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

Describing the largest prospective study of the preclinical stages of chronic bronchitis yet reported, this book is remarkable in two ways. First, it illustrates the value of relatively simple observations continued over a number of years by a dedicated team of workers in elucidating the natural history of a common disease. Secondly, it shows that if a study ofthis type is carefully planned and diligently executed expert statistical analysis can produce straightforward, unambiguous, and often unexpected conclusions. The authors have employed these techniques most effectively in their study of chronic bronchitis and have made a cogent case for a drastic reappraisal of traditional views on the pathogenesis of that disease. Of particular importance is the distinction made between the obstructive and hypersecretory forms of chronic bronchitis, and their. conclusion that acute infective exacerbations of chronic obstructive lung disease do not, contrary to popular belief, cause a permanent loss of ventilatory capacity. Another pertinent observation is that some cigarette smokers are more liable than others to develop airflow obstruction. Smoking is clearly responsible for most cases of chronic bronchitis, and the incidence of the disease would undoubtedly diminish dramatically if, by some miracle, the habit were to be abandoned by the majority of the population. It would appear, however, that other as yet undertermined factors are implicated in the production of airflow obstruction and mucus hypersecretion. If we could identify those which predetermine the increased susceptibility of some smokers to chronic bronchitis, particularly the obstructive form of the disorder, this knowledge would, as the authors point out, be of considerable value in predicting the course of the disease, and it might also provide the basis for a more rational approach to the practical aspects of prevention and treatment. This book could well be a milestone in the bibliography of chronic bronchitis and related disorders. It deserves to be widely read not only in Britain but also in those countries where the problems posed by this common disease are even less clearly understood.

https://www.bmj.com/content/bmj/1/6059/515.3.full.pdf

Handbook of Enzyme Electrophoresis in Human Genetics

'Summary'	347	
I.	'Introduction'	348	
II.	'Origin of the cospeciation concept'	349	
III.	'Theoretical framework and methods for testing for cospeciation'	349	
IV.	'Studies of natural associations reveal the prevalence of host shifts'	355	
V.	'Relationship between host–symbiont coevolution and symbiont speciation'	378	
VI.	'Conclusion'	381	
 	'Acknowledgements'	381	
 	References	381	
 	Glossary	379	


Summary
Hosts and their symbionts are involved in intimate physiological and ecological interactions. The impact of these interactions on the evolution of each partner depends on the time-scale considered. Short-term dynamics – ‘coevolution’ in the narrow sense – has been reviewed elsewhere. We focus here on the long-term evolutionary dynamics of cospeciation and speciation following host shifts. Whether hosts and their symbionts speciate in parallel, by cospeciation, or through host shifts, is a key issue in host–symbiont evolution. In this review, we first outline approaches to compare divergence between pairwise associated groups of species, their advantages and pitfalls. We then consider recent insights into the long-term evolution of host–parasite and host–mutualist associations by critically reviewing the literature. We show that convincing cases of cospeciation are rare (7%) and that cophylogenetic methods overestimate the occurrence of such events. Finally, we examine the relationships between short-term coevolutionary dynamics and long-term patterns of diversification in host–symbiont associations. We review theoretical and experimental studies showing that short-term dynamics can foster parasite specialization, but that these events can occur following host shifts and do not necessarily involve cospeciation. Overall, there is now substantial evidence to suggest that coevolutionary dynamics of hosts and parasites do not favor long-term cospeciation.

https://nph.onlinelibrary.wiley.com/doi/pdf/10.1111/nph.12150

Cospeciation vs host-shift speciation: methods for testing, evidence from natural associations and relation to coevolution

18. The Phylogenetic Dimension Of Insect-Plant Interactions: A Review Of Recent Evidence

Sympatric populations can diverge when variation in phenology or life cycle causes them to mate at distinctly different times. We report patterns consistent with this process (allochronic speciation) in North American gall-forming aphids, in the absence of a host or habitat shift. Pemphigus populi-transversus Riley and P. obesinymphae Aoki form a monophyletic clade within the North American Pemphigus group. They are sympatric on the eastern cottonwood, Populus deltoides (Salicaceae), but have distinctly different life cycles, with sexual stages offset by approximately six months. Field evidence indicates that intermediate phenotypes do not commonly occur, and mitochondrial and bacterial endosymbiont DNA sequences show no maternal gene flow between the two species. Because a genetically distinct population of P. obesinymphae occurs in the southwestern United States on Populus fremontii, we consider the possibility of an initial allopatric phase in the divergence. We discuss the likely origins of the host use patterns in P. obesinymphae, and the larger sequence of evolutionary changes that likely led to the sympatric divergence of P. populi-transversus and P. obesinymphae. A plausible interpretation at this stage of investigation is that a shift in timing of the life cycle in an ancestral population, correlated with an underlying phenological complexity in its host plant, spurred divergence between the incipient species.

https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.0014-3820.2004.tb01677.x

Phylogenetic and molecular evidence for allochronic speciation in gall-forming aphids (Pemphigus).

Summary

1. The endophagous weevil Lixus elongatus was studied on two Carduus species in order to look for the impact of a structural gradient in host plant stem diameter on the life cycle and the genetic structure of this species.



2. Body size of L. elongatus females was correlated positively with their fecundity and the stem diameter at the location of oviposition.



3. Emerging individuals showed a significantly positive relationship of body size with the stem diameter at their pupation and emergence site. Mating behaviour was positively size-assortative.



4. Allozyme analyses indicated a gradient of genetic differentiation along body size classes and a decreasing heterozygosity with decreasing body size.



5. The results led to the conclusion that a structural gradient in plants can promote body size variability in an endophagous herbivore through phenotypic constraints of the host plant and the herbivore. As size-assortative mating translates this variability into differential gene flow between individuals of different body size, this could provide the raw material for speciation processes. Within a single host plant patch, however, further processes are necessary to split the existing genetic gradient into separate gene pools.

Endophagous insects and structural niches on plants: ecology and evolutionary consequences

Synchronous speciation of hosts and herbivorous insects predicts a congruent topology of host and insect phylogenies and similar evolutionary ages of host and insect taxa. To test these predictions for the specialized herbivorous fly genus Urophora (Diptera: Tephritidae), we used three different approaches. (i) We generated a phylogenetic tree of 11 European Urophora species from allozyme data and constructed a phylogeny of their hosts from published sources. Superimposing the Urophora tree on the host-plant tree we found no evidence for general congruence. (ii) We correlated genetic distances (Nei distances) of the host plants vs. the genetic distances of associated Urophora species. Overall, the relationship was not positive. Nevertheless, for some pairs of Urophora species and host plants genetic distances were in the same order of magnitude. (iii) We collected allozyme data for pairs of thistle taxa and pairs of herbivores on thistles together with independent time estimates. With these data we calibrated a molecular clock. There was a non-linear relationship between phylogenetic age and genetic distance, rendering the dating of deep events in thistle–insect evolution difficult. Nevertheless the derived molecular clock showed that the split of insect taxa lagged behind the split of hosts. © 2005 The Linnean Society of London, Biological Journal of the Linnean Society, 2005, 84, 775–783.

Flies on thistles: support for synchronous speciation?

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Endophagous insects and structural niches on plants: ecology and evolutionary consequences

Flies on thistles: support for synchronous speciation?