ON THE EDGE OF EXTINCTION - A - THE TIGER (Panthera tigris)

[peter]

GENETIC ANCESTRY OF THE EXTINCT JAVAN AND BALI TIGERS (Xue et al, 2015)

The article is interesting in more than one way. I especially liked the introduction (pp. 2) and the discussion (pp. 6-7). This post has a number of conclusions regarding the evolution of tigers. For now, they can be considered as the final word on tiger evolution. In order to prevent problems, I decided for quotes only.  

01 - Fossils and the common ancestor of modern tigers

" ... The earliest tiger fossils found in northern China and Java ... date back to 2 million years ago ... in the early Pleistocene. Molecular genetic imputation traces all living tigers back to a common ancestor as recent as 72 000-108 000 years ago. It has been speculated that the Toba vulcano super eruption in Sumatra approximately 73 500 years ago may have contributed to this recent coalescence for modern tigers ... " (pp. 2).

02 - Tiger subspecies

" ... The most important application of genetic techniques has been in resolving taxonomic uncertainty surrounding tiger subspecies ... " (pp. 2).

" ... current tiger taxonomy, informed by molecular genetic evidence, now recognizes 6 living subspecies ...; Bengal tiger (Panthera tigris tigris), Amur tiger (Panthera tigris altaica), south China tiger (Panthera tigris amoyensis), Sumatran tiger (Panthera tigris sumatrae), Indochinese tiger (Panthera tigris corbetti), and Malayan tiger (Panthera tigris jacksoni) ... " (pp. 2).  

03 - The Sunda Shelf and Sunda tigers

" ... Populations from islands are generally given subspecies status in taxonomy; however, named island subspecies may not carry significant genetic distinctiveness, particularly if gene flow between them occurred recently. During the Pleistocene, sea level fluctuations repeatedly exposed vast areas of the Sunda Shelf, forming land bridges intermittendly among the islands of Sumatra, Java and Bali as recently as the Last Glacial Maximum (LGM, c. 20 000 years ago) and enabling recent population connectivity and possible gene flow. However, other studies indicate that the ability of animals to move across the exposed Sunda Shelf may have been restricted and populations or subspecies have deeper divergence dating back to even millions of years ... (pp. 2-3).
   
04 - Sunda tigers and Mainland Asia tigers

" ... Geographic distribution of mtDNA haplotypes among voucher tiger specimens with recorded origins from the islands of Sumatra, Java, and Bali indicates a recent common ancestor among these subspecies. SON and BAL were 2 haplotypes exclusively found in Java and bali; none of the 8 haplotypes from the living voucher Sumatran tigers (SUM1-8) existed in Java or Bali. No mtDNA haplotype is shared among voucher Sumatran, Javan and Bali tigers. All 9 voucher Javan tigers carry haplotype SON and both voucher Bali tigers carry BAL ... " (pp. 6).

" ... Phylogenetic analysis of mtDNA haplotypes representing all 9 tiger subspecies ... produced congruent topologies corresponding to major geographic partitions. Tigers from the 3 Sunda Islands clustered into a monophyletic group with 11 haplotypes. The 2 haplotypes (SON and BAL) in vouchers P.t. sondaica and P.t. balica formed a statistically robust subgroup within the Sunda tiger population ... " (pp. 6).

" ... Mainland Asia tigers parse into 5 distinct groups corresponding to major phylogeographic clustering and prior subspecies recognition ... P.t. tigris on the Indian subcontinent is genetically distinct from the other mainland subspecies, corresponding to an early divergance ... " (pp. 6).

05 - Estimated coalescence time

" ... The estimated coalescence time of mtDNA haplotypes for all tiger subspecies ... using Panthera ssp. divergence times as calibration ... is 94 500 years ..., highly consistent with a previous estimation of 108 000 years. The recent coalescence in modern tigers, as compared to some other Panthera species ..., is consistent with a late Pleistocene bottleneck in tigers ... " (pp. 6).

06 - Taxonomic status of Javan and Bali tigers

" ... Our results provide consistent molecular genetic evidence that tigers on Bali, Java, and Sumatra recently derived from a common matrilineal genetic lineage. This mtDNA similarity needs to be validated ... yet the close association among the 3 subspecies is supported by the clustering of all mtDNA haplotypes from the 3 Sunda Islands leading to a strongly supported monophyletic clade distinct from other mainland subspecies; the existence of a recent ancestral Sunda tiger lineage that later evolved into present populations on Java, Bali, and Sumatra ... " (pp. 6).  

07 - The Sumatran tiger

" ... Previous paleontological and morphological studies have suggested that the Sumatran tiger is a hybrid of mainland tigers and the Javan tiger; however, this conclusion is not supported by maternal mtDNA ... markers ... " (pp. 6).

