Neville Handel
Biology 112
Fall 2005
The Dispersal and Evolution of an Idea
In April of 1926, M.L. Fernald of the Gray Herbarium at Harvard University
published a paper titled “The Antiquity and Dispersal of Vascular Plants”. The paper
was a direct and bitingly sarcastic affront to the “age and area” hypothesis that had been
put forth by one of Fernald’s contemporaries, Dr. J.C. Willis. The age and area
hypothesis asserted that species with the largest distributions are old, having gradually
spread out in all directions and expanding their range (given no barriers to dispersal). In
contrast, young species have smaller distributions because they have not had time to
expand their ranges fully. The idea immediately drew criticism from a number of
scientists, including Fernald, who hoped to finally pronounce the theory “a dead issue”
(Fernald, 1926) with this paper.
Fernald systematically picks apart Willis’ arguments, starting with the rate of
migration of plants, which he argues is much faster than Willis asserts. He argues that
the widespread boreal forest colonized its’ current range in a matter of a few thousand
years after the retreat of the glaciers, which is certainly problematic for age and area and
the assertion that migration is a slow process. He points out further that the idea calls for
the continual creation of new species but does not deal with the accumulation of older
species and extinction, which the expansion of ranges would eventually necessitate.
Other points that Fernald takes issue with Willis on are: the world is not static, and
therefore no single factor (i.e. age) is “all-controlling” (Fernald, 1926); he bases many of
his ideas on information from books and herbaria rather than collection of data from the
field, which can lead to misrepresentation of actual distribution, species identification and
commonness; he includes many other scientists’ work as supporting his hypothesis, when
they themselves make no such claims and often distance themselves from it; many of the
scientists who do support his work have shown their incompetence through their own
work; he bases much of his ideas upon an inaccurate knowledge of geologic history and
ignores that which does not fit into his scheme; evaluations of his hypothesis in the field
have cast great doubt upon its’ validity; paleontological evidence shows that many of the
oldest species (especially monotypes and endemics) were once widespread and had more
congeners, but are now some of the most restricted in range and have fewer or no extant
relatives, as extinction is a common occurrence; many of the most widespread species
appear to be very young or newly introduced (i.e. weeds); areas that were not glaciated
(and are therefore old) tend to have many more endemics than the areas that were
glaciated, but Willis’ theory would argue the opposite; some old genera (Rubus,
Ranunculus, Crataegus) are very aggressive and vigorous colonizers while others
(Liriodendron, Sassafras, Hamamelis) are not expanding their ranges at all, showing that
there are no clear correlations between age and area.
When discussing endemic species in northeastern North America, Fernald
expresses some information gathered from his own work. His conclusion is that the 100
or so endemic species of the Gaspe Peninsula and Western Newfoundland were once
widespread across northern North America and even Siberia, but are now relegated to
small un-glaciated pockets because the advance of the glaciers removed them from much
of their former range. Their closest relatives can be found in un-glaciated locations over
2000 miles to the west in Alaska and British Columbia. Thus, we can witness through
such a vicariance event the process of allopatric speciation and formation of endemism or
epi-biotic species (Fernald, 1926). Fernald hypothesized that these species failed to
colonize the newly exposed habitats after the glaciers retreated because they had waned
into a phase of senescence, while the younger more aggressive species quickly moved in.
Fernald did acknowledge that old taxa can still be aggressive and vigorous, citing the
examples mentioned above in regards to their colonization of newly exposed moraines in
New Zealand. These aggressive older taxa subsequently evolved into new species via
sympatric speciation in the newly available habitats. Fernald’s ideas on the causes of
endemism and disjunctions were based on keen observations and critical thinking, and
they led to an explosion of research on these topics, which I will discuss below.
In his 1942 review paper, H.M. Raup called Fernald’s ideas on endemism,
disjunctions, and senescence some “of the most important developments in floristic
geography” (Raup, 1942). Fernald’s concept of senescence of plants after glaciation as
an explanation of wide disjunction to distant relatives was a powerful concept. Raup
noted that subsequent genetic studies had hinted that the lack of aggressiveness of the
relictual “nunatak” species (as observed by Fernald) was not “a function of age
necessarily, but rather as a result of reduction in genetic variability” (Raup, 1942).
Fernald himself did not claim age to be the cause of this senescence, but he believed
senescence to be real. Raup believed that advances in genetics and cytology would help
to clarify many of the problems that had been plaguing plant bio-geographers, such as
relationships between disjunct populations, causes of endemism, and taxonomic
classification.
In their 1965 paper “Endemism and Speciation in the California Flora”, G.L.
Stebbins and Jack Major show this hope beginning to have come to fruition. As they
noted, genetic research was allowing botanists to learn more accurately the relationships
between different species. In California, a state with a large number of endemic species,
the implementation of genetic research helped to clarify the picture of their evolutionary
histories. Stebbins and Major clarified the distinction between paleo- and neo-endemics.
The former are defined as “relatively old… known in essentially their present form from
fossil remains… their present endemic condition is the result of the restriction of a
formerly more extensive geographic distribution” (Stebbins and Major, 1965). In
contrast, they note that “some of the most narrowly restricted endemic species are closely
related to neighboring species of the same genus, occur in ecologically recent habitats,
and so are almost certainly recently evolved species which have never been widespread”
(Stebbins and Major, 1965). They stated that paleo- and neo-endemics are at opposite
ends of a spectrum of endemism and that various types of endemism likely exist between
the two. It was their conviction that areas with high levels of endemism should be
extensively studied to find out the ecological factors that contribute to this phenomenon,
a sentiment that is still held and being researched today (Heads, 2004; Tribsch, 2004).
