Molecular Systematics
Molecular Systematics
Systematics is the science of classification and reconstruction of phylogeny (evolutionary relationships) of organisms (Organic Evolution and Classification and Phylogeny of Animals ). Systematics has traditionally depended on detailed analyses of morphology (structure) and development as criteria for distinguishing groups of organisms and for reconstructing phylogenies. Since the advent of practical methods for determination of DNA base sequences and for isolation of specific genes in a genome, systematics has had a powerful new tool added to its arsenal. The polymerase chain reaction has made it possible to sequence genes from very tiny DNA samples. These techniques have spawned an enormous number of studies that increase our understanding of animal relationships, and the discussions of phylogeny in Part III of this book will cite many examples of such studies.
The rationale for molecular systematics depends on the accumulation of mutations in genes over evolutionary time as lineages of animals diverge from their common ancestor. Some genes are amazingly similar (conserved) in organisms that are only very distantly related and so do not lend themselves well to this use. Sequences of genes that encode a variety of proteins and especially the gene encoding the small subunit of ribosomal RNA have been analyzed. In many instances sequence analyses support phylogenies based on morphological and developmental evidence, but sometimes they do not (for example, as in the phylogenetic position of the chaetognaths, see Chaetognaths and Hemichordates). Such disagreement should encourage further studies in an effort to clarify the questions raised. In many instances sequence analysis has provided the only evidence for relationships between organisms because no evidence was provided by morphology and development.
Systematics is the science of classification and reconstruction of phylogeny (evolutionary relationships) of organisms (Organic Evolution and Classification and Phylogeny of Animals ). Systematics has traditionally depended on detailed analyses of morphology (structure) and development as criteria for distinguishing groups of organisms and for reconstructing phylogenies. Since the advent of practical methods for determination of DNA base sequences and for isolation of specific genes in a genome, systematics has had a powerful new tool added to its arsenal. The polymerase chain reaction has made it possible to sequence genes from very tiny DNA samples. These techniques have spawned an enormous number of studies that increase our understanding of animal relationships, and the discussions of phylogeny in Part III of this book will cite many examples of such studies.
The rationale for molecular systematics depends on the accumulation of mutations in genes over evolutionary time as lineages of animals diverge from their common ancestor. Some genes are amazingly similar (conserved) in organisms that are only very distantly related and so do not lend themselves well to this use. Sequences of genes that encode a variety of proteins and especially the gene encoding the small subunit of ribosomal RNA have been analyzed. In many instances sequence analyses support phylogenies based on morphological and developmental evidence, but sometimes they do not (for example, as in the phylogenetic position of the chaetognaths, see Chaetognaths and Hemichordates). Such disagreement should encourage further studies in an effort to clarify the questions raised. In many instances sequence analysis has provided the only evidence for relationships between organisms because no evidence was provided by morphology and development.
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