From the archives!
One of the most unusual and interesting of amphibians has to be the Olm (Proteus anguinus), an unusual long-bodied cave-dwelling salamander from SE Europe [adjacent image from the Devon Karst Research Society]. Olms were the first specialised cave-dwelling animals (so-called stygobionts or troglobites) to be discovered, they were traditionally identified as dragon larvae by local people, and they remain mysterious and the source of controversy, debate and discovery. I've had a special affinity for olms since seeing them (live) in the former Yugoslavia in 1987, and after a colleague published a brief article on them in 2004 I ended up compiling and publishing my olm-related thoughts. In the interests of re-cycling that text I reproduce it here, in updated form.
What might be the most fascinating fact concerning olms is the most poorly-known and least mentioned one: the 1986 discovery of a surface-dwelling olm, described in 1994 by Boris Sket and Jan Willem Arntzen. So olms aren't just 'unusual long-bodied cave-dwelling salamanders' - they now exist in two forms, the cave-dwelling White olm Proteus anguinus anguinus and the surface-dwelling Black olm or Brown olm P. a. parkelj. Unlike the unpigmented nominal form with its skin-covered eyes, P. a. parkelj (presently known only from Bela Krajina in SE Slovenia) is dark brown or black and has externally visible (albeit small) eyes. Because White olms produce melanin when kept in sunlight (and are thus not albinistic as sometimes implied), the difference in colour between the two forms is not unexpected. However, there are also other, more important differences separating the two. P. a. parkelj differs from the nominal form in also having a proportionally shorter, broader and more muscular head, fewer teeth, a proportionally longer body and a proportionally shorter tail and limbs (Sket & Arntzen 1994) [image of P. a. parkelj below from wikipedia].
Most of the features which distinguish P. a. parkelj from P. a. anguinus are plesiomorphies [= features not unique to olms, but present also in related salamanders] and hence P. a. parkelj may be the ancestor of the White olm. Having said that, one of the most interesting contentions made recently about olms (Sket 1997) is that the different cave-dwelling olm populations may have evolved independently from different ancestral populations. If this is correct it may be that the different White olm populations represent different species which resemble one another by convergent evolution, and which have partly or mostly fused as they have met up within the Dinaric karst system. Sket (1997) thought that morphological and genetic differences observed among olms might provide support for this view and, ironically, if correct it would mean that several old species names proposed for different cave-dwelling olms might need to be resurrected. Fitzinger (1850) named seven new olm species within his genus Hypochthon (H. zoissii, H. schreibersii, H. freyeri, H. haidingeri, H. laurentii, H. xanthostictus and H. carrarae), though given that the type localities for some of them were just a few km apart, it's unlikely that they really were distinct taxa.
It's worth saying that olms almost certainly aren't ancient relicts, or living fossils. In fact, they must be young and recently evolved. Why? During the Pleistocene, the Dinaric area was so close to areas that were fully glaciated that temperature there must have been at or below freezing. This is far too cold for olms, which require temperatures of 6-18ºC for their eggs and larvae to develop (and toward the upper end of that range is best). Furthermore, karstification and the development of underground streams only began in the Dinaric region during the late Pliocene at the earliest, apparently. In view of these problems, olms either (1) survived in surface waters in the region, where summer temperatures were just about tolerable (but where winter temperatures would have made life difficult), or (2) moved into the area from a warmer, southerly refuge (Griffiths 1996, Sket 1997). It isn't yet known which was the case: more research is needed [UPDATE: in a comment added to the original (2006) version of this article, Tommy Tyrberg noted that olms would not have had such a hard time during the Pleistocene as a diverse Pleistocene avifauna from Sandalja in Istria contains a surprising list of warm-adapted species (such as the Middle spotted woodpecker, an oak forest specialist). Olms may not, therefore, have had to move far to find suitable refugia].
Whatever, troglobitic olm populations must have evolved within the last 10,000 years or so, and presumably the specialised troglobitic morphology of living olms evolved during this time. Similarly recent invasions of cave systems appear to have occurred among various troglobitic fishes.
