2.1. Geographical isolation
The study of biogeography deeply shaped our understanding of speciation
processes. It is of no surprise that the very broad pattern of
geographic distribution of Daphnia appears to be influenced by
vicariant processes such as the fragmentation of ancient continental
landmasses or glaciation cycles. The genus Daphnia displays a
geographic dichotomy, where species within the subgenusCtenodaphnia occur in the southern hemisphere while the subgenusDaphnia contains species commonly found in the northern
hemisphere and rarely in the tropics (Hebert, 1978; Benzie, 2005; Popova
& Kotov, 2013). Fossils from the Jurassic-Cretaceous boundary indicate
that daphniids originated more than 145 Mya ago (Kotov & Taylor, 2011),
consistent with estimates of molecular dating (Lehman, Pfrender, Morin,
Crease, & Lynch, 1995; Colbourne & Hebert, 1996; Adamowicz, Petrusek,
Colbourne, Hebert, & Witt, 2009). This line of evidence suggests that
the two subgenera (Daphnia and Ctenodaphnia ) likely
originated during the breakup of Pangea into Laurasia and Gondwana
(Kotov & Taylor, 2011). Based on phylogenetic evidence, it was
estimated that allopatric speciation accounts for ~42%
of the speciation processes in Daphnia (Adamowicz et al., 2009;
Figure 2), of which the majority (30%) represent intercontinental
splits, while 12% represent intracontinental isolation. Species
counterparts of D. pulex , D. pulicaria , D. obtusa ,D. curvirostris , and D. magna in European and North
American displayed high sequence divergence indicating splits between
2–3 Mya (Colbourne et al., 1998; Cerny & Hebert, 1999; Hobæk &
Weider, 1999; Weider et al., 1999a; De Gelas & De Meester, 2005;
Markova, Dufresne, Rees, Cerny, & Kotlik, 2007). Intercontinental
divergence between North and South America is also observed between
daphniid species (Adamowicz, Hebert, & Marinone, 2004). As an example,
South American and North American populations of the broadly distributedDaphnia ambigua show a split from a common ancestor of roughly 2
Mya (Hebert, Witt, & Adamowicz, 2003).
At the intracontinental level, lineages within species complexes often
show disjointed distributions, suggestive of allopatric speciation. A
phylogeographic survey of D. ambigua in North America showed
regional divergence with distinct central and eastern lineages that
coincide with the Appalachian mountain range barrier (Hebert et al.,
2003). Genetic differentiation has also been found to occur between
populations of D. pulex and Daphnia arenata on either side
of the Rocky Mountains (Crease et al., 1997). The Daphnia
carinata species complex, thought to have diversified roughly 70 Mya
during the Mesozoic (Benzie, 1987), includes species that are endemic to
various habitat-specific regions in southern Australia on either side of
a montane barrier, with some of the more broadly distributed species
showing regional divergence (Hebert & Wilson, 1994).
The repeated glacial cycles of the Pleistocene have been viewed as a
‘speciation pump’ (sensu Haffer, 1969) that has shaped current
geographic distributions of Daphnia . Glaciation events have
likely shaped the distribution of the D. pulex species complex in
North America (Weider & Hobæk, 1994; Crease et al., 1997; Haileselasie
et al., 2016), with many lineages originating during the Pleistocene
(Colbourne et al., 1998; Crease et al., 2012). Arctic lineages in theD. pulex species complex display a biogeographic pattern
consistent with a shift to a high-Arctic refuge during the last
glaciation event, likely due to northward movement via passive
dispersal. This dispersal colonized parts of Greenland, Iceland,
Svalbard, and northern Europe (Van Raay & Crease, 1995; Weider, Hobæk,
Crease, & Stibor 1996; Weider & Hobæk, 1997; Colbourne et al., 1998;
Weider et al., 1999a; Weider & Hobæk, 2003). Populations from glaciated
regions typically show decreased genetic divergence compared to
populations from unglaciated regions (Ishida & Taylor, 2007a).
Glaciation events have also shaped the distribution of closely related
species Daphnia parvula and Daphnia retrocurva , withD. retrocurva distributed in northern regions of North America in
large post-glacial regions (Costanzo & Taylor, 2010). Despite its broad
distribution, D. obtusa shows cryptic allopatric lineages in
North America (Hebert & Finston, 1996), which diverged roughly
<1 Mya, suggesting recent range expansion from refugia
following Pleistocene glaciation (Penton, Hebert, & Crease, 2004). In
North America, molecular analyses revealed multiple cryptic lineages
within the Daphnia laevis species complex, correlated with the
emergence of glaciation events, followed by geographic dispersal along
major migratory flyways (Taylor et al., 1998). In Europe, theDaphnia longispina species complex was found to originate roughly
5-7 Mya (Schwenk, 1993; Taylor, Hebert, & Colbourne, 1996), due to
range expansion from multiple glacial refugia (Petrusek, Seda, Machacek,
Ruthova, & Smilauer, 2008; Zuykova, Simonov, Bochkarev, Taylor, &
Kotov, 2018a; Zuykova et al., 2018b). However, the majority of the
lineages within this species complex are restricted to one continent,
which distributions largely shaped by geographic barriers (Schwenk,
Posada, & Hebert, 2000), although there are some exceptions (Ishida &
Taylor, 2007a; Ishida & Taylor, 2007b). Recent phylogeographic surveys
in Asia have uncovered a new cryptic species complex, Daphnia
mitsukuri , with multiple lineages that span a wide altitude range due
to multiple colonization events stemming from an eastern Palearctic
refugia (Ma, Petrusek, Wolinska, Hu, & Yin, 2019). In the Daphnia
carinata species complex, the restricted distribution of Daphnia
nivalis in alpine glacial lakes in the Snowy Mountains of Australia was
attributed to post-Pleistocene glaciation (Benzie, 1987).
Despite the overwhelming evidence that geography played an important
role in driving speciation in Daphnia , particularly when looking
at deep phylogenetic splits within daphniids, geographic processes
cannot account for the great biodiversity of this genus. Recent studies
on unglaciated regions show endemism, uncovering multiple cryptic
lineages and relict groups (Benzie, 1986; Benzie & Bayly, 1996; Ishida
& Taylor, 2007a; Kotov & Taylor, 2019) and these regions are
considered ‘hotspots’ for biodiversity. Thus, geographic barriers could
not completely account for current biogeographic patterns in certain
species complexes (Kotov & Taylor, 2010), suggesting that other
speciation processes, such as ecological forces might have a strong
influence. Unfortunately, delineation between geographic and ecological
processes is notoriously difficult to make (Taylor et al., 1996; Weider
et al., 1999b; Schwenk et al., 2000; Taylor, Sprenger, & Ishida, 2005),
and it has been suggested that multiple processes often shape the
current biodiversity of this genus.