This technique, though, is not entirely sound, because there is no clear reason that 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 these are 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.

The University of California Museum of Paleontology provides an excellent explanation of cladistic methods and terminology. Also be sure to check out the museum's glossary of phylogenetic terms for some quick definitions.

www.ucmp.berkeley.edu/clad/clad4.html
www.ucmp.berkeley.edu/glossary/gloss1phylo.html

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