As mentioned earlier, olms were the earliest troglobites to be discovered. While it's been stated on occasion that the species was discovered as recently as 1875 (Laňka & Vít 1985, Keeling 2004), olms first became widely known in 1689 when Baron Johann Weichard Valvasor wrote about the animals in his book on the Yugoslavian province of Carniola (on which see below). However, it wasn't until the mid-1700s that the animals become the subject of proper scientific debate. At this time Slovenian scientist Giovanni Scopoli 'discovered' olms and realized just how extraordinary they were (Scopoli 1772). He planned to describe the animal scientifically and enhance his reputation by doing so. We know that Scopoli sent pictures of olms to Carl von Linne and that Linne and Scopoli disagreed as to whether the animals were a distinct new genus (Scopoli's view), or the juveniles of something else (Linne's view). However, the Austrian anatomist J. N. Laurenti became very interested in olms at the same time as Scopoli (apparently because of a specimen Scopoli had sent to one of the Laurenti's friends) and, in 1768, published the first scientific description of the species (Laurenti 1768). This is the 'official' date of the White olm's scientific discovery. Laurenti's choice of generic name for the olm (Proteus) is based on the Greek god Proteus but it is not Proteus' shape-shifting ability that Laurenti had in mind, but rather his status as shepherd of sea creatures. Laurenti's work on the olm did not actually become that well known and it was Karl von Schreibers' work of the 1790s and early 1800s which made olms well known among scientists [Laurenti's olm drawing is shown above, from wikipedia].
Olm distribution has been the subject of much confusion and speculation. Presently, olms are restricted to the Dinaric Karst, a region that extends from the Soca (formerly the Isonzo) River (near Trieste) in SE Friuli-Venezia Giulia, Italy to the Trebišnica River in eastern Herzogovina. In between Italy and Herzogovina, olms also occur in southern Slovenia, southern Croatia and parts of Bosnia. Little known is that the species has been recorded from localities in France (Moulis) and Germany (Harz). These extralimital records are all apparently due to human introduction however. They are also found in the Parolini Grotto, Vicenza, northern Italy, but their presence here is due to human introduction also. Of further interest, the locality mentioned by Valvasor (1689) - the spring of Lintvern, near Vrhnika - is actually outside of the Dinaric Karst, and is unlike the other areas inhabited by olms in geology and geomorphology. It seems that Valvasor made the logical (but incorrect) assumption that Lintvern (which is a garbled form of the German word Lindwurm, meaning dragon) was so named because it was the source of olms (which were fancifully regarded as dragon larvae at the time).
Keeling (2004) implied that Carniola (note: not Carinola) might be the Italian part of the olm's range and also wondered if Carniola might still be part of Austria. Carniola is today called Kranjska and was controlled by Austrian royalty until 1918 (consequently, the ruling classes there spoke German until the 20th century). It is today part of central Slovenia and is thus not either the Italian part of the olm's range, nor an Austrian extension of the species' range.
Bizarrely, olms were traded during Victorian times as exotic pets and were apparently available in Britain as such (which raises the remote possibility that they might have been introduced to British cave systems in the same way that they were in French, German and Italian ones). During the 1950s it was reported that olms were present in the Carpathian karst of eastern Serbia, and in 1960 a team of speleologists from Ljubljana led an expedition to the region to investigate this possibility. They didn't find any olms there, and nor has anyone else since.
Olms have been horrendously over-collected for scientific use and were also apparently collected by farmers for use as pig food. One of the greatest problems facing olms today is metal poisoning caused by industrial pollution and a number of populations have declined as a result of such. P. a. parkelj is under strict legal protection. Olms have been protected in Slovenia at least since 1949 and elsewhere in their range they are widely recognized as deserving protection [image below shows olm larva, from the Devon Karst Research Society].
Finally, regarding diet and breeding, olms apparently mostly detect their prey using chemical clues and the detection of water currents but they also possess electroreceptive organs in the head and thus presumably employ electroreception. Despite their vestigial nature, the eyes of White olms are not completely useless and are able to detect light. Olms appear to mostly prey on aquatic crustaceans but also eat snails and insect larvae. Captive specimens have eaten worms and adults may be cannibalistic on occasion. When I visited Postojina we were told that the olms on display were not fed both because their food proved hard to procure, and because they were quite able to survive for years without feeding. Indeed there was apparently a specimen kept at the Faculty of Biotechnology in Ljubljana which survived for an astonishing 12 years without food. Olms are long-lived, reaching sexual maturity between 7 and 14 years, and almost certainly ordinarily live for more than 50 years, though ages twice this have been suggested by some writers.