" ... Our molecular phylogenetic and phylogeographic analysis supports geographic subdivision within the Sunda tiger group, with haplotypes SON and BAL exclusevely found from Java and Bali respectively. The genetic differentiation between Sumatran, Javan, and Bali tigers is also significant, indicating a restriction or lack of matrilineal gene flow among the three islands ... " (pp. 6).

" ... Nevertheless, the similarity among the Sunda tiger mtDNA haplotypes ... suggest common origin and rapid divergence of island subspecies, and may reflect that the somewhat distinctive morphological features in each subspecies have evolved rapidly after each island was colonized ... " (pp. 7).

08 - The Javan tiger

" ... All estimates consistently put the coalescence time of modern tiger lineages within the last 100 000 years. Mitochondrial DNA haplotypes from tigers from Java and Bali fall into the Sunda tiger clade that belongs to modern tigers. Therefore, these genetic data do not support the hypothesis that the modern Javan tigers is an autochthonous descendant of a historic tiger population, whose fossils were found in Java and dated to 1.3-2.1 MYA. Instead, early to middle Pleistocene tiger populations in the Sunda Islands may have been eliminated because of drastic biogeographical events associated with glacial-interglacial and/or the Toba vulcano super eruption ... (pp. 7).

09 - Modern tiger evolutionary history

a - " ... We are now able to construct, for the first time, the intraspecific phylogeny for the tiger based on all the 9 recognized subspecies. The position of P.t. amoyensis and its relation P.t. sumatrae/P.t. sondaica/P.t. balica ... suggests a once widespread tiger population from China to the Sunda Shelf that became isolated, likely by rising sea levels during interglacial periods ... " (pp. 7).

b - " ... A second wave of tiger expansion and divergence (P.t. tigris, P.t. corbetti, P.t. jacksoni, P.t. altaica and P.t. virgata) replaced much of the range of P.t. amoyensis on the mainland and evolved into modern populations in Indochina, the Indian Subcontinent, the Caucasus, and the Russian Far East, where tiger fossils are only found from the Holocene ... " (pp. 7).

10 - Conclusions

This paragraph was added to get to a general picture on tiger evolution. Although based on the article discussed in this post, I added a few ideas I've had for some time.        

- Although tiger fossils of up to 2.1 million years old have been found, all modern tigers can be traced to a common ancestor.
- Modern tigers evolved between 72 000 - 108 000 years ago.
- From -a- and -b- follows that tiger evolution probably had at least one population bottleneck. 
- The Late Pleistocene population bottleneck probably was a result of the Toba vulcano super eruption. 

- The common ancestor of all modern tigers evolved in south China or northern Indochina.
- From south China or northern Indochina, tigers first spread south to the Sunda Shelf.
- During an interglacial period, Sunda tigers and south China tigers became seperated.
- In the last 100 000 years or so, Sunda tigers occupied different regions.
- Over time, they left Palawan and Borneo, but they survived on Bali, Java and Sumatra until recently.
       
- Towards the end of the Pleistocene, south China and northern Indochina tigers tigers spread west.
- India was reached about 12 000 years ago. The other regions were occupied in the Holocene.
- In the Holocene, 5 distinct types or subspecies evolved. The most recent subspecies probably is the Amur tiger.

- Although Bali, Java and Sumatra tigers belong to one subgroup, they are distinctly different from each other.
- The three types probably evolved when the islands they occupied became isolated as a result of rising sea levels.
- Although all three subspecies are different, Javan and Bali tigers are closer to each other than to Sumatran tigers.
- Sumatra could have been the last island colonized, as they differ markedly from all other subspecies.

11 - Panthera tigris jacksoni

How about a hand for our newest member? Jacksoni, after he was severely tested by our most respected senior members, is here to stay, so it seems. One wonders why we didn't see him in all these years, as he was there long before we were. Different from both corbetti and sumatrae, he must have occupied the southernmost tip of Malaysia unseen somewhere between 72 500 - 12 000 years ago.

He reminds me of Sunderban tigers in more than one way. Jacksoni is a small animal as well and he too seems to have shrunk with age in the last century or so. Some years ago, I bought an article that was based on an interview with the Sultan of Johore. The Sultan was a hunter. Tigers. His bag, in spite of the modest average, had a few specimens well over 9 feet 'between pegs' and there's at least one skull well over the average for adult Indian males. A few decades later, however, those with experience in Perak, Terengganu and Johore, however, agreed not one male exceeded 8.8  'between pegs'. A result of overhunting or isolation?

Time for a little research, I'd say. I'll post what I have on jacksoni and propose a small debate. Is he really different from Terengganu and Perak tigers? If so, in what respects? Most likely causes? Reply