Evolving together

In much of the last two chapters we attempted to explain the outcome of ecological interactions by assuming one species evolving in relation to constraints provided by another, evolutionarily static, species. Of course there is no reason why this process need be so one-way, and if it is not, then it is essential that the reciprocal evolution of both species in an interaction be considered. Such reciprocal evolution of interacting species is called co-evolution.

The aim of co-evolutionary studies is to predict or understand the evolutionary dynamics and evolutionary outcomes that result from species interactions. The subject actually has one of the longest histories of any in evolutionary ecology, with basic concepts and theory dating to the 1950s (see Thompson 1999). However, the intensity of research has only picked up in the last two decades, aided, particularly, by the molecular and cladistic revolutions. The evolutionary dynamics of interacting species do not readily lend themselves to study. Although use of the fossil record has been attempted, such studies on their own are often deficient in key data relating to the interaction. By far, the most promising progress has been made with living species, sometimes in combination with the fossil record.

To observe the dynamics and outcomes from living species, evolutionary ecologists take two approaches: a longitudinal (historical) one that reconstructs the past, normally using cross-species phylogenies to estimate the pattern of changes over time. Sometimes, however, the evolutionary dynamics are sufficiently rapid that long term field studies can detect them.A complementary (transverse) approach is to observe the interaction at different points in space, which highlights possible alternative long-term temporal outcomes. Recently, it has been suggested (Thompson 1994, 2001) that the spatial dynamics are also the key to understanding the temporal dynamics. Before we proceed to examining that hypothesis, let us have a look at some of the evidence that describes how co-evolution can proceed.

From several case studies of interacting species in the field, several alternative dynamical outcomes are now known or may be implied (Table 11.1). They are doubtless just a small sample of the co-evolutionary universe. These outcomes have previously been referred to as the 'modes of co-evolution' (Thompson 1989,1994).I restrict myself here to modes that relate specifically

Table 11.1 Some alternative modes of co-evolution, following Thompson (1989, 1994)

Empirical model

Example

Properties

Diversifying co-evolution

Escape-and-radiation co-evolution

Arms race

Co-evolutionary alternation

Mutual dependence

Co-evolutionary successional cycles

Co-evolutionary turnover

Maternally inherited symbionts (Thompson 1987)

Umbelliferae and their phytophages (Berenbaum 1983)

Flower morphology and morphology of pollinators (Steiner and Whitehead 1990)

Cuckoos and their hosts (Davies and Brooke 1989)

Eukaryotes and their mitochondria/chloroplasts (Margulis and Bermudes 1985)

Trifolium repens and grasses (Turkington 1989)

Body size of Anolis lizards in the Caribbean (Roughgarden and Pacala 1989)

One species stimulates speciation in another

Radiation in one lineage occurs while it temporarily escapes the interaction with another

Directional change in quantitative traits, perhaps reaching stable or local equilibria

The identity of interacting species changes as a result of an adaptive response in one species which initially decreases but later increases the interaction A mutualistic relationship shows temporal stability with increasing specialization and obligation

Local succession creates changing patterns of association between species creating local or temporal interactions in which co-evolution occurs Asymmetric competition creates cycles of invasion, co-evolution, extinction, evolution, and reinvasion to outcomes (rather than assumptions). They are distinguished from each other by four characteristics (Table 11.2).

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