Refs - -
Fitzinger, L. 1850. Ueber den Proteus anguinus der Autoren. Sitz.-Ber. Akad. Wiss., Math.-naturw. Cl. 5, 291-303.
Griffiths, R. A. 1996. Newts and Salamanders of Europe. T & A D Poyser, London.
Hanken, J. 1999. Why are there so many new amphibian species when amphibians are declining? Trends in Ecology & Evolution 14, 7-8.
Keeling, C. 2004. Olm. Mainly About Animals July 2004, 20-21.
Laňka, V. & Vít, Z. 1985. Amphibians and Reptiles. Hamlyn, London.
Laurenti, J. L. 1768. Specimen Medicum Exhibens Synopsis Reptilium Emendatum. Joan. Thomae (Vienna).
Scopoli, J. A. 1772. Annus Quintus Historico-Naturalis. C. G. Hilscher, Lipsiae.
Sket, B. 1997. Distribution of Proteus (Amphibia: Urodela: Proteidae) and its possible explanation. Journal of Biogeography 24, 263-280.
- . & Arntzen, J. W. 1994. A black, non-troglomorphic amphibian from the karst of Slovenia: Proteus anguinus parkelj n. ssp. (Urodela: Proteidae). Bijdr. Dierk. 64, 33-53.
Valvasor, J. W. 1689. Die Ehre des Herzogthums Crain. W. M. Endtner, Nuernberg.
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More like this
We've all had them, haven't we?
Seriously though, I had never heard of those surface-dwelling 'normal' olms. Thanks for bringing that to my attention!
That too was news to me. I'm not sure if I wanted to know, though...
Very cool. I too was able to see the "human fish" at Postojna in the mid-1980s. Many fascinating parallels between these urodeles and various cave fishes, including (possibly) multiple parallel evolution, extant surface forms, extremely low metabolic rates, loss of pigmentation and eyes, etc.
Sitting here chanting "Olm mane padme olm"
Sorry - just had to do it
"Bizarrely, olms were traded during Victorian times as exotic pets and were apparently available in Britain as such (which raises the remote possibility that they might have been introduced to British cave systems in the same way that they were in French, German and Italian ones)"
Hmmm. When was Bram Stoker's "Lair of the White Worm" written?
Might be worth a look in the Peak District...
Lair Of The White Worm was published in 1911,
( Which makes it Edwardian, yes? ) and isn't nearly as
much fun to read as Dracula. Sadly, Super-sized
salamanders seldom sally in science fiction...( or horror*)
Informative and interesting post,as always.
Hmmmmm. There was no Slovenia before about 1945, and judging from his entirely Italian name, I don't think Scopoli was an ethnic Slovene either...
Soča. Or just "the Emerald River" ;-)
(That's not what it means, but it's famous for being remarkably green.)
Still Isonzo where it flows into Italy.
Postojna.
Same when I visited in 1993.
Again, Darren, you save the best for last. Surviving 12 years without feeding, and living 50+ years, both astonish this ignoramus. One such as I might speculate that individuals simply waited out the inhospitable Pleistocene climate.
Tatzelwurm!
Off-topic, but check out the open access issue of The Anatomical Record devoted to "The Paranasal Sinuses: The Last Frontier in Craniofacial Biology" (Vol 291(11); http://www3.interscience.wiley.com/journal/117927936/grouphome/home.html ). Mostly on primates (various papers) but one paper on dinosaurs (Witmer & Ridgely) with very nice imagery (amazing curly airways in ankylosaurs!).
And getting back to urodeles, Craig sibilantly said
but didn't mention Karel Äapek's War with the Newts. One of the classics, quite Stapledonian, and definitely recommended.
Dragon larvae ... I can see why, freaky looking things.
And 10,000 years? Wow, that's fast evolution.
I really love these articles!
Damn, it looked fine in preview. You know who I mean: see http://en.wikipedia.org/wiki/War_with_the_Newts .
Why on earth would an organisation like a Faculty of Biotechnology keep an animal for 12 years without feeding it anyway?
Ian,
if mad scientists didn't ever do mad stuff, how would we find out these cool facts about longevity and survival-without-food? It's like that classic demo with the funnel of pitch; every now and then an olm drops... There may well be important applications when we're in that rocket ship to Mars.
