Global climate has fluctuated greatly during the past three million years, leading to the recent major ice ages. An inescapable consequence for most living organisms is great changes in their distribution, which are expressed differently in boreal, temperate and tropical zones. Such range changes can be expected to have genetic consequences, and the advent of DNA technology provides most suitable markers to examine these. Several good data sets are now available, which provide tests of expectations, insights into species colonization and unexpected genetic subdivision and mixture of species. The genetic structure of human populations may be viewed in the same context. The present genetic structure of populations, species and communities has been mainly formed by Quaternary ice ages, and genetic, fossil and physical data combined can greatly help our understanding of how organisms were so affected.

http://webpages.icav.up.pt/PTDC/BIA-BEC/105083/2008/hewitt2000.pdf

The genetic legacy of the Quaternary ice ages

The genetic effects of pleistocene ice ages are approached by deduction from paleoenvironmental information, by induction from the genetic structure of populations and species, and by their combination to infer likely consequences. (1) Recent palaeoclimatic information indicate rapid global reversals and changes in ranges of species which would involve elimination with spreading from the edge. Leading edge colonization during a rapid expansion would be leptokurtic and lead to homozygosity and spatial assortment of genomes. In Europe and North America, ice age contractions were into southern refugia, which would promote genome reorganization. (2) The present day genetic structure of species shows frequent geographic subdivision, with parapatric genomes, hybrid zones and suture zones. A survey of recent DNA phylogeographic information supports and extends earlier work. (3) The grasshopperChorthippus parallelusis used to illustrate such data and processes. Its range in Europe is divided on DNA sequences into five parapatric races, with southern genomes showing greater haplotype diversity — probably due to southern mountain blocks acting as refugia and northern expansion reducing diversity. (4) Comparison with other recent studies shows a concordance of such phylogeographic data over pleistocene time scales. (5) The role that ice age range changes may have played in changing adaptations is explored, including the limits of range, rapid change in new invasions and refugial differentiation in a variety of organisms. (6) The effects of these events in causing divergence and speciation are explored usingChorthippusas a paradigm. Repeated contraction and expansion would accumulate genome differences and adaptations, protected from mixing by hybrid zones, and such a composite mode of speciation could apply to many organisms.

https://www.fc.up.pt/mestr_biodiv/aulas/hewitt_1996.pdf

Some genetic consequences of ice ages, and their role in divergence and speciation

The Quaternary cold periods in Europe are thought to have heavily influenced the amount and distribution of intraspecific genetic variation in both animals and plants. The phylogeographies of 10 taxa, including mammals (Ursus arctos, Sorex spp., Crocidura suaveolens, Arvicola spp.), amphibians (Triturus spp.), arthropods (Chorthippus parallelus), and plants (Abies alba, Picea abies, Fagus sylvatica, Quercus spp.), were analysed to elucidate general trends across Europe. Only a small degree of congruence was found amongst the phylogeographies of the 10 taxa, but the likely postglacial colonization routes exhibit some similarities. A Brooks parsimony analysis produced an unrooted area phylogram, showing that: (i) the northern regions were colonized generally from the Iberic and Balkanic refugia; and (ii) the Italian lineages were often isolated due to the presence of the Alpine barrier. The comparison of colonization routes highlighted four main suture-zones where lineages from the different refugia meet. Some of the intraspecific genetic distances among lineages indicated a prequaternary divergence that cannot be connected to any particular cold period, but are probably related mainly to the date of arrival of each taxon in the European continent. As a consequence, molecular genetics so far appears to be of limited use in dating Quaternary events.

Comparative phylogeography and postglacial colonization routes in Europe

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

Population structure is the result of both present processes and past history. Molecular markers are proving of great value in describing the former, and it is important to similarly determine the latter in order to understand their respective contributions. The study of palaeo-climates has also advanced significantly, and in particular that of the Pleistocene ice ages, which modified species ranges considerably. The last ice age and rapid post-glacial colonization of Europe is summarized. Possible population genetic consequences of expansion northward from southern refugia, and those of remaining in these mountainous regions are discussed. A series of recent case studies are detailed where DNA sequence information has been used to describe species genetic variation and subdivision across Europe. These include a grasshopper, the hedgehog, oak trees, the common beech, the black alder, the brown bear, newts, shrews, water vole, silver fir and house mice. These molecular data confirm southern peninsulas of Europe as major ice age refugia, and in most cases demonstrate that genetically distinct taxa emerged from them. They can thus define genomic differences and so greatly augment previous fossil data. The refugial genomes contributed differently in various species to the re-colonization of Europe, with three broad patterns described as paradigms—«grasshopper», «hedgehog» and «bear». These different expansion patterns produced clusters of hybrid zones where they made contact, and it is argued that many species genomes may be further cryptically subdivided. A reduction in diversity from southern to northern Europe in the extent of allelic variation and species subdivision is seen; this is attributed to rapid expansion northward and the varied topography of southern refugia allowing populations to diverge through several ice ages. The differences in DNA sequence indicate that some species have been diverging in refugial regions for a few ice ages at most, whilst distinct lineages in other species suggest much more ancient separation.

