Ghost lineage

The Comoros coelacanth ( Latimera chalumnae ), one of two recent representatives of the coelacanth ...

... and Macropoma lewesiensis from the Upper Cretaceous . There is a 70 million year ghost lineage between the two species.

In paleontology and phylogenetics, lines of descent are referred to as ghost lineage ( German, for example, "ghostly descent") , the presence of which in a certain period (or stratigraphic unit) has not been directly proven, but only indirectly developed. The concept and phrase go back to paleontologist Mark Norell . The term “ghost range” is also used for the duration of a certain line. Ghost lineages are fundamentally problematic because they each correspond to an additional auxiliary hypothesis. However, their use in research is inevitable in some areas. In practice, according to Ockham's razor , when interpreting the data, preference is usually given to the hypothesis that results in fewer or at least shorter ghost lineages .

Ghost lineages arise as a result of a number of different methods and theoretical derivations. Mostly they are based on the fact that the same problem, e.g. B. the age or the relationships of a certain group of organisms, different and independent methods are used. Different age estimates are based primarily on absolutely dated fossils, age estimates derived with the methods of the molecular clock or sequences of splitting events of evolutionary lineages derived from the cladistic analysis of the relationships of a group .

Interpolations

The simplest case of generating ghost lineages is based directly on the incompleteness of the fossil record. If a certain taxon (a species, a genus or a higher taxonomic unit) is found in a deposit of a geological time epoch and in another that has a different age, the taxon must also have existed in the intervening time. In paleontology, the application of this obvious principle is often referred to as the “ range through method ”. In the same way, a taxon can be found both fossil and living ( recent or extant). A prominent example of this are the coelacanth , which has only been preserved in fossil form as far as the Cretaceous period and which, up to the representatives living today, has a gap of over 70 million years.

Phylogenetic ramifications

Ichthyosaur family tree . Thick gray and black bars represent the fossilized occurrence of the genera and species. The thin black lines correspond to the ghost lineages of the various taxa.

While ghost lineages based on interpolation only quasi fill the life span of a taxon, it can be extended beyond the fossil-documented area through the use of phylogenetic branching patterns (or cladograms). In extreme cases, the age of groups can be estimated from which no fossils were found.

The method is based on the application of cladograms in the context of phylogenetic systematics or cladistics . The topology and sequence of cladogenesis are identified on the basis of original characteristics that different members of the group have in common due to common inheritance from a parent species (the so-called autapomorphies ). While fossils can also be used, this can also be done solely with living species.

Since all living species have a common ancestry, new taxa can only arise when existing ones change their shape and characteristics and finally split into several new taxa. Two related lines that were newly created by splitting up such a parent line must necessarily be of the same age. If the age of one of these lines is known from dated fossils, the other line must be at least as old. Naturally, extension with this method is not possible beyond the oldest fossil of the oldest lineage.

Molecular clocks

In addition to the dating of fossils, the age of a lineage can be determined independently by the method of the molecular clock . The age of the branch is estimated from the difference between two DNA sequences and an average mutation rate . According to the neutral theory of molecular evolution , the sequence change should be roughly proportional to the time that has passed, so that if the rate of change is known, the age can be read off directly.

In practice, this method is often difficult because the mutation rate often varies significantly depending on the group and sequence and the age of the clades is therefore incorrectly estimated. The calibration of the respective molecular clocks via the reference of fossils or tectonic events is therefore complex and has a strong influence on the length of ghost lineages calculated in this way .

Estimation of past biodiversity

Ghost lineages must also be taken into account when determining the biodiversity of the flora and fauna of earlier eras. For only a handful of deposits, this is so well documented in fossil form that it can be read directly from the fossil record with some reliability. Do you want to find out whether in a certain epoch z. If, for example, the number of species increased or decreased, whether a certain geological event could have been related to adaptive radiation or whether an alleged mass extinction was real or only faked by coincidences of tradition, it is necessary to reconstruct the species population of the corresponding epoch. For this, the ghost lineages must be included, as otherwise the number of species would be massively underestimated.

Problems

Since ghost lineages are by definition only indirectly determined constructs, all hypotheses that are based on their existence are critically based on the fact that the hypotheses on which the construction is based are correct. If a fossil is incorrectly dated, a cladogram incorrectly constructed, a molecular clock incorrectly calibrated, extensive ghost lineages are generated, the (shadowy) existence of which coincides with the application of correct methods. Fossils can be incorrectly dated or, more commonly, poorly preserved or incomplete fossils can be mistakenly or wishfully assigned to a known lineage.

The age of correctly dated and assigned fossils must inevitably always underestimate the age of the lineage (since it was not the first representative that was fossilized). On the other hand, there is serious evidence that the molecular clock method overestimates the age of stem lines as a rule. In addition, this method is based on the calibration of critical branching events, which could be both fossilly incorrectly dated and phylogenetically incorrectly interpreted (cf.).

swell

literature

• Demetrio Boltovskoy: The Range-through Method and First-Last Appearance Data in Paleontological Surveys. In: Journal of Paleontology 62 (1), 1998. pp. 157-159.
• Lionel Cavin, Peter L. Forey: Using Ghost Lineages to Identify Diversification Events in the Fossil Record. In: Biology Letters 3 (2), 2007. doi : 10.1098 / rsbl.2006.0602 , pp. 201-204.
• Mark A. Norell: Tree-based Approaches to Understanding History: Comments on Ranks, Rules and the Quality of the Fossil Record. In: American Journal of Science 293-A, 1993. pp. 407-417.
• Alexandr P. Rasnitsyn : Testing Cladograms by Fossil Record: The Ghost Range Test. In: Contributions to Zoology 69 (4), 2000. pp. 251-258. ( Full text )

Web links

• Matt Wedel: Ghost lineages. University of California Museum of Paleontology, www.ucmp.berkeley.edu, May 2010.

Individual evidence

1. ^ MA Norell (1992): Taxic origin and temporal diversity: the effect of phylogeny. In: MJ Novacek, QD Wheeler (eds.): Extinction and phylogeny . Columbia University Press, New York: 88-118.
2. ↑ Rasnitsyn 2000, p. 251.
3. ↑ Boltovskoy 1998, pp. 157-159.
4. ↑ Norell 1993, p. 409.
5. ^ Cavin & Forey 2007, p. 201.
6. ^ Andrew B. Smith (2003): Getting the measure of diversity . Paleobiology, 29 (1): 34-36.
7. ↑ Philip CJ Donoghue, Michael J. Benton (2007): Rocks and clocks: calibrating the Tree of Life using fossils and molecules . Trends in Ecology and Evolution, Vol. 22, No. 8th
8. ^ Gregory A Wray (2001): Dating branches on the Tree of Life using DNA . Genome Biology 2001, 3
9. ↑ Michael J. Benton, Francisco J. Ayala (2003): Dating the Tree of Life . Science 300: 1698-1700.