Species in trouble

Modern genetics has advanced our understanding of how evolution comes about at the cellular and molecular level. What is not as clear is the statistical way in which nature actually selects out of random changes at the molecular level – do organisms undergo continuous change? Do the changes occur at separate events, either genetic or environmental? 

The classical belief is what is known as the Red Queen theory, which is that the change occurs continuously, to keep in step with the environment. A group at the University of Reading, UK, has published a report that confirms the Red Queen theory, but with a difference.

Red Queen theory
The name derives from Lewis Carroll’s Alice through the Looking Glass, in which the young girl lands in a fantasy world where chess-board characters come to life; the queen (of the chess-board) and Alice are continuously running but stay at the same place. This condition has become an accepted technical term in different fields.
In the field of evolution theory, it refers to the random genetic mutations in a species that must continuously happen to match changes in the environment. But there is no way the changes can be sufficient, as the environment keeps changing, sometimes due to genetic adaptation or extinction of other species.

The species, thus, never really adapts, but must sooner or later. If it fails to keep in step with the change that is required, it becomes extinct. The name was proposed in 1973 by Van Valen, an American evolutionary biologist who noticed from extensive fossil records of the changes in species, that there seemed to be a constant probability that a family of related organisms would lose the race against the constantly changing environment.

While Van Valen and many studies that followed did favour the constant rate model (of continuous change), it was not actually tested against other mechanisms that could have led the same results – except one suggestion that the probability of changing species decreases with the age of an organism in one form, while the probability of extinction increases.

Other models of speciation
There can be different ways for species to separate and diverge. While gene flow, or the intermingling of individuals in a group, would prevent one genetic strain from growing apart from the remainder, the preferred survival of one strain needs ‘genetic drift’ and ‘natural selection’. The new strain needs to be kept separate and also be given an advantage to multiply.
 
Geographical separation
One way this can happen is when parts of a species are physically separated, for example if a mountain range appears or a river changes its course. Or it could happen if a group of individuals migrate and move to a different geographical location. When the populations are physically apart like this, there is no gene flow and the two groups evolve separately.

Another way is when the individuals are all in the same location, but there is a division of the area by some physical feature, like soil type. Here, although gene flow can occur, strains may have high fitness in one region and low fitness in the other and separate species will arise. An example of such evolution in action has been seen in the Mount Carmel area in Israel, where the two sides of a valley have different conditions, one arid the other lush, because of the shadow of the E-W mountain range.

A separate species can occur in the same region and area if there are reasons to prevent gene flow, like parasites specialising on different hosts or plants favouring different pollinators. The Monkey flower is an example, the pink variety is pollinated by bumblebees while the red by hummingbirds.

Statistical analysis
Chris Venditti, Andrew Mede and Mark Pagel of the University of Reading used statistical methods to distinguish between the factors of evolution, exploring 750 genetic family trees, covering different animal, plant and fungal species, including bumblebees, cats, turtles and roses.

The genetic family tree is the history of how a genetic group evolves together and periodically separates and branches out. The analysis was of the probability of branching and of the length of the branches till they separated again. The reasons for branching could be different ones, and groups of causes could be simply additive or more drastic and multiplicative. Additive causes would lead to well known ‘normal’ probability distribution. Multiplicative causes would lead to stronger bunching, in the ‘logarthmic normal’ distribution.

Yet another cause is the ‘rare’ or accidental cause, that is to say, by events unrelated to each other. Such events follow the Poisson distribution, used to study lightning strikes or the number of calls at a call centre, or even the occurrence of galaxies in the universe. A variant is the Weibull distribution, which can take different parameters for conditions like ‘infant mortality’ (failure is likely early in the experiment and gets less likely as time passes) or ‘ageing’, where there are more failures as time passes.

Results of the study
Rigorous analysis of the data has amounted to a stringent test of the hypothesis that new species arise at a constant rate, which suggests a mechanism by which different strains will arise among independently evolving groups. But the Red Queen theory is seen to arise even by considering that the causes of species arising are many and ‘rare’, and where each one is capable of leading to speciation by itself. The Red Queen theory appears to hold, but not the ‘constant rate’ assumption – it is the separated and unrelated causes that bring on the speciation.

The authors say that this way of thinking has implications for understanding why some groups have so many more species than others.

If speciation is driven by rare and somewhat unpredictable events, then it will be the number of such events that sets the rate of speciation. Researchers seeking explanatory theories of speciation should focus on how many sufficient causes are shared by a group of organisms, rather than on special driving forces or how these forces might combine.