Species

236 Species

Lineages

A version of the evospecies is Kevin de Queiroz’s general lineage conception, sometimes
misleadingly referred to as the Universal Concept. The key term lineage is taken from
Hull’s metaphysics,^37 based on Simpson’s evolutionary species definition,^38 as revised by
de Queiroz.^39 It represents the generative aspect of species that we saw from early in the
history of the concept. It broadly means

... a series of entities forming a single line of ancestry and descent. ...

Lineages in the sense described above are unbranched; that is, they follow a sin-

gle path or line anytime an entity in the series has more than one descendant. ...

Consequently, lineages are not to be confused with clades, clans, and clones—though

the terms are often used interchangeably in the literature.^40

... a lineage (an ancestral-descendant sequence of populations) ...^41

A problem with this is that while every species is a lineage (that is, forms generative
sequences), not every lineage forms a species. Essentially a lineage is any part of the
evolutionary tree that is unbroken (see Figure 8.1), but we know that species have within
them subordinate lineages, of populations, of genes and of kin groups. As we shall see
with phylogenetic species concepts in the next chapter, this leads to problems in specify-
ing what level of lineage one wants to call a species.
For example, harbor seals and albatrosses turn out to have extensive within-species
variations that form lineages.^42 The decision of the level of diversity of lineages is informed
by other considerations, including what the taxonomist considers a “significant” amount
of genetic divergence or not. But this means that a lineage, or indeed an evolutionary
species, is determined in the end by phenetic criteria: the rank depends on arbitrary (but
not necessarily subjective) lines of divergence in the characters or genetic matrix used.
One attempt to deal with this is the Genealogical Concordance species concept of
Avise and Ball.^43 De Querioz considers this a phylogenetic conception of species,^44
but in my opinion, it is better thought of as an evolutionary conception. It is basically
the view that population subdivisions concordantly identified by multiple independent
genetic traits constitute the population units worthy of recognition as phylogenetic taxa.
Relying upon genetic coalescence theory, in which genetic lineages are traced back to
original populations,^45 a species is considered to exist when it shares a number of genetic
coalescents. More recently, multispecies coalescence (MSC) has been used to attempt to
delimit species.^46

(^37) Hull 1980, 1981, 1984, 1988b, 1988a, 1989, 1992, 1997.
(^38) Simpson 1951, 1961.
(^39) de Queiroz 1998, 1999.
(^40) de Queiroz and Donoghue 1990, 50.
(^41) Simpson 1951.
(^42) Burg 1999, Burg and Croxall 2004.
(^43) Avise and Ball Jr. 1990.
(^44) de Queiroz 2007.
(^45) Avise and Wollenberg 1997, Wang et al. 1997, Knowles and Carstens 2007.
(^46) Carstens and Dewey 2010, Carstens et al. 2013, Yang and Rannala 2014, Lamanna et al. 2016, Leavitt
et al. 2016, but see Sukumaran and Knowles 2017 for criticism, arguing that MSC delineates popula-
tion structure, not species.