1.3 Relationships between species
Using the idea of blood relationships in people as an analogy, can you think of two distinct types of relationship between species?
One is the relationship of descent – as in the parent–child relationship – and the other is that of shared parentage, as with brothers and sisters. Similarity between species may reflect either of these two kinds of relationship. It would be misleading to push the analogy too far, though, for two reasons. First, with the exception of allopolyploid hybrid species, new species are derived from single parental species. Secondly, a newly evolving species derives its characteristics directly from those currently present in the ancestral population: there is nothing in a species population that corresponds to the unchanging germ line of individual parent organisms. Two fundamental patterns of change in a phylogeny thus give rise to the differences between related species: anagenesis refers to descent with modification, within any given single lineage; and cladogenesis refers to the evolutionary division of lineages causing a proliferation of species. The grouping of species to form higher taxa can emphasise either or both of these components of phylogeny (Figure 1).