"What Determines Species Diversity?" is one of the top 25 questions from the 125th anniversary issue of Science magazine [Science, July 1, 2005]. The complete reference is ...Pennisi, Elizabeth (2005) What Determines Species Diversity? Science 309: 90. [Text] [PDF]Elizabeth Pennisi is a news writer for Science magazine. She has been publishing articles there for at least ten years. She had previously written about genes and genomes, including earlier articles about the number of genes in the human genome and one of the other top 25 Science questions [Why Do Humans Have So Few Genes?]
The question is poorly phrased because it's really about speciation.
Understanding what shapes diversity will require a major interdisciplinary effort, involving paleontological interpretation, field studies, laboratory experimentation, genomic comparisons, and effective statistical analyses. A few exhaustive inventories, such as the United Nations' Millennium Project and an around-the-world assessment of genes from marine microbes, should improve baseline data, but they will barely scratch the surface. Models that predict when one species will split into two will help. And an emerging discipline called evo-devo is probing how genes involved in development contribute to evolution. Together, these efforts will go a long way toward clarifying the history of life.This question is also related to a more fundamental question; namely, what is a species?
Paleontologists have already made headway in tracking the expansion and contraction of the ranges of various organisms over the millennia. They are finding that geographic distribution plays a key role in speciation. Future studies should continue to reveal large-scale patterns of distribution and perhaps shed more light on the origins of mass extinctions and the effects of these catastrophes on the evolution of new species.
This certainly counts as one of the top questions in biology. If we ask it in the form "What causes speciation?" then it gets us into a discussion about punctuated equilibria, founder effects and all kinds of other controversial problems. It also brings up the issue of the role of natural selection and environment in speciation. While there may not be anything new to discover, there are many open questions concerning the mechanisms of speciation. Does sympatric speciation happen, for example?
There are many who think that natural selection plays only a minor role in most speciation events and there are many who think that environmental change is not correlated with speciation [Adaptation and Accident in PNAS, Evolution of Mammals].
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On the most fundamental level the determinant of species diversity is the balance between speciation rate and extinction rate. Of course on a more pragmatic level we have to understand what determines these two components (speciation vs. extiction rate) and how they interact (e.g. how speciation rate is affected/correlated with extinction rate in different ecological and evolutionary settings).
With the current anthropomorphically cause global environmental change (global warming, habitat fragmentation, etc.) understanding the causes of extinctions are of great importance. I believe, however, that understanding the causal basis of speciation is of a particular challenge because it occurs at a much slower time scale than, at least many of today's extinction events and is simply harder to study. These days there is a rich theoretical literature addressing various aspects of the speciation process. Although these models provide many important insights there are still many unanswered questions that remain to be addressed, e.g. the role of genetic correlations in multiple interacting species for (co)evolutionary dynamics and speciation as well as the interplay between ecological dynamics and evolutionary dynamics.
There are many who think that natural selection plays only a minor role in most speciation events and there are many who think that environmental change is not correlated with speciation
There are many who think that natural selection plays an appreciable role in many speciation events. For instance, the following book is well known:
D. Schluter, 2002. The Ecology of Adaptive Radiation (Oxford Series in Ecology and Evolution).
A good book is: S. Gavrilets 2004, Fitness Landscapes and the Origin of Species (MPB-41) (Monographs in Population Biology). And, of course: J.A. Coyne & H.A.Orr, 2004.Speciation.
So, whereas there are lots of random processes, don't overdo the dumping on selection.
I think that this is one of the more fascinating avenues of discovery in evolution; and one that is completely missed out on by people who focus on Evo-Creo because of ID/Creationist misdirection.
That being said it also is a topic that I think creates the biggest problem for those who claim to accept "micro-evolution" but claim "macro-evolution" is impossible.
I have asked ID/Creationists for a definition of what specifically prevents micro-evolutionary "creep" into macro-evolution, and how they can define it when speciation is still not easily defined.
No answer, yet. (Perhaps the question is poorly phrased.)
mario says,
I have asked ID/Creationists for a definition of what specifically prevents micro-evolutionary "creep" into macro-evolution, and how they can define it when speciation is still not easily defined.
Asking IDiots to define something in scientific terms is not likely to be a successful strategy.
However, I have attempted to explain the difference between microevolution and macroevolution and to show why microevolution is insufficient as an explanation of macroevolution [Macroevolution].
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