David:
As far as I know, Scopoli was indeed an ethnic Italian. (Although during his lifetime, 1723-1788, there was technically no 'Italy'.) Scopoli was born in Tyrol, which at the time was part of the Habsburg Empire. So at least in principle, one could also call him an Austrian.
Complicated things, these ethnic/national affinities.
"Super-sized
salamanders seldom sally in science fiction...( or horror*)"
Rupert Gould, one of the earliest writers to investigate the Loch Ness Monster,actually conjectured that Nessie was, in fact, a gigantic Newt. Even then, this view didn't find popular support.
It nonetheless provided the basis for the first ever Loch Ness Monster film, "The Secret of the Loch", released in 1934.
Just to confuse matters, the monster was played by a photographically enlarged Iguana!
Since we're discussing 'salamanders' and 'science fiction', the pedant in me feels compelled to point out that in one rather notable science fiction film, namely Aliens, there was a character called 'Newt'. (Although she was hardly super-sized.)
"Super-sized
salamanders seldom sally in science fiction"
I saw that line and thought it had the makings of a good tongue-twister, good work Craig :D
Something new, pleaaase! About dinosaur nest in North America.
I read it in a local newspaper and had interesting time wondering, how journalist tangled it and what was the original... A bit like reconstructing mouse skeleton from an owl pellet. ;-)
David:
> There was no Slovenia before about 1945, and judging from
> his entirely Italian name, I don't think Scopoli was an
> ethnic Slovene either...
Slovenia was divided between several counties and duchies in the 18th century, all part of the Holy Roman Empire, and all ruled by the Habsburgs (with the exception of a small coastal strip that belonged to the Republic of Venice).
There was, however, some early form of Slovenian "romantic nationalism" in the age of enlightenment, especially among the intellectuals and scientists of the Zois circle. I don't know if Scopoli was a member, most scientists active in the region were, but Scopoli might have been a little bit too old.
> As far as I know, Scopoli was indeed an ethnic Italian.
> (Although during his lifetime, 1723-1788, there was
> technically no 'Italy'.) Scopoli was born in Tyrol, which
> at the time was part of the Habsburg Empire. So at least
> in principle, one could also call him an Austrian.
Before the end of the Holy Roman Empire, and the founding of the Austrian Empire in 1804, there was no formal political union between the territories ruled by the house of Habsburg-Lorraine, even if they acted as a de-facto state. Perhaps it would be best to call Scopoli a Tyrolean, the population of some valleys spoke Italian, or at least Romanic, while others spoke German, but new-fangled ideas like ethnic nationalism meant little in 18th-century Tyrol, a very conservative and traditionalist place.
>> "Super-sized
>> salamanders seldom sally in science fiction...( or
>> horror*)"
> Rupert Gould, one of the earliest writers to investigate
> the Loch Ness Monster,actually conjectured that Nessie was,
> in fact, a gigantic Newt. Even then, this view didn't find
> popular support.
A large lissamphibian or albanerpetontid would make sense as an ambush predator in places too cold for crocs.
Has the Ness ever frozen over?
Frozen over? It's a creation of inland ice. All of Scotland was under ice last glacial maximum, and in the Highlands the ice even returned during the Younger Dryas!
(Incidentally, this still holds, though probably not to that extent anymore.)
Lair of the White Worm was made into a movie in 1988, directed by the (very strange) Ken Russell. It's really bizarre, and well deserves its R rating.
@johannes: Antarctic Australia had a giant carnivorous temnospondyl, Koolasuchus, in the Antarctic. I don't know why the modern amphibians never produced anything of the sort. The Cryptobranchid "giant salamanders" are carnivores, but eat fish and invertebrates, not large animals.
OK, David, fair enough. Has the Ness frozen over in recorded history? In living memory? I haven't been able to find any indication, although I found one official assertion that its "waters ... never freeze over", or at least haven't lately. The Hudson River, next to Manhattan, used to freeze over frequently before the dams were put in.
I've just wasted half an hour with the internet trying to figure out what nationality we should credit Scopoli to. He was born in Cavalese, now in the province of Trento (or does one say Trentino for the province) in Italy. I ***think*** his home town is probably now mainly Italian-speaking (no guarantee that it was in his day, though!), but it is fairly close to the area where Ladin is spoken. (Ladin is a Romance language, but not all that close to Italian: I think I have read that when Ladin-speakers learn a language of widerr usefulness, German rather than Italian is their first choice.) One website said he was "of Italian extraction," which fits with his name but doesn't really tell us very much.
