Chapter 4: 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. Chapter 3 covered the sorts of evolutionary changes that occur 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 that 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 eighteenth century. Ironically, Linnaeus never believed that species change over time. His strict creationist views informed his research, which he saw as structuring an accurate description 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, a 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.
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. The most important issue is resolving variable traits into two classes, of which only one type is phylogenetically informative.
- Ancestral traits: an ancestral trait is one that characterizes the common ancestor of more than one species. For example, egg laying is a characteristic of the ancestors of chickens, platypuses, and humans. It has been lost in humans, however. These characters do not provide useful information for reconstructing phylogenies.
- Derived traits: a derived trait is one that has evolved since the time of the last common ancestor of the species in question. Live birth and lactation, which characterize all placental mammals to the exclusion of other groups, are examples of derived traits. These are the only characters that can be used to reconstruct phylogenies.
As you learned in previous chapters, some adaptations evolved more than once in different species, leading to convergence. An example is the structure of the eye. Systematists need to be able to distinguish between species that share the same adaptation and those that developed it independently. Characters that are similar because of convergence are called analogous, while those that result from descent from a common ancestor are homologous. A chicken and a platypus, for instance, both reproduce by laying eggs. This trait is homologous because the chicken and platypus are descended from a common ancestor. The chicken and humans are both bipedal, yet they do not share a common bipedal ancestor, and thus the trait is analogous.
Problems arise when there is convergence of characters, which may be incorrectly designated as homologous traits. Familiar examples of this are identifications 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.
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Figure 4.1. Drawings of the forelimbs of three mammals. Credit: Richard Owen, On the Nature of Limbs (1849) J. van Voorst: London. |
So how do researchers decide which characters should be classified as homologies? This question hinges on the issue of character polarity—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 out-group. For example, siamangs and gibbons (Hylobatidae) could be used as an out-group 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 we should assume the gibbons 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