The complete answer to your question would require several volumes to explain. So I’m going to take one single, tiny piece and attempt to show how evolution (macroevolution for those who insist on this arbitrary distinction), forms a part of the bedrock upon which my work rests. As I said before, I don’t “do” evolution on a daily basis. Rather, what I do on a daily basis would be impossible if others hadn’t done the hard bits - using evolutionary theory to develop an understanding of nature that allowed them to generalize from specifics. Those generalizations form the basis for what I do. The taxon cycle appears to fit your request. It is one of - not the only - the bases for biogeographical patterns that have direct bearing on the work done by those of us who operate in fragmented habitats, because these habitats are functionally equivalent to islands. Islands are where the taxon cycle was first identified.
The taxon cycle is one of the foundational parts of island biogeography. It explains the pattern observed in biological succession on islands - a pattern that has been confirmed by observations of Krakatao and Surtsey, as well as older islands such as Madagascar. I will be substituting “fragment” for island here, as the exact same patterns have been observed in fragmented habitats. The taxon cycle is a macroevolutionary explanation that shows how over evolutionary timescales colonization leads to increasing endemicity and vulnerability to subsequent colonization. The cycle couples paleontology to observation of modern species. Besides the areas that are pertinent to my work (vulnerability to invasion), it also provides an explanation for such observations as character displacement, adaptive radiation, relictual populations in fragmented/island-type habitats, rapid speciation, and faunal turnover.
There are four stages of the taxon cycle:
I. In stage one, organisms invade the fragment from outside. These may be populations which are related to those already present, or may represent non-native exotics. IOW, depending on distance, species dispersal ability is the key criteria.
II. In stage two, the invader may expand its niche by invading other habitats. Species gradually evolve local forms that become restricted to a subset of the available habitat. They then become vulnerable to being out-competed in their original niche by further specialized invaders.
III. Species can become highly differentiated endemics that ultimately become extinct and are replaced by new invaders. Stage three species thus evidence a longer history of evolution in isolation, being found in scattered endemic forms.
IV. The final distributional pattern in the cycle is when a highly differentiated endemic species persists as a relic OR is found only as fossil or sub-fossil remains.
One reason for the success of invaders, as outlined above, is that a recent arrival may have left behind predators, parasites, and competitors on colonizing the new habitat, enabling it to flourish despite the existence of local forms with a longer period of evolutionary adaptation. We can tell relatively when a species has established itself and roughly what stage it is in by examination of the fossil record (this is especially true on large, old islands such as Hawaii and Madagascar). Distribution patterns of existing species are also indicative of relative “age” and stage, as invasives will have a greater effect on population distribution as earlier species get “squeezed” into smaller habitat patches. This is also a way to tell how impacted the habitat is by the invasive (what stage it is in).
How is this macroevolution? We can trace changes in faunal assemblages on islands through the paleontological and paleoecological records. An example, New Providence Island in the Caribbean has changed from a highly xeric environment in the Late Pleistocene, to a moist tropical environment today. Faunal assemblages have changed as much as 50% in that time frame. In those areas where a good sub-fossil record exists, we can trace the gradual change over fairly recent time frames due to environmental shifts or waves of invasives. None of this, of course, depends on absolute dates - relative dates are sufficient. Plus, we can get a pretty fair “timeframe” from observations of new islands such as Anak Krakatao to show the amount of time necessary for short term adaptations and subsequent waves of colonization to affect the local systems.
So where does this lead in my work? As I noted, I don’t try and “re-prove” the taxon cycle in my daily work. I don’t have to - someone else (many someones, actually), has already done that. However, I can and do use the concepts “proven” by the hypothesis as a foundation for some questions I need to answer. For instance, using the
Agave americana example from my previous post, I can finally answer the question: Should I be worried?. Using the concepts developed under the pertinent aspects of the taxon cycle hypothesis, coupled with other observations from biogeography, I can look at a comparison of the
Agave’s native habitat and my site (both xeric, for instance). I can look at a comparison between the community composition of the species’ native range, and the one I’m concerned about (
Agave parasites, diseases, herbivores, etc) to determine if there are any species with counter-adaptations in my site that would control or limit expansion of
Agave populations. Finally, I can look at the historical record of
Agave distribution patterns in other areas and formulate a risk assessment in mine (i.e., is it susceptible to ecological release, etc). Each of these
concepts is based on a macroevolutionary theory developed from applying paleontological and paleoecological patterns to existing species and populations. I’m not concerned about macroevolution in my daily work, but that work would not be possible - or worse, would be wrong - UNLESS the macroevolutionary concepts had proven accurate over the long term. From the distribution patterns of
Molothrus bonairiensis in the Lesser Antilles to the invasion risk from an exotic import, it all depends on macroevolutionary theory. Not using “macroevolution” daily, but the concepts developed under the theory are the ones I DO apply every day. And remember, the taxon cycle is only one, teeny tiny aspect of biogeography, which is indeed only one aspect of my work. I can make a similar case for just about all the rest of what I do - at least as far as the theoretical underpinnings I rely on are concerned.
Wave it away as you wish, the framework provided by the ToE - not just population genetics - is critical for decision-making in current applied ecology.
Excuse me, I need to go dig up that damn
Agave patch before it eats my reserve.
{The outline of the taxon cycle above was derived in part from Whittaker, RJ, 2002,
Island Biogeography, Oxford Uni Press. I have used the conceptual framework of this book so often as a reference that I've had to replace it twice.}
™ Version 4.2
