All evolutionary biologists are familiar with evolutionary units that evolve by vertical descent in a tree-like fashion in single lineages. However, many other kinds of processes contribute to evolutionary diversity. In vertical descent, the genetic material of a particular evolutionary unit is propagated by replication inside its own lineage. In what we call introgressive descent, the genetic material of a particular evolutionary unit propagates into different host structures and is replicated within these host structures. Thus, introgressive descent generates a variety of evolutionary units and leaves recognizable patterns in resemblance networks. We characterize six kinds of evolutionary units, of which five involve mosaic lineages generated by introgressive descent. To facilitate detection of these units in resemblance networks, we introduce terminology based on two notions, P3s (subgraphs of three nodes: A, B, and C) and mosaic P3s, and suggest an apparatus for systematic detection of introgressive descent. Mosaic P3s correspond to a distinct type of evolutionary bond that is orthogonal to the bonds of kinship and genealogy usually examined by evolutionary biologists. We argue that recognition of these evolutionary bonds stimulates radical rethinking of key questions in evolutionary biology (e.g., the relations among evolutionary players in very early phases of evolutionary history, the origin and emergence of novelties, and the production of new lineages). This line of research will expand the study of biological complexity beyond the usual genealogical bonds, revealing additional sources of biodiversity. It provides an important step to a more realistic pluralist treatment of evolutionary complexity.biodiversity structure | evolutionary transitions | lateral gene transfer | network of life | symbiosis

We argue that a fashionable interpretation of the theory of natural selection as a claim exclusively about populations is mistaken. The interpretation rests on adopting an analysis of fitness as a probabilistic propensity which cannot be substantiated, draws parallels with thermodynamics which are without foundations, and fails to do justice to the fundamental distinction between drift and selection. This distinction requires a notion of fitness as a pairwise comparison between individuals taken two at a time, and so vitiates the interpretation of the theory as one about populations exclusively. # British Society for the Philosophy of Science 2004 2 Some recent advocates of the 'central tendencies' approach substitute one of the PNS's deductive consequences for the PNS itself. Instead of the PNS, Matthen and Ariew ([2002], p. 73) treat Fisher's fundamental theorem of natural selection as the central explanatory principle of Darwinism. This theorem states that the fitness of a population increases at a rate proportional to the genetic variance in fitness present in the population (cf. Strickberger [1985], p. 728). Matthen and Ariew's version of the theorem, is attributed to C .C. Li: 'In a subdivided population the rate of change in [overall population] growth rate [i.e. fitness] is proportional to the variance in growth rates [i.e. fitnesses].' 'Variance', of course, is a population-level property, and so suits Fisher's theorem to express the central tendency thesis that Matthen and Ariew defend. However, the claims to be made here about the PNS pop could equally well be made with respect to Fisher's theorem. 3 Cf. also Walsh, Lewens and Ariew ([2002], p. 460): 'The objective of natural selection is to explain and predict changes in the relative frequencies of heritable traits within a population. The change that selection explains is a consequence of variation in fitness (citing Lewontin [1970, 1974], Brandon [1990]).' And further, Walsh, Lewens and Ariew ([2002], p. 469): 'natural selection theory explains changes in the structure of a population, but not by appeal to the individual-level causes of births, deaths, and reproductions.'

Ecological fitness has been suggested to provide a unifying definition of fitness. However, a metric for this notion of fitness was in most cases unavailable except by proxy with differential reproductive success. In this article, I show how differential persistence of lineages can be used as a way to assess ecological fitness. This view is inspired by a better understanding of the evolution of some clonal plants, colonial organisms, and ecosystems. Differential persistence shows the limitation of an ensemblist noncausal understanding of evolution. Causal explanations are necessary to understand the evolution by natural selection of these biological systems.

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