In a world without natural selection and no vast mountain ranges dividing populations, one might expect biodiversity to remain forever stagnant. But according to a study published this week in Nature, new species can arise arbitrarily and without provocation, challenging the widely held notion that physical isolation and selection are the driving forces behind speciation.
Image: Wikimedia commons"So much of ecology and evolutionary biology is based on this idea of adaptive divergence leading to
In a world without natural selection and no vast mountain ranges dividing populations, one might expect biodiversity to remain forever stagnant. But according to a study published this week in Nature
, new species can arise arbitrarily and without provocation, challenging the widely held notion that physical isolation and selection are the driving forces behind speciation.
|Image: Wikimedia commons|
"So much of ecology and evolutionary biology is based on this idea of adaptive divergence leading to speciation," said evolutionary biology linkurl:Charles Goodnight;http://www.uvm.edu/%7Ebiology/Faculty/Goodnight/Goodnight.html of the University of Vermont, who was not involved in the work. "What this [study] is saying is that speciation may just be a result of random processes."
In 2001, linkurl:Stephen Hubbell;http://www.plantbio.uga.edu/%7Eshubbell/Webpages/Members/steve_wp.htm of the University of Georgia proposed the neutral theory of biodiversity, in which the patterns of biodiversity across the globe are explained largely by chance. The idea brought into question the traditional, niche-based view of ecological community structure, which posits that organisms diffuse across a variable environment as a result of competition for resources. Hubbell's theory, explained physicist linkurl:Amos Maritan;http://www.padova.infm.it/sezioneb/PeopleMaritan.htm of the University of Padova in Italy, who wrote an accompanying review to the current study, demonstrated that this type of species segregation can happen "in a spontaneous way."
However, neutral theory described the spatial distribution of species once they form, but not how or why they arise in the first place. Complex systems biologist linkurl:Yaneer Bar-Yam;http://necsi.org/faculty/bar-yam.html of the New England Complex Systems Institute in Cambridge, Mass., and colleagues expanded this model to explain the process of speciation. They found that starting with a population of genetically identical individuals in a homogeneous environment, sexual reproduction, mutation, and limited dispersal led to the splitting of species -- as defined by a threshold genetic distance -- after just 300 generations, in the absence of physical barriers and selection.
"Traditionally, it was believed that most species arise because physical barriers prevent mating for long enough for the populations to diverge," said Bar-Yam. Similarly, natural selection in a heterogeneous environment can explain species divergence, as spatially divided populations adapt to their local environments. "But what our work shows is that's not necessary," he said.
"That doesn't mean that [geographic barriers and selection] are not playing a role," Bar-Yam added. It's like a spontaneous traffic jam, he explained. An accident is not necessary for traffic to back up. "It's enough just to have heavy traffic, and you'll have jams forming," he said. But if there is an accident, there's no doubt the traffic will slow. Likewise, "if there is a barrier, you expect that species will form," he said, "[but our results suggest that] the underlying process of spontaneous formation of species is so strong that it's overwhelming [such local] processes."
As in previous models of neutral selection, the patterns of biodiversity estimated by this new model accurately reflected the observed patterns in nature, Bar-Yam said. From speciation rates to patterns of species richness and abundance, the model produced spatial dynamics that approximated the empirical data known for a variety of species, including plants, birds, and fish. The universality of these results raises "the possibility that something really simple could be underlying many of the patterns seen," said physicist linkurl:Jayanth Banavar;http://www.phys.psu.edu/people/display/?person_id=19 of Penn State University, who coauthored the accompanying review with Maritan. Species may arise and coexist simply as a result of spatial and genetic diffusion, he said.
However, "more study is needed to assess whether the assumptions are in fact justified in real field data," Banavar cautioned, such as how genetically similar individuals must be in order to successfully produce offspring and the distance those offspring disperse after birth. Additionally, the model must be expanded to include how species interact with each other, Maritan added, as "interactions are relevant to understanding biodiversity."
Still, this simplified model is "a step forward," Banavar said. It examines "speciation in a more natural way than has been done previously [while] retaining many of the patterns that [are] seen in nature. It's the next step in considering realistic speciation processes."
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[2nd October 2008]