Diversity Within and Among Species

Thoughts on diversity, ecology and evolution

Andrew L. Hipp, PhD
Plant Systematist and Herbarium Curator
The Morton Arboretum

In the fourth chapter of The Origin of Species, Darwin presents a phylogenetic tree depicting the relationships among a hypothetical set of individuals. Natural selection, Darwin explains, causes the branches on this tree to diverge from each other in ecology, behavior, and form. The tree thus portrays how a single population can give rise to a multitude of varieties.

After describing how natural selection creates varieties from a single population, Darwin writes:

If we suppose the amount of change . . . in our diagram to be excessively small, these three forms may still be only well-marked varieties; but we have only to suppose the steps in the process of modification to be more numerous or greater in amount, to convert these three forms into doubtful or at least into well-defined species. Thus the diagram illustrates the steps by which the small differences distinguishing varieties are increase into the larger differences distinguishing species.

Natural selection, Darwin tells us, causes populations to become adapted to their environment, and, consequently, to differentiate from one another, and it is this same process that eventually leads to the formation of new species, new genera, new families, and even phyla and kingdoms. Stated another way, the wide range of biological and ecological diversity we see today is a product of natural selection and lots of time. Darwin’s insight was that the mechanism by which populations diversify within species is the same mechanism by which species, genera, or families diversify. Microevolution—evolution within species—and macroevolution—evolution at the species level and higher—work hand-in-hand.

I have been thinking recently about how macroevolutionary and microevolutionary approaches provide complementary views on biological diversification. Let me give you an example from our work in the evolution of sedges (Carex: Cyperaceae), a fascinating group of plants that represent more than 5% of the diversity of the Great Lakes region.

In sedges, chromosome rearrangements evolve rapidly within and among species. Many species possess numerous chromosome arrangements, but each species still appears to be genetically, ecologically, and morphologically coherent, often across a wide geographic range. This is striking: with so much chromosomal variation, we might expect sedges to have splintered into even more species than they have. Sedges are quite diverse, with more than 2,000 species worldwide, and chromosome evolution seems likely to have played an important role in their diversity. In our lab, we use molecular methods to estimate evolutionary relationship between species, and statistical methods to estimate how chromosome rearrangements evolve between species. These are primarily macroevolutionary approaches. We have found using these approaches that chromosome breakages and fusions evolve rapidly to different equilibrium points in different sedge lineages, such that different branches on the sedge tree of life are marked by different dynamics of chromosome evolution. We know that one such transition in chromosome evolutionary dynamics occurred between 2-8 million years ago, somewhere near the ancestor of a lineage of more than 35 sedges that characterize wetlands, grasslands, and woodlands of the Great Plains and eastern North America. These shifts in evolutionary dynamics have the effect of driving chromosome rearrangements apart in different lineages, and they consequently limit the opportunities for hybridization between lineages. We find, using a macroevolutionary approach, a plausible mechanism by which changes in the dynamics of chromosome evolution may lead to increased species diversity.

This story from a macroevolutionary perspective raises the question of whether chromosome rearrangements play a role in genetic diversification within species. To get at this, we are working with collaborators Paul Rothrock and Richard Whitkus  to investigate how chromosome rearrangements affect gene flow among populations within two sedge species, one from eastern North America and one from the West Coast. In both species, we find a striking pattern: the difference in chromosome number between individuals or populations correlates significantly with their genetic distance. Furthermore, it is not the existence of a single rearrangement that limits gene flow, but the interaction between rearrangements. What this suggests is that as populations accumulate different chromosome rearrangements over time, they lose the ability to exchange genetic material. Chromosome evolution consequently has the potential to work together with ecological diversification to create new species. Moreover, we find that there is almost no genetic structure within the eastern North American species, in contrast to the deep genetic breaks found between closely related species. These findings tell us that chromosome rearrangements may play a role in allowing populations to diverge from one another by limiting hybridization between those populations, but that the chromosome rearrangements we find within species are insufficient on their own to cause speciation. They are most likely working hand-in-hand with ecological divergence or the evolution of reproductive incompatibilities to form the diversity we see today.

We know many details now about the generation of biodiversity that Darwin didn’t know, and the relative importance of natural selection and neutral genetic drift in diversification has been argued at length throughout the latter half of the 20th century. But the importance of evolution by natural selection as a unifying mechanism of biological diversification cannot be overstated. When we talk about plant biodiversity, we are really talking about diversity both within and among species.

For further reading:
 Darwin’s phylogenetic tree, from chapter four of The Origin of Species.
The Origin of Species, 6th (1872) edition.
Hipp et al. 2009. The most current review of what we know about the pattern and process of chromosome evolution in sedges.

What were Darwin’s key ideas and how are modern scientists using them? Who was Darwin? Try this video clip that features Dr. Andrew Hipp, Plant Systematist and Herbarium Curator at The Morton Arboretum speaking about the implications of Darwin’s work on his won research. And we have more. Click here for questions, answers, videos and a whole new learning experience!

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