Although researchers widely agree that species are lineages, the exact natures of both species and lineages remain unclear (Haber 2013, 341; Richards 2010, 142; Barker and Wilson 2010). The key innovation in this paper is the idea that some, perhaps many, evolving lineages are positive feedback systems. This leads to testable hypotheses that help integrate otherwise discordant views about species.
May aim in this paper is to motivate my feedback systems idea, and associated hypotheses, by showing how they can solve six striking puzzles, and be further tested empirically.
Six Striking Puzzles about Lineages and Species:
- Newton Puzzle:
Defenders of various species concepts still use words, in often vague and ambiguous sentences, to define the term ‘species’. Newton used numbers, in precise mathematical formulas, to define key terms in physics. That was crucial for advancing physics. It is striking that we still await such advance for the terms ‘species’ and ‘lineage’.1 Can we begin to fill this gap?
- Lineage Puzzle:
Rather than settling disputes about the nature of species, the Darwinian revolution added a new puzzle. It suggested, at least to many, that species are actively evolving lineages, yet it has yet to clarify the nature of such lineages. It is common for systematists to assume that any lineage just is a monophyletic clade, but it has been shown that this is typically impossible for the types of actively evolving lineages that species are supposed to be (de Queiroz 1998, 60–61). So we need a fuller theory about what I call active lineages, the sorts of things Hull had in mind when he said that a species are cohesive groups of populations that do things, “the sorts of things which evolve, split, bud off new species, go extinct, etc.” (1981, 146).
- Specialness Puzzle:
Research on species remains torn between the idea that species are taxa, just as genera, families, and so on are (Baum 2009; Ereshefsky 1991), and the idea that there is something special about species that all other taxa lack (Mayr 1963; Sites and Marshall 2004). It is puzzling that both ideas should persist if they are not reconcilable. Can we formulate a view on which species are special in some ways, though also like other lineages in others?
- Sober Puzzle:
Evolutionary theory is famous for replacing Aristotelian thinking about species with “population thinking” (Mayr 1976). The most sophisticated and well-argued
1 True, mathematical means for species delimitation are sophisticated, important, and proliferating, especially within so- called integrative taxonomy. But that is about the epistemic issue of knowing that some group is a species, not about the ontological issue of why that group is a species—what makes it one (de Queiroz 2007).
elaboration of this is found in Elliott Sober’s “Evolution, Population Thinking, and Essentialism” (1980). But readers have not recognized that the concept SPECIES simply drops out of the picture on this account, rather than being changed or improved. Sober shows how population thinking takes us from the level of organisms, to dynamic processes at the level of populations. But this ascent stops at the population level, rather than continuing onto involve the species level. It is puzzling that researchers continue to assert the evolutionary importance of species (Agapow 2005; Cracraft 2002), when population thinking has yet to be developed to that level. Can it be?
- Similarity Puzzle:
Although it is often said that a main evolutionary lesson for species theorizing is that the nature of species involves distinctive causal and ancestry relations, rather than similarity relations (Ghiselin 1974; Hull 1976; Ereshefsky 2001), many appeals to similarity continue to crop up in species concepts and discussions of them (Kornet and McAllister 2005; Mallet 1995; Cracraft 1983). Is there a way to scratch the stubborn itch of similarity, by showing from an evolutionary point of view what is important about it?
- Integration Puzzle:
Notoriously, we have a plurality of species concepts (Mayden 1997; Wilkins 2009), with none enjoying widespread support, not even the biological species concept when speaking just of birds (Sangster 2014). It may be that this simply reflects discordance in nature itself (Dupré 1993). But there is a surprising lack of good argument for that view (Wilson 2005; Barker 2016). Species concepts that attempt to integrate complementary parts of other concepts are on offer (e.g., de Queiroz 1998; Wiley and Mayden 2000). But they remain vague and underdeveloped (Richards 2010). Can we do better?