On the other hand, he got his medical degree at Innsbruck: most likely a lot of his textbooks would have been in Latin, but he would at the very least have had a fair number of German-speaking classmates. And he worked for a significant part of his adult life (16 years) in what is now Slovenia.
He died before the French Revolution: probably our modern idea of nationalism would have seemed strange to him. He probably thought of himself as a loyal subject of the Habsburgs. (A Yugoslav-- he admitted that he was really an ethnic Serb, but as a point of honour insisted on "Yugoslav" rather than "Serb" in the slot on his passport that specified nationality-- friend in the 1980s once said to me that, if he could choose, he would prefer to be a citizen of the Austro-Hungarian empire. Nationalism in some parts of the world doesn't have a happy history.)
William Miller:
thank yu very much
> Lair of the White Worm was made into a movie in 1988,
> directed by the (very strange) Ken Russell. It's really
> bizarre, and well deserves its R rating.
Starring Catherine Oxenberg, who, coming back to the topic of Balkan politics, is a member of one of the two rival Serbian royal dynasties (either Karageorgevic or Obrenovic, I don't know).
I'm very sure that the comment left by "erotik videolar" was made by a spambot...
Catherine Oxenberg is a Karadjordjevic, or so says Wikipedia.
There were Miocene cryptobranchids that got to something like 2.5 m total length.
That doesn't matter. 10,000 years is probably not enough for a salamander to evolve giant size in that cold, food-poor lake.
Though that may have historical and geographic reasons more than how similar Italian is.
Certainly. -lar/-ler is the Turkish plural ending.
> Perhaps they would be too vulnerable, at least as
> juveniles, to mammalian predators (e.g., otters and bears)?
Dartian,
selective pressure by mammalian predators would probably just lead to the development of more potent poison glands in lissamphibians.
Albanerpetontids were scaly AFAIK, so their ability to develop poison glands in their skin might have been limited, thus making them more vulnerable to predation by mammals. On the other hand, the equally scaly temnospondyls co-existed with otter-like mammaliforms in Jurassic China.
David: I have no truck with a notional amphibious Nessie. (I'm on record promoting an even more notional scansorial plesiosaurid.) I just wondered whether the lake used to freeze over in cold winters before dams went in upstream.
What other big lakes used to freeze over but don't, any more?
Do you think the spambot was responding to the discussion of giant carnivorous temnospondyls or the Slovenian and Italian history ?
Why? We're talking fish-style scales here, bone plates inside the skin, not sauropsid-type scales, thickenings of the horny layer of the epidermis.
Hmmm. Temnospondyls are known in Asia till the Late Jurassic. The Daohugou bed, where Castorocauda comes from, is Late Jurassic or early Early Cretaceous (up to Barremian), with dates toward the younger end of this range being more probable; it has not yielded any temnospondyl remains so far.
The European Haldanodon is Late Jurassic, but more desman- than otter-like, especially in size!
Wait, why would an albanerpetontid make sense as an ambush predator? As far as I can tell they're rather small (<10 cm), terrestrial and likely burrowers.
Johannes:
That's probably right. Although at least the Recent giant salamanders seem to be palatable to mammals (=humans). So much so, in fact, that the Chinese giant salamander Andrias davidianus is classified as 'Critically Endangered' due to overharvesting for food (Feng et al., 2007).
But on closer thought, perhaps mammals aren't really that significant amphibian predators anyway? Teleost fish are probably much more serious predators/competitors of lissamphibians, particularly at early developmental stages.
Reference:
Feng, X., Lau, M.W.N., Stuart, S.N., Chanson, J.S., Cox, N.A. & Fischman, D.L. 2007. Conservation needs of amphibians in China: A review. Science in China Series C: Life Sciences 50, 265-276.
> Why? We're talking fish-style scales here, bone plates
> inside the skin, not sauropsid-type scales, thickenings of
> the horny layer of the epidermis.
This is why I wrote "might have been limited" rather than "impossible".
> Wait, why would an albanerpetontid make sense as an ambush
> predator? As far as I can tell they're rather small
Wich doesn't mean it had to stay that way. Many clades start with small body size and then get larger during their evolution - that's Cope's rule. Are there any inherent size limits in albanerpetontid anatomy?
