What is a Relict Population

Generally, a relict population can be defined as a part of an evolutionary lineage which has been isolated geographically.

Three types of relicts have to be considered:

1. Geographical relicts without significant genetic differentiation. This type of relict is usually found in "exclaves" beyond the Northern margin of a species' range. Typical representatives of this type are the minute German and Czech populations of N. tessellata and Z. longissimus. Both occupy geographically restricted, climatically favorable habitats along river valleys, preferably the South-facing (sunny) banks and adjacent hills of rivers flowing in latitudinal direction. The same areas are occupied by other "submediterranean" relict species such as L. bilineata and Podarcis muralis. Other such relicts (L. viridis, E. orbicularis) are found in subcontinental areas of Eastern Germany, which have more insolation in summer. All these species must have reached their presently isolated Northern territories in post-glacial times. In climatically favorable times, such as the climatic optimum 5,000-7,000 years ago, they must have extended their range to a much larger territory (e.g., Z. longissimus reached the Baltic Sea Coast (Peters 1977)). Yet the territories they occupy today are small and isolated. They are classified as Holocene relict populations (Table 3). Due to their very restricted occurrence and low population sizes today, these populations must be considered true relicts in Germany, and they are consequently classified in the highest national conservation categories. Yet, if the available genetic data are considered, their extinction would not mean significant loss to the species' total genetic diversity. With the exception of the above-mentioned East German haplotype of E. orbicularis and some single mutations in other species, all major haplotypes found in the isolated German populations also occur in the larger territories of the respective species further South. Of course, caution has to be taken not to rely solely on a limited amount of genetic data - usually of mitochondrial genes. Theoretically, a wide genomic screening could change the observed pattern.

2. Genetic relicts: This type of relicts is usually found at the Southern or Eastern edge of a species' range. Genetic relicts can be defined as possessing a unique, significantly differentiated haplotype or a haplotype group of their own. To qualify as a relict, a historical loss of territory is also necessary. Typically, these relicts have a long evolutionary history in one particular region and were unable to extend their range post-glacially. In some cases, such as the Alpine population of V. berus, the range probably shifted from lowland (Northern Italy) to highland (the Swiss and Austrian Alps), but the range extension to the North was accompanied by loss of territory in the South. This type of relict is older than Holocene. It is classified as Pleistocene relict populations (Table 3), if it is not considered taxonomically distinct. Pleistocene relict populations have often been isolated by ecological change (sometimes with human interference, as the aquatic N. tessellata in the desertifi-cated Aral Sea region, and E. orbicularis in North Africa). Others were isolated from the main distribution area by a competing species, as the L. bilineata population of Cres Island, surrounded by L. viridis, which is thought to have expanded its territory. Another case is V. berus in Greece, restricted to high mountain habitats by both climatic warming and competing V. ammodytes.

3. Endemic relict taxa: If the genetic isolation has prevailed over long enough time, a taxonomic recognition of the isolated population as an endemic subspecies or

Table 3 Endemics and relicts among the investigated taxa

Meta-Taxon

Regional endemic subtaxa

Endemic relict subtaxa

Pleistocene relict populations

Holocene relict populations

Emys orbicularis

E. trinacris

E.o.persica Caspian Area

E. orbicularis

E.o.orbicularis

complex

Sicily

E.o.ssp.

North Africa

E. Germany.

S. Anatolia

E.o.orbicularis

Lithuania.

Turkey

Kazakhstan

Lacerta viridis complex

L. bilineata ssp.

L. viridis ssp.Turkey

L. bilineata Balkans. Cres

L. bilineata

S. Italy, Sicily

Isl.

W. Germany

L. viridis ssp.Balkans

L. viridis

E. Germany

Lacerta agilis complex

La.tauridica Crimea

L.(a.)boemica Caucasus

La.chersonensis

L. <3g/7/sBiitain.

La.bosnica

Carpathians

Sweden

Balkans

La.garzonii

Pyrenees

Hierophis viridiflavus

H.v.xanthurus

S. Italy, Sicily

Zamenis longissimus

Z. lineatus

Z. longissimus

complex

S. Italy, Sicily

Germany.

Czech Rep.

Matrix maura

undescribed

undescribed

Matrix maura

North Africa

Andalucia

Southern Spain

N. tessellata

undescribed

undescribed

N. tessellata

N. tessellata

Middle East

Greece.

Crete.

W. Germany. Czech Rep.

Middle East

Aral Sea region

Vípera bems complex

V.b.bosniensis Balkans

V. barani Turkey

V. berus

V. nikolskii

Alpine Region

Black Sea area

V. berus

V. seoanei

Greece

Pyrenees

V.b.berus

Carpathians

species may be justified. Many of these taxa may also be termed Pleistocene relicts. Table 3 shows some examples. A distinction must be made between an endemic relict taxon and an endemic taxon which is not considered a relict. This decision must be based on the phylogeographic position of the taxa. For example, E. trinacris is an early offspring of the Tertiary Emys population in Europe. It was isolated in Sicily, where it differentiated genetically, but there is no evidence that it ever had a larger range, or that it had descendants which migrated elsewhere.So it is an endemic but not a relict. On the other hand, E. orbicularis persica (group VII in Figs 1 and 2) and the Southern Turkish subspecies (group VIII) must have had a larger range in the past. They were probably linked to Mediterranean groups before their range was restricted by Pleistocene expansion of E.o.orbicularis. A comparable fate may be assumed for L. viridis ssp. from Turkey and L. (agilis) boemica from Caucasus. Their phylogenetic position at the base of their respective radiation suggests that these taxa remained in the area of origin of those species, where they survived in a small area only, while more derived subspecies successively occupied new territories. On the other hand, several L. viridis ssp. from Balkans and L. agilis tauridica from Crimea are endemics, but not isolated, and are not likely to have experienced shrinking of their range. In Natrix, a number of distinct haplotype groups have been identified at the Southern margin of the ranges of N. maura (Andalucia, North Africa) and of N. tessellata (Greece, Iran, Arabia), but no taxonomic consequences have yet been drawn from that observation. We can expect both endemics and endemic relicts among those undescribed taxa, which occur in or near the probable areas of origin of those species (Guicking et al. 2006a). Finally in the V. berus complex, relicts occur at the Southern margin of the distribution area of this cold-adapted vipers. V. barani is completely isolated in Turkey, and V. nikolskii has apparently lost territory by intrograding V.b.berus from the North (Zinenko, unpublished data). V. seoanei, retreating in the Pyrenees, may be under pressure from two warm-adapted species which occur parapatrically: V. aspis and V. latastei (Stümpel et al. 2005).

How will the expected change of climate (global warming) affect relict reptile species? In short, there will be advantages for relicts of thermophilic species in the North, which may enlarge their territories, as is already observed with some plants and invertebrates.

Pleistocene relicts in the South, now confined to mountain areas like V. seoanei in the Pyrenees and V. berus in the Greek mountains, will have to retreat to higher altitudes, while lowland species invade into their former territory. Relicts of European species in North Africa (E. orbicularis) or Asia Minor (L. viridis, V. barani) may die out, while other species take their place.

Thus future climate change will favor some species to the disadvantage of others.

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  • Severino
    What is relict population?
    7 years ago

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