In a 1972 paper titled “Morphology and Phytogeography: The Classical Approach
to the Study of Disjunctions”, C.E. Wood noted that much of the geologic and taxonomic
work that Fernald had based his ideas upon had been incorrect or later revised, so that his
hypothesis on senescence of nunatak species did not represent what in reality likely
happened. Nevertheless, Fernald’s general point that glacial advancement and retreat had
fractured a once widespread flora and led to endemism and disjunctions on opposite ends
of North America was still validated. Wood noted that as knowledge of floras around the
world changed, it was likely that our notions of species migrations and relationships
would change as well. He lamented that the patchy nature of the fossil record would
never give us a complete or wholly accurate picture of past conditions, thus hindering our
understanding of the historical influences on species distribution and endemism. Indeed
this sentiment is still echoed today by authors such as A.S. Harold and R.D. Mooi, who
note that “areas of endemism are fundamentally historical entities, not distributional ones,
and their definition should take history into account” (Harold and Mooi, 1994). In a
study of glacial disjuncts in the Alps, Schonswetter et al conclude that “interpretation of
the phenomenon ‘rarity’ must take into account the history of the investigated taxa and
the ecological factors leading to their present day restriction” (Schonswetter et al, 2003).
Fortunately, advancements in genetic research and in our abilities to understand the
nature of past climatic conditions has allowed us to more accurately decipher the nature
of historical influences on species distributions and endemism.
Numerous studies have utilized updated historical information and genetic
research to uncover the causes of endemism and the phylogeny of taxa. Studying the
origin of the Streptanthus glandulosus complex on serpentine soil-based habitat “islands”
in California, Mayer et al calculated genetic diversity between and among populations to
uncover their relationships. They concluded that the complex was likely the result of
both paleo- and neo-endemism, with S. glandulosus being historically widespread across
both serpentine and non-serpentine habitats as a single species. However, the nonserpentine habitats were gradually lost to land use change while the serpentine soils were
not cultivated. As a result, the isolation of the widely separated serpentine habitats and
the lack of gene flow between populations allowed for the evolution of neo-endemic subspecies with high intra-population diversities, a factor that further ruled out long-distance
dispersal by a small population as a cause of their founding (Mayer et al, 1994). Morrell
et al also found support for both paleo- and neo endemism in their genetic evaluation of
the relationship between a rare California and Oregon coastal endemic, Gilia millefoliata
and two South American Gilia species. Unlike the previous study, this example involved
a bird-facilitated long distance dispersal event in which a G. millefoliata-like ancestor’s
seeds were transported to coastal Chile, where in turn two new Gilia species evolved in
isolation (Morell et al, 2000). While these cases show that genetic analyses can help
clarify the phylogenetic picture with regards to endemic species, other authors have noted
that our understanding of the evolution of endemic species, and thus our classification of
them, are most accurate when genetic analyses are coupled with morphological analyses
(Debussche and Thompson, 2002).
Returning more directly to the ideas first asserted by Fernald, a slew of papers
concerning the relationship between glaciation and endemism have recently been put
forth, drawing heavily upon the advances in genetics, geology, biogeography, and paleoclimatology I alluded to above. Reisch et al used genetic analyses to verify the
commonly-held belief that disjunct Saxifraga paniculata populations in central Europe
are glacial relicts (Reisch et al, 2003). Schonswetter et al found that disjunctions in
Androsace wulfeniana populations in the Alps are the result of glaciation and not recent
long distance dispersal events, as they contain high levels of intra-population genetic
variation (Schonswetter et al, 2003). They argue that these high levels of genetic
variation, coupled with A. wufleniana’s lack of post-glacial colonization, refute the claim
that low diversity caused the “senescence” described by Fernald. Tribsch used biogeographic analysis and paleo-climatic information to determine that “vicariance
resulting from Pleistocene glaciations is the most important factor causing distributional
patterns of endemic plants and the formation of areas of endemism” in the eastern Alps.
(Tribsch, 2004). However, it must be taken into account that environmental variables
such as edaphic conditions can be essential in determining present ranges by limiting
dispersal and establishment of particular species (Clark and Funk, 2005; Schonswetter et
al, 2003). These examples are the modern progeny of the evolution of Fernald’s ideas
regarding species distributions, endemism, and disjunct populations. Advances in
genetics, technology, and our understanding of geologic and climatic history have
enabled us to more accurately determine the causes of endemism and disjunctions than
Fernald was capable of doing in his time, though his work was nevertheless brilliant
given the information he had access to.
In light of this, I believe that as advances in our ability to reconstruct paleoenvironments and climates occur, we will be able to more accurately understand why and
how the process of extinction occurred, and what the effects of these processes on current
species distributions is. I believe that learning the likely effects of invasive species and
disrupted or enhanced disturbance regimes on native ecosystems is an essential question
that we will need to deal with if we want to maintain “natural” ranges of species
distributions and minimize the loss of narrow endemics. Lastly, I think it would be
interesting to learn what is really at the cause of the “senescence” that Fernald describes,
as it appears to be neither related to age nor genetic intra-population genetic diversity, but
plays a vital role in the inability to disperse to newly available habitats.
Bibliography
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diversity: II An analysis of five florulas from northeastern South America. Proceedings
of the Academy of Natural Sciences of Philadelphia 154: 29-37.
Debussche, M; Thompson, JD; 2002 Morphological differentiation among closely related
species with disjunct distributions: a case study of Mediterranean Cyclamen L. subgen.
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Fernald, ML; 1926 The Antiquity and Dispersal of Vascular Plants, The Quarterly
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Heads, M. 2004 What is a node? Journal of Biogeography, 31 (12):1883-1891.
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complex (Cruciferae): Genetic Divergence and Gene Flow in Serpentine Endemics,
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Well thought through analysis of the development of research based on Fernald’s ideas.
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