How, If, and Why Species Form

Biologists have struggled for centuries to properly define what constitutes a “species.” They may have been asking the wrong question—many smaller organisms might not form species at all.

By Axel G. Rossberg, Tim Rogers, and Alan J. McKane | November 1, 2013

WIKIMEDIA COMMONSEvolution is not concerned with species, but with individuals. The survival and reproduction of those individuals that are best adapted to their environment determine the characteristics of subsequent populations, but neither the process nor the theory requires that these populations be organized into species. The formation of species, when it occurs, is a phenomenon that needs to be explained.

On closer inspection, the very notion of a “species” is difficult to formally define. This has been a matter of debate since before Darwin, who himself concluded that “we shall have to treat species in the same manner as those naturalists treat genera, who admit that genera are merely artificial combinations made for convenience” (Origin of Species, Chapter XIV). Many subsequent authors have proposed more formal definitions, known as species concepts, each useful for particular applications but not without idiosyncrasies.

For example, in sexually reproducing populations, the biological species concept refers to distinct groups of organisms that can only mate successfully with other members of the same group. This definition at first seems reasonable, but situations have been observed where members of group A can mate with group B, and B with C, but not A with C. Formation of biological species is therefore not an evolutionary necessity: under some conditions it happens, under others it does not.

Two other species concepts that are frequently employed are ecological species and genetic species. An eco-species is a distinct group of related organisms that perform a specific ecological function, and so occupy a particular niche. The loss of that species from a community means loss of that function, and so identity and number of eco-species are important characteristics of ecosystem health. On a more practical level, the rapid advance of genetic sequencing technologies has focused attention on a species concept based solely on genotype: a genetic species is a collection of organisms whose genomes are very similar to each other, but distinct from those of organisms belonging to other species. The genetic separation of species is known as the DNA barcoding gap.

The biological intuition about species formation is mostly based on observation of birds, mammals, and other larger creatures. However, it is the meiofauna (of body size around 0.05–1 mm) and smaller microorganisms that dominate biodiversity and ecosystem functioning on Earth. It is therefore important to ask if our current understanding of species formation is applicable to these organisms.

One way to approach this problem is through the study of simplified evolutionary models. Analysis of these models can give insights into real-world processes happening over many millions of years that cannot be directly observed. In this context, species formation can be interpreted as a particular instance of the general phenomenon of self-organized pattern formation. Just as a jet of water can break up into little drops, but will do so only under certain conditions, the ancestral tree of life on Earth may, near the tips of its branches, become structured into distinct species when the conditions are right.

Whether or not this happens will depend on the precise criteria for delineating species. In a recent series of articles, we argue that the formation of both ecological and genetic species may be highly dependent on global population sizes. Put simply, in very large populations, organisms that might otherwise belong to the same species can be very distantly related to each other, sometimes to the point of harboring too much genetic variation to occupy the same ecological niche. What’s more, in mature communities in which most niches are occupied, interactions among organisms can have a much bigger role in determining individual survival than the external environment does, making the forces of evolution less coherent. As a result, populous organisms display a range of genotypes and phenotypes, seemingly unconstrained by ecological niches. For these organisms, then, the ecological species concept does not apply and, as a result, clearly identifiable genetic species cannot form either.

Smaller organisms tend to be much more numerous than larger ones—global populations of insect species, for example, can be several million times larger than those of rodents—but the rate of appearance of new mutations is not much different. We predict that at around the millimeter scale of body size and smaller, populations become large enough to prohibit species formation. This is consistent with results of recent work in the field of environmental genomics, in which organisms are collected from environmental samples and parts of their genomes are read using high-throughput sequencing. The resulting sequences are then organized into a hierarchy of clusters, based on genetic similarity. For meiofauna, the outputs of these clustering algorithms reveal a relatively smooth distribution of genomes, which do not fall easily into clearly defined and well-separated groups. The clustering patterns are consistent with the predictions of our model for the case of large population sizes, where species do not form.

Just as evolution does not critically depend on species formation, ecology as a science does not critically depend on it either. Species richness as a predictor for ecosystem functioning, for example, can be replaced by other measures of biodiversity, such as functional or phylogenetic diversity, that do not depend on the species concept. The biggest challenges are likely to arise in the fields of ecological theory and modeling: the functional distinctness of species is the starting point of most theoretical ecology. If species formation among small organisms is indeed less prevalent than currently assumed, an alternative conceptual basis for community ecology is needed. 

