New model for speciation

A new model for how species arise in the absence of natural selection and physical separation of gene pools suggests the patterns observed in some cases can be attributed to mere chance. This questions the long held view that species arise solely as a result of competition for resources and reproductive isolation.

A single gene mutation has just been identified in a melanocortin-1 receptor gene that appears to lead to underly the changes in plumage and mating habits in Monarch fly catching birds in the Solomon islands off Papua New Guinea that has lead to a speciation being observed in the wild. Interesting articles.

http://www.eurekalert.org/pub_releases/2009-07/uocp-sct071409.php

See: http://www.nature.com/nature/journal/v460/n7253/full/nature08168.html

From: http://www.the-scientist.com/blog/display/55825/

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 Charles Goodnight 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, Stephen Hubbell 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 Amos Maritan 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 Yaneer Bar-Yam 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 Jayanth Banavar 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."

Originally posted by Diodorus Siculus
A new model for how species arise in the absence of natural selection and physical separation of gene pools suggests the patterns observed in some cases can be attributed to mere chance. This questions the long held view that species arise solely as a result of competition for resources and reproductive isolation.

A single gene mutation has just ...[text shortened]... e next step in considering realistic speciation processes."

This may be true but it doesn't eliminate the selection pressure of isolation as a mechanism. It just adds to the list.

Originally posted by sonhouse
This may be true but it doesn't eliminate the selection pressure of isolation as a mechanism. It just adds to the list.

I have perhaps confused things here by presenting two important new papers in evolutionary biology at the same time. The work on the single point mutation in MC1R in the bird species appears to be classic allopatric speciation - geographical isolation and genetic drift leading to a reproductive boundary and subsequent speciation event.

What is surprising about the study is that as few as one heritable alteration affected behaviour leading to an evolutionary branching. The caveat is that they have looked at genes likely affecting plumage so the genetic analysis is non-exhaustive so occult changes in the unanalysed portion of the genome may have contributed to behavioural modifications in mating observed. Pyrosequencing in the future will likely allow whole genome sequencing in these circumstances, however, epigenetic factors may also plausibly be involved in speciation events which would not be detected even in these instances.

The study looking at chance as a mechanism in speciation events is theoretical and is importantly in groups without reproductive boundaries imposed by extrinsic boundaries like mountains, islands etc. and also the absence of predation and competition for scarce resources. In these instances, speciation still appears to occur over quite short time intervals which is a new, exciting idea and counter-intuitive as the prevailing idea is that without overt selection pressure, "evolutionary stasis" and no or few new species would appear.

Originally posted by Diodorus Siculus
I have perhaps confused things here by presenting two important new papers in evolutionary biology at the same time. The work on the single point mutation in MC1R in the bird species appears to be classic allopatric speciation - geographical isolation and genetic drift leading to a reproductive boundary and subsequent speciation event.

What is s ...[text shortened]... ithout overt selection pressure, "evolutionary stasis" and no or few new species would appear.

So the bottom line might be there is a reason for mass extinctions, makes room for something better, eh.

Originally posted by sonhouse
So the bottom line might be there is a reason for mass extinctions, makes room for something better, eh.

Reason doesn't come into speciation events or patterns of diversification after mass extinctions and anthropomorphising nature holds up progress in my opinion.

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.

This is the scenario that comes to mind - is this correct?

Say you have a large population of Plant A that stretches uniformly across an entire continent with no physical barriers. If dispersal is limited, over many generations it is likely (by random chance) that the plants on the west coast will become very distinct from the plants on the east coast (with a range of intermediates occurring in between). As more generations go by all of these differences would intensify until at least the west coast and east coast plants became distinct enough from each other to become separate species.

As such, the "physical barrier" would be the large range of the species itself - which would make complete mixing impossible for a species where "dispersal is limited"

Originally posted by Melanerpes
They found that starting with a population of genetically identical individuals in a homogeneous environment, sexual reproduction, mutation, and [b]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.

This is the scen ...[text shortened]... elf - which would make complete mixing impossible for a species where "dispersal is limited"[/b]

That's similar to how I view it too. The error-prone proof reading of DNA polymerase I generates genetic variation allied with recombination during sexual reproduction at least in this model for a branching to occur even in the absence of natural selection and physical barriers. The increasing appreciation of horizontal gene flow also likely contributes greatly to speciation by generating intrinsic barriers to reproduction.