https://www.fc.up.pt/mestr_biodiv/aulas/hewitt_1999.pdf

Post-glacial re-colonization of European biota

From a study of the geographic occurrences of contemporary hybridization among North American animals, it has become apparent that most of the hybrids are produced in a few relatively localized zones, with little hybridizing in the vast areas between these zones of mixing. The hybrids tend to be at least moderately fertile and therefore to be a source of significant gene exchange between the typically allopatric pairs of species and semispecies. There is a wide variety of consequences from this introgression, with greater or lesser influence on the parental populations, and large portions of the fauna and probably flora are involved. An appropriate term for such a belt of interfaunal and interfloral linkage is suture-zone.1

Suture-Zones of Hybrid Interaction Between Recently Joined Biotas

Males of two closely related, co-occurring species of Lycaena butterflies have dorsally blue (Lycaena heteronea) or red-orange plus ultraviolet (Lycaena rubidus) wings. Males are selectively territorial against conspecific males. Virgin females accept only conspecific males, probably chosen by wing color. Females are nonterritorial and spend most of their adult activity ovipositing on the correct larval food plants. Eyes of both species contain four spectral types of visual pigments (P360, P437, P500, and P568) but the distribution of these pigments within the receptor mosaic is quite different between both species and sexes. The ventral eye region of L. heteronea is tetrachromatic but that of L. rubidus is trichromatic, lacking the blue-sensitive visual pigment P437. The dorsal eye region of males of both species is dichromatic (P360 and P437). Visual-pigment spectra and wing-reflectance spectra are well matched for effective discrimination of wings of conspecific males from those of other species. The dorsal region of female eyes is trichromatic, containing P360, P437, and P568. The third visual pigment, P568, is important for long-range detection by ovipositing females of red coloration on Eriogonum and Rumex food plants. P568 has the same absorbance spectrum as the human red-cone and is considerably red-shifted compared to the P530 possessed by most insects. That the sexes and closely related species can have such major differences in distribution of visual pigments indicates that the visual system is as readily altered as wing coloration in the course of adaptive evolution.

https://www.pnas.org/content/pnas/88/7/2783.full.pdf

Color vision in Lycaena butterflies: spectral tuning of receptor arrays in relation to behavioral ecology.

The Population Genetics of Insect Introduction

Publisher Summary The Lepidoptera known cytologically are characterized by the ZO and ZW type of sex chromosomes, with the female heterogametic (ZO or ZW) and the male homogametic (ZZ). Secondary sexual characters are independent of gonadal control or hormones circulating in the hemolymph. A new bilateral gynandromorph of colias philodice is figured. The right side is female and alba and the left side is male and normal yellow except for alba patches in the distal part of the hindwing. It is suggested that these patches are produced over cells that have become femalelike by losing one Z chromosome late in development, with the entire insect presumed to have a uniform Aa or AA genotype for the alba genes. It is not known whether colias females are ZO or ZW. The chromosome number for the five species studied in this chapter is typically 31 or 32. As in Diptera, it appears that crossing over in Lepidoptera may be confined to the homogametic sex (male).

The Genetics of Colias (Lepidoptera)

Since the end of the Nineteenth Century the taxonomy of the small order Thysanura has been extensively treated by a few specialists, notably F. Silvestri, P. W. Wygodzinsky, and to a lesser degree K. W. Verhoeff, K. Escherich, H. W. Womersley, and J. Stach. All of these men have recognized and named various new genera and numerous new species. Among them only Verhoeff and Escherich have attempted to develop a major classification of the species into supra-generic groups. Verhoeff's classification of the saltatorial Thysanura (Machilidae of most textbooks) and that of Escherich for the non-saltatorial forms (Lepismatidae of authors) are still followed with little change by recent students of the order.

Charles L. Remington

Papers

Suture-Zones of Hybrid Interaction Between Recently Joined Biotas

Color vision in Lycaena butterflies: spectral tuning of receptor arrays in relation to behavioral ecology.

The Population Genetics of Insect Introduction

The Genetics of Colias (Lepidoptera)

The Suprageneric Classification of the Order Thysanura (Insecta)