Axel G. Rossberg is an ecologist working at the Centre for Environment, Fisheries & Aquaculture Science in Lowestoft, U.K. Tim Rogers is a mathematician at the University of Bath, and Alan J. McKane is a theoretical physicist at the University of Manchester. All three share a deep interest in complex natural systems.

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Avatar of: James V. Kohl

James V. Kohl

Posts: 516

November 4, 2013

Ecological, social, neurogenic, and socio-cognitive niche construction are unequivocally nutrient-dependent and pheromone-controlled in species from microbes to man. Current examples from yeasts, invertebrates, nematodes, other mammals and a human population in what is now central China, support a model of experience-dependent themodynamically-altered non-random stochastic gene expression that links the epigenetic 'landscape' to the physical landscape of DNA. The link is evident in the organized genome of all species via the conserved molecular mechanisms that enable their abiltiy to self-organize and maintain nutrient-dependent organism-level thermoregulation controlled by the effects of population size on the physiology of reproduction.  

A recent report on Mosaic Copy Number Variation in Human Neurons incorporates what is known about  nutrient-dependent pheromone-controlled copy number variation in yeasts. "The mechanism by which one signaling pathway regulates a second provides insight into how cells integrate multiple stimuli to produce a coordinated response." The signaling pathway is the gonadotropin releasing hormone (GnRH) neuronal system in vertebrates, and the yeast alpha mating pheromone is so similar to mammalian GnRH that it elicits a luteinizing hormone (LH) response from the cultured pituitary cells of rats.

Given Darwin's proposal that 'conditions of life' must be considered before moving forward to consider natural selection, and works that clearly show Feedback loops link odor and pheromone signaling with reproduction, it is amazing to see so many theorists not recognize the obvious fact that selection first occurs for nutrients that metabolize to pheromones, which control the physiology of reproduction and thus control nutrient-dependent speciation throughout ecological, social, neurogenic, and socio-cognitive niche construction.

Isn't the problem with speciation that's adressed here the problem with portrayals that random mutations are the substrates on which natural selection acts? If so, let's just admit there is no experimental evidence for that, instead of attempting to "reinvent the wheel" of niche construction.

Avatar of: Curculio


Posts: 61

November 4, 2013

The authors should ponder and incorporate metapopulation issues. Populations are not single entitities but are a mix of islands and mainlands.  Hence the statement: "We predict that at around the millimeter scale of body size and smaller, populations become large enough to prohibit species formation." doesn't seem to incorporate spatial fragmentation.  On the other hand, there are many, many larger species that also form large single fragment populations.  Locusts, much larger than 1 mm form enormous populations.  Of course, these peaks are temporary and the population recede or are split off to spin into new radiations, as did the African locusts in the New World.

Avatar of: Lalo


Posts: 1

November 8, 2013

Why should we frame nature or even universe to our perception? We should drop this as a factor that has our development drag instead of run.  The study relates to purely tangible enviromental factors. What about the inmense world of not so defineable factors that scientists all over avoid because the scientific world is so focused on tangible, easily probable phenomena. What if some species in the past could have developed through many changes of enviroment and competition purely on their mental capacity to adapt to the enviroment and keep their best at all times?  We are infants next to dinosaurs or meiofauna.

Avatar of: J Ireland

J Ireland

Posts: 1

November 10, 2013

While the authors model would explicitly apply to bacteria also the model has already been disproven there.  Studies of Wolbachia species have shown definite and persistent species isolation even upon mixing of populations that are closely related.  Likewise, work in the Bacillus cereus group has shown ecological and niche isolation and speciation.  I think the authors are over generalizing and purposely avoidied microbes (which would also fit in their model) because of the wealth of evidence to contradict them.

November 18, 2013

It seems to me that species are just as artificial as races. It may be useful to talk about races in certain circumstances and it may be useful to talk about species in certain circumstances, but that doesn't mean the actually exist as sharply outlined categories. In the end, every creature is simply a collection of properties. Some creatures have many in common and as such are logically closely related. Others have fewer in common and are less closely related. Nothing more.

Avatar of: James V. Kohl

James V. Kohl

Posts: 516

Replied to a comment from Bart B. Van Bockstaele made on November 18, 2013

November 18, 2013

Nutrients metabolize to species-specific pheromones that control the nutrient-dependent physiology of reproduction.  Remove those biological facts from any further considerationr and you automagically arrive at differences in species. "This will restrict causal analyses to differential cases (a difference that causes a difference)."