Troglobytes were studied in the early 20's&30's in Romania by Prof. Emil Racoviță (pronounced "racovitza"), 1868-1947. Prof. Voinov, the author of an article included in a bullettin published in "La vie scientifique en Roumanie" wrote that Dr. Racoviță demonstrated that troglobytes were "living fossils, and they often show -nowadays- ancient degrees of [primitive] life, even tertiary or secondary [geological] stages". In the end, Caves do really retain primitive stages of ancient, primitive life.
May I use informations from this article to quote them?*
Refs.:
Eliade, M., "Fragmentarium"; Ital. ed., Milan, Jaca Book, 2008, pp. 53-56 [published in 1939]
> If anybody out there know something more about Dr. Racoviță [e.g. bibliography, studies published, theories, etc...] please let me know...
*(I need some information for my M.Sc. thesis!). Thanks a lot, and keep up the good work, Darren Naish!
Leo
Troglobytes were studied in the early 20's&30's in Romania by Prof. Emil Racoviță (pronounced "racovitza"), 1868-1947. Prof. Voinov, the author of an article included in a bullettin published in "La vie scientifique en Roumanie" wrote that Dr. Racoviță demonstrated that troglobytes were "living fossils, and they often show -nowadays- ancient degrees of [primitive] life, even tertiary or secondary [geological] stages". In the end, Caves do really retain primitive stages of ancient, primitive life.
May I use informations from this article to quote them?*
Refs.:
Eliade, M., "Fragmentarium"; Ital. ed., Milan, Jaca Book, 2008, pp. 53-56 [published in 1939]
> I can't tell no more about those researches, but I need to know more! If anybody out there know something more about Dr. Racoviță [e.g. bibliography, studies published, theories, etc...] please let me know...
*(I need some information for my M.Sc. thesis!). Thanks a lot, and keep up the good work, Darren Naish!
To the reader(s): If you own a .pdf file of ancient Racoviță's works, and you wish to send it /share it, please feel free to ask me my e-mail address. Thanks in advance!
Leo
I just don't see how the scales could limit the development of poison glands.
Not that I know of. However, they were burrowers.
And forget Cope's "rule". Every time someone takes a serious look at any particular clade (due to the fact that capable methods were only developed recently, that hasn't happened often yet), it is found to fail.
Er, no. The olm is a Tertiary relict (for Europe -- not worldwide, see Necturus, the mudpuppies of North America), but the black olm still exists, and there are most certainly no Mesozoic ("Secondary") relicts in caves!
All I know is that a journal was named after him.
- "In the end, Caves do really retain primitive stages of ancient, primitive life."
> Er, no. The olm is a Tertiary relict (for Europe -- not worldwide, see Necturus, the mudpuppies of North America), but the black olm still exists, and there are most certainly no Mesozoic ("Secondary") relicts in caves!
------------------------------------------------------
> Well, it was the summary ending of what they wrote (the lines I've quoted from prof. Voinov) "in illo tempore" (30's), not my thoughts!!!Sorry, if it was not clear
"Caves do really retain primitive stages of ancient, primitive life":
THEY [e.g. at least, scientists in Eastern Europe during the 20s/30s] thought this was possible, and this view was widely accepted and scientifically adopted.
Leo
What about a burrowing Nessie?
;)
Loch Ness is very deep, and of constant tempreture, it has never frozen over (unlike LaKe Champlain, which is 400ft max dept to Nesses 900+)
In fact it acts as a hot water bottle to the surrounding land. I have been in the Great Glen midwinter, snow all around but get lochside and green as ever.
Loch Ness was the site of one of GBs fist multidiscliplinary lake studies in 1904..no monster then...but they devote an entire chapter to mirages.
Jerzy: you mean a burrowing beakless azhdarchid Nessie, right?
Tengu: Thank you, that's what I needed.
question: if olms are THAT young [10.000 yrs is definitely younger than, let's say, australopithecus], shouldn't they supposedly live a shorter lifespan and breed like hell? -like rats do-
i've read about it in some evolutionary biology book. younger species tend to live shorter lifespans and breed like rabbits...
The other way around: high turnover makes high rates of evolution easier. It's not, however, necessary. Small population size can even help (by making drift much more powerful).