Biology Reference
In-Depth Information
Patterns of divergence are most common in larger lakes.The aquatic sys-
tems involved possess diverse littoral, benthic, and pelagic environments,
where divergent selection is apparently able to overcome the tendency for
gene flow to maintain genetically uniform populations.
Races of northern freshwater fish adapted to shallow-water and open-
water feeding niches have evolved in 37 species (Robinson and Wilson
1994). Schluter (2000) summarized information on eight sympatric pairs
and one trio of fish occupying postglacial lakes in Europe and North
America. The ancestors of these species colonized newly formed lakes
from refugia south of the glacial front or from the ocean within the past
15,000 yr. In these cases, ecological races of a single species coexist but
show genetic divergence and differential ecological specialization. Typi-
cally, one form is specialized for feeding on planktonic organisms, the
other on benthic organisms. In the case of the trio of divergent forms, one
was a plankton-feeder, and the other two were large and small benthic
feeders. Several other cases of multiple ecomorphs of individual fish
species have been reported but not yet subjected to genetic analysis.
Ecological forms adapted to feeding in different microhabitats and on
different prey are common in the arctic char (
Salvelinus alpinus
), a post-
glacial colonist of many lakes in Eurasia and North America (Johnson
1980). Typically, a large open-water feeder and a smaller benthic feeder
coexist, but occasionally, other forms occur (Griffiths 1994). In Thing-
vallavatn Lake, Iceland, for example, four forms can be recognized: small
and large benthic, snail-feeding forms that occur in water of different
depths, and planktivorous and piscivorous forms that feed in open water
(Jonsson et al. 1988;Terje et al. 1992).The four forms all spawn in the lit-
toral zone, where the juveniles live and feed together.The different forms
tend to be partially separated in spawning behavior and timing, especially
the two benthic forms, which tend to spawn in different months. The
degree of genetic difference among these forms is very small, being great-
est between the benthic forms.The differentiation of these forms appears
to have occurred in sympatry.
A somewhat similar pattern of divergence can be seen for the rainbow
smelt (
Osmerus mordax
) in streams and lakes along the Atlantic coast of
New England and eastern Canada (Taylor and Bentzen 1993). Typically
an anadromous fish that lives in coastal waters and ascends streams to
spawn, this species has given rise to lacustrine forms in formerly glaciated
regions. Some of these lake populations also exhibit dwarf and normal
forms. Similar sets of dwarf and normal populations exist in lakes in
Maine, New Brunswick, Nova Scotia, and Quebec. Dwarf smelt exhibit
