Lineage (evolution)

An evolutionary lineage (also called a clade) is composed of species, taxa, or individuals that are related by descent from a common ancestor. Lineages are subsets of the evolutionary tree of life. The concept of an evolutionary lineage is grounded in the science of cladistics. Lineages are often determined by the techniques of molecular systematics. A lineage can be distinguished from a mere collection of species by the fact that it contains only and all individuals that share a common ancestor.

Phylogenetic representation of lineages

File:PhylogeneticTree.jpg

Fig. 1: A rooted tree for rRNA genes

Lineages are typically visualized as subsets of a phylogenetic tree. For example, the tree in Figure 1 shows the separation of life into three ancient lineages: bacteria, archaea, and eukaryotes. Phylogenetic trees are typically created from DNA or protein sequence data. Sequences from different individuals are collected and their similarity is quantified. Mathematical procedures are used to cluster individuals by similarity.

Just as a map is a scaled approximation of true geography, a phylogenetic tree is an approximation of the true complete evolutionary relationships. For example, in Figure 1, the entire lineage of animals has been collapsed to a single "leaf" in the tree. However, this is merely a limitation of rendering space. In theory, a true and complete tree for all living organisms or for any DNA sequence could be generated.

The "lineage" definition of human races

Many contemporary biological definitions of race conceptualize races as evolutionary lineages within the human species. Genetic data can be used to infer population structure and assign individuals to groups that often correspond with their self-identified geographical ancestry. An example of this concept is represented in Figure 2.

Fig. 2: An example of the lineage concept of human races in the context of higher levels of categorization. This rendering is a simplification.

A primary motivation for categorizing human genetic variation in this way comes from biomedical research. In this context, human races represent different genetic backgrounds that may influence the association of diseases with their causes (genetic or environmental).

The concept of racial lineage is very similar to the concept of familial lineages in genealogy. This has led some commentators to describe races as extended families.

A major objection to the view that contemporary humans can be categorized into lineages is the existance of individuals with ancestry from multiple lineages. To accomodate admixture, the definition of races is expanded beyond straightforward lineages to include the possibility of fractional lineage membership. This is often represented graphically as a triangle plot (see Figure 3). The fuzziness of racial lineages has led to the description of races as fuzzy sets (Sarich & Miele 2004).

File:Admixture triangle plot.png

Fig. 3: Triangle plot shows average admixture of five North American ethnic groups.

References

• Sarich, Vincent, and Frank Miele. Race: The Reality of Human Differences. Westview Press, 2004.