Conservationists are far from able to assist all species under threat, if only for lack of funding. This places a premium on priorities: how can we support the most species at the least cost? One way is to identify 'biodiversity hotspots' where exceptional concentrations of endemic species are undergoing exceptional loss of habitat. As many as 44% of all species of vascular plants and 35% of all species in four vertebrate groups are confined to 25 hotspots comprising only 1.4% of the land surface of the Earth. This opens the way for a 'silver bullet' strategy on the part of conservation planners, focusing on these hotspots in proportion to their share of the world's species at risk.

Biodiversity hotspots for conservation priorities

So far in this course we have dealt entirely with the evolution of characters that are controlled by simple Mendelian inheritance at a single locus. There are notes on the course website about gametic disequilibrium and how allele frequencies change at two loci simultaneously, but we didn’t discuss them. In every example we’ve considered we’ve imagined that we could understand something about evolution by examining the evolution of a single gene. That’s the domain of classical population genetics. For the next few weeks we’re going to be exploring a field that’s actually older than classical population genetics, although the approach we’ll be taking to it involves the use of population genetic machinery. If you know a little about the history of evolutionary biology, you may know that after the rediscovery of Mendel’s work in 1900 there was a heated debate between the “biometricians” (e.g., Galton and Pearson) and the “Mendelians” (e.g., de Vries, Correns, Bateson, and Morgan). Biometricians asserted that the really important variation in evolution didn’t follow Mendelian rules. Height, weight, skin color, and similar traits seemed to

Human biochemical genetics

Endostatin: an endogenous inhibitor of angiogenesis and tumor growth.

Angiostatin: A novel angiogenesis inhibitor that mediates the suppression of metastases by a lewis lung carcinoma

Hybrid zones are, roughly speaking, narrow regions in which genetically distinct populations meet, mate, and produce hybrids. They are often only a few hundred meters wide and yet may be several hundred kilometers long. They are found in a wide variety of organisms: Tables 1 and 2 list about 150 reasonably clear examples, in which there is a spatial transition between two hybridizing forms. Such a widespread and striking phenomenon requires explanation; more importantly, it offers us several ways of understanding the nature and origin of species. First, hybrid zones pose interesting questions for the taxonomist, for they contrast two views of the species: as a set of populations delimited by genetic barriers to gene exchange; and as a set of populations maintained in a particular stable equilibrium by selection. Second, the wide range of genotypes found in a hybrid zone can be used to analyze the genetic differences and selective forces that separate the taxa involved. This may allow some inferences about the way these differences evolved and, by extrapolation, about the way fully isolated species diverge from each other. Finally, models of parapatric speciation, and of Wright's "shifting balance," involve the formation, move­ ment, and modification of hybrid zones. Hybrid zones must be understood before the plausibility of these models can be judged. After clarifying our terminology, we will summarize the relevant theory and then see how this can be used to make inferences from field data. A notable increase in the number of detailed genetic studies of hybrid zones occurred over

Analysis of Hybrid Zones

/pdf/the-extinction-of-partula-on-moorea-28ikl9750k.pdf

The extinction of Partula on Moorea.

https://www.genetics.org/content/genetics/54/5/1261.full.pdf

The inheritance of polymorphic shell characters in partula (gastropoda).

Land snails of the genus Partula, inhabiting the high islands of the Pacific Ocean, have provided exceptional opportunities for studies oil the origin and differentiation ofspecies: The endemic taxa ofMoorea, in French Polynesia, have been particularly well studied. In an attempt to control the numbers of the giant African snail, Achatina fulica, which is an agricultural pest, a carnivorous snail, Euglandina rosea; has been introduced into Moorea. It is spreading across the island at the rate ofabout 1.2 km per year, eliminating the endemic Partula. One species is aiready extinct in the wild ; and extrapolating the rate of spread of Ezigltmdina , it is expected that all the remaining taxa (possibly excepting P. exigua) will be eliminated by

/pdf/the-extinction-of-endemic-species-by-a-program-of-biological-lclgj87jg3.pdf

The Extinction of Endemic Species by a Program of Biological Control

Land snails of the genus Partula Ferussac offer several advantages for the study of micro-evolutionary change. They are highly variable both within and between populations. Their mobility is low, so that there are genetic differences between natural populations only short distances apart (20 m or less). They are ovoviviparous, and easy to rear in the laboratory, so that these genetic differences can be investigated experimentally. Finally, they show, at least in some places, a very curious pattern of speciation.



Since 1962 we have been studying the population genetics of Partula species on the island of Moorea in French Polynesia.



The late Professor H. E. Crampton, in a classic monograph, recorded ten species from the island. Later, he added an eleventh. One of the species (P. dendroica Crampton) is an allopatric replacement of P. suturalis Pfeiffer and probably deserves only the rank of geographical race, since the two forms cross freely in the laboratory. We have evidence that P. tohiveana Crampton, P. olympia Crampton, and P. mooreana Hartman will all, in some localities, hybridize with P. suturalis. We also have evidence of natural hydridization between P. aurantia Crampton and P. suturalis, and between P. exigua Crampton and P. taeniata Morch.



There are thus two major-species groups on Moorea (the suturalis complex and the taeniata complex). The status of the other ‘species’ (P. mirabilis Crampton, P. solitaria Crampton and P. diaphana Crampton and Cooke) remains uncertain, although they are probably members of the taeniata complex.



Two members of a species-group may behave as distinct species at one locality, but hybridize freely or intergrade at another. Such changes can take place over distances of 200 m or less. Some of the changes seem to be related to geographical barriers, There is, for instance, an apparent ring-species with a diameter of about 5 km. Other changes are more difficult to interpret, since they occur without obvious relation to geographical features.



Ecological and genetic studies on P. taeniata have led to the suggestion that striking divergence between adjacent populations can take place even in the absence of geographical barriers. Whether this can continue to the point of speciation is still uncertain, but an explanation in these terms would clarify many of the puzzling phenomena we have observed in populations of Partula.

Ecological genetics and speciation in land snails of the genus Partula

Land snails of the genus Partula Fer. inhabiting the island of Moorea in French Polynesia show complex patterns of partial speciation. The late H. E. Crampton described eleven species from the island. Two have since been relegated to the genus Samoana . Of the remaining nine taxa, as many as four may coexist at a single locality without evidence of hybridization. Nevertheless an allele appearing in any of the taxa is potentially capable of spreading to any of the others, although in the process it may have to pass through several intervening taxa. Large morphological, ecological and behavioural differences have evolved despite this potential for gene flow. The biological differences are not reflected in 20 enzyme loci surveyed by conventional starch-gel electrophoresis, nor in the numbers of chromosomes. It is clear that a significant part of the morphological divergence between taxa has taken place of Moorea. Our observations are consistent with the view that all the Moorean taxa (excepting Samoana ) are descendants of a single ancestral invasion, although the possibility of more than one invasion cannot be excluded. The pattern of differentiation shows at least two cases of circular overlap (‘ring species’), with diameters of 2 and 3 km, and several apparent examples of character displacement. Coexisting taxa tend to partition the available habitat, although the separation is never complete.

James Murray

Papers

The extinction of Partula on Moorea.

The inheritance of polymorphic shell characters in partula (gastropoda).

The Extinction of Endemic Species by a Program of Biological Control

Ecological genetics and speciation in land snails of the genus Partula

The Genus Partula on Moorea: Speciation in Progress