Carl Zimmer probably said it best: "Others maintain that as random mutations arise, complexity emerges as a side effect, even without natural selection to help it along. Complexity, they say, is not purely the result of millions of years of fine-tuning through natural selection—the process that Richard Dawkins famously dubbed “the blind watchmaker.” To some extent, it just happens.' If so, perhaps we should abandon the concept of species in the context of organismal complexity.

We can then move forward with snake-centric evolution of the human brain (e.g., in the one species of Homo that has been here for the past 1.8 million years).  Natural selection must, in theory, somehow occur. Snake predation theory may be essential to consider if mutation-driven evolution is replaced by nutrient-dependent pheromone-controlled adaptive evolution.

Replied to a comment from James V. Kohl made on November 18, 2013

November 18, 2013

I would agree that complexity is not purely the result of natural selection. If we accept that random mutation occurs, and I don't think we have any choice in the matter as the evidence is just too overwhelming, we must also accept increasing complexity as a logical and predictable result. It seems quite logical and predictable that natural selection will weed out at least some of those random mutations, and favour others. It also seems logical that natural selection would be less important for current humans, since we have eliminated natural selection to some degree, certainly not entirely, and therefore, natural selection still occurs.


Avatar of: Valerie Franck

Valerie Franck

Posts: 1

November 18, 2013

There is a very fundamental flaw here. Natural selection works at the level of the individual, whereas evolution works at the level of the population. It is variation among populations that typically leads to new species, not variation within populations. The fact that genetic variation within large populations tends to create a spread of phenotypes, rather than discrete phenotypes, has nothing to do with the ability to form species. All you need is two large populations that become genetically isolated over a long period of time and, when re-introduced, remain genetically isolated. The easiest systems to work in with respect to reproductive isolation are ones where reproduction is strongly linked to courtship behavior: hence the focus on birds, mammals, and fish.

Avatar of: Andy Brower

Andy Brower

Posts: 8

November 20, 2013

Several points:

"Evolution is not concerned with species, but with individuals."  Well, organisms are the things that survive or not, but unless they are asexual, they do not persist or evolve.  So really, from a Dawkinsian perspective, evolution is not concerned with individuals, either, but with replicators (DNA).

"... the biological species concept refers to distinct groups of organisms that can only mate successfully with other members of the same group. This definition at first seems reasonable, but situations have been observed where members of group A can mate with group B, and B with C, but not A with C."  This problem was specifically dealt with in Mayr's original formulation of the BSC:  “A species consists of a group of populations which replace each other geographically or ecologically and of which the neighboring ones intergrade or hybridize wherever they are in contact or which are potentially capable of doing so (with one or more of the populations) in those cases where contact is prevented by geographical or ecological barriers.” (Mayr, 1940. Speciation phenomena in birds. Amer. Nat. 74:249-278).  This seems to invalidate the authors' objections.

"The genetic separation of species is known as the DNA barcoding gap."  That is one very specific type of "genetic species" concept, but there is an enormous literature on species delimitation based on multiple genes or genomic evidence, and equating a genetic species concept with DNA barcoding is extremely misleading.

"One way to approach this problem is through the study of simplified evolutionary models. Analysis of these models can give insights into real-world processes happening over many millions of years that cannot be directly observed."  Another way is through systematics.  Comparison of characters among taxa can also "give insights into real-world processes happening over many millions of years that cannot be directly observed."

Questions:  How does phylogenetic diversity not depend on some species concept?  What are those things at the tips of the branches?

It seems that the authors have a problem with some of the ontological claims of certain species concepts and therefore wish to reject the idea of species concepts altogether.   However, there are other concepts, such as the "phylogenetic species concept" states that species are self-replicating lineages that have fixed character differences that allow their members to be identified, but implying nothing about their metaphysical reality or role in the economy of nature independent of our ability to recognize them.  Such concepts are coherent and applicable across all life, and the authors might find them useful for their own questions.


Avatar of: Joao Furtado

Joao Furtado

Posts: 1

December 31, 2013

The more science-instrumental technology becomes sophisticated, the more the species concept appears as a “management-thinking model” of a research realm in which the inhabitants occupy compartmented, non-integrated, non-systemic discipline-oriented loci and try to interpret how life forms are “catalogued” but not organically structured into the web of life.

More than in the past time, the man-made concept of species resides in a broad spectrum of conjectures and still fits into the very old artificial statement that a species is what the taxonomist considers to be a species.

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