Species and Lineages as Positive Feedback Systems:
My key idea that evolving lineages, including those of species types, are positive feedback systems gets its start from the familiar claim that each species exhibits a kind of evolutionary cohesion that sets it apart from other species and groups (Simpson 1961; Mayr 1963; Wiley 1978; Templeton 1989; de Queiroz 1998; Morjan and Rieseberg 2004; Barker and Wilson 2010).
But authors have erred in thinking of such cohesion as an effect — an effect of causal variables such as gene flow, niche sharing, genetic similarity, and so on. This view has led to researchers disputing which of these variables is the main cause of species cohesion (Ereshefsky 2001).
I propose that species cohesion is a type of evolutionary cohesion more generally, and that this is not an isolable effect of causal variables such as gene flow and niche sharing. Rather, what these causal variables produce are greater amplitude and frequency of later instances
of these variables themselves. They are thus forming positive feedback systems within groups of populations over evolutionary time. The evolutionary cohesion of such a group simply is the repeated instantiation or cycling of such a system.
For example, gene flow between three populations in an active lineage during one time period tends to increase the amplitude and frequency of gene flow between the populations at a later time period, by integrating the populations genetically and thereby enhancing their reproductive compatibility (Mayr 1963). Moreover, the different causal variables tend to interactively enhance each other; gene flow tends to enhance later niche- sharing, and vice versa (Morjan and Rieseberg 2004). Rising amplitude and frequency of such relations over evolutionary time simply is an increase in degree of evolutionary cohesion; drops are decreases.
Active population-level lineages can then be defined as those lineages that feature evolutionary cohesion of this sort. They feature the positive feedback systems just outlined.
Obviously, there will be a spectrum of active lineages, from those with lower degrees of evolutionary cohesion to those with higher. Within this spectrum will be a particular range cohesion values that corresponds to the active lineages of the species type.
More formally, the following variables form positive feedback relations within groups of populations over evolutionary time:
g = gene flow between populations within the group.
n = niche-sharing between populations in the group.
s = relative phenotypic similarity between populations within the group.
There will also be variables that disrupt the feedback relations:
m = mutations isolated to only part of the group populations.
d = genetic drift localized to only some populations in the group.
Further empirical work may show that further variable should be added to these lists. But the general idea is that we can then mathematically characterize (with updates in light of the empirical work) the evolutionary cohesion in question, C, as follows:
C ∝gns/md (1)
One hypothesis is then that species exhibit a particular grade of C — what authors are
getting at with talk of ‘species cohesion’ — which can be represented as CS: CS = X < C < Y (2)
This shows CS falling between a discoverable minimum threshold, X, and a discoverable maximum threshold, Y.2 Lineages in which C < X would be candidate higher taxa (such as genera) of the active lineage sort, and those in which C > Y would be candidate lower taxa (such as sub-species) of the active lineage sort.
Combining (1) and (2) would then allow us to define a feedback species concept (FSC) exactly: such a feedback species is an active population-level lineage for which C = CS.
As I will argue in the paper, the conjunction of (1) and (2) within the FSC will help answer the Newton Puzzle for species.
That puzzle for lineages, and the Lineage Puzzle itself, will be addressed by elaboration of (1).
I will help answer the Specialness Puzzle by suggesting that those who view species as special among other active lineages should hypothesize that the spectrum of instances of evolutionary cohesion found in nature features a) relatively large gaps between X and lesser values of cohesion, and b) also relatively large gaps between Y and greater values of cohesion.
Showing how (1) and (2) extend population thinking from the population to the species level will help answer the Sober Puzzle.
Elaboration of variable s will explain how to justify the importance of similarity to specieshood, from an evolutionary perspective.
And to address the Integration Puzzle I will show how the FSC integrates key insights from advocates of many of existing species concepts.
I will close by drawing on existing empirical data from other researchers to support my views, then detail how further empirical testing can and should be carried out.