352
P. 9. MEADOWS AND J. I. UAXPBELL
(George, 1967). Populations from other areas produced free swimming
and demersal larvae from one and the same egg batch, and while the
proportion of the two types was constant in different samples from a
single brood it varied between broods. George was unable to separate
the larvae morphologically but recorded a biochemical difference in
the esterase pattern of the adults which might prove relevant. He
suggested (loc. cit. 1967) that the population in Southampton Water
which produced only demersal larvae might be genetically isolated
from other populations and might have arisen from an initial colonization by a few demersal larvae from one brood. Unfortunately there
are no genetic experiments on any freshwater or marine species on which
to base this reasoning. The work of Bartel and Davenport (1956) and of
Hickok and Davenport (1957) on different populations of the hesionid
polychaete Podarke pugettensis living either free or commensal with
the starfishes Patiria miniata or Luidia foliata has been referred to
already in the section on commensalism and will not be discussed
further here. The final example is of the isopod Asellus communis
which lives in ponds among herbage and vegetation and also in streams
and rivers. Animals collected from streams are consistently more
rheopositive than animals from ponds (Allee, 1912).
We consider this particular field of habitat selection very important, and the few references we have been able to find suggest that
populations of a species that live in contrasting habitats do show
preferences in accordance with these contrasts. It is not known how
the populations might have become initially separated.
The
separation might have been a result of the development of a geographical
barrier, or might have been caused by mutant behavioural changes in
the population or by the colonization of different habitats by a few
aberrant individuals. Some of these processes at least are likely to
be taking place as populations sporadically split off from a parent
stock and move into new habitats. Indication of the way the initial
separation may have arisen might come from a study of intertidal
animals which have an extended vertical distribution on the shore and
which, therefore, may be considered as living in different habitats when
at either extreme of their range. The behaviour of individuals of three
species of Littorina collected from extremes of their ranges accords
with their habitats (Gowanloch and Hayes, 1927 ; Hayes, 1927) ; for
example, those collected from higher levels of the shore were more
photopositive and geonegative than those from lower levels.
The successful colonization of habitats that are new to a species
cannot be very common, otherwise a large proportion of the range of
habitats within a particular environment would eventually be colonized
P. 9. MEADOWS AND J. I. UAXPBELL
(George, 1967). Populations from other areas produced free swimming
and demersal larvae from one and the same egg batch, and while the
proportion of the two types was constant in different samples from a
single brood it varied between broods. George was unable to separate
the larvae morphologically but recorded a biochemical difference in
the esterase pattern of the adults which might prove relevant. He
suggested (loc. cit. 1967) that the population in Southampton Water
which produced only demersal larvae might be genetically isolated
from other populations and might have arisen from an initial colonization by a few demersal larvae from one brood. Unfortunately there
are no genetic experiments on any freshwater or marine species on which
to base this reasoning. The work of Bartel and Davenport (1956) and of
Hickok and Davenport (1957) on different populations of the hesionid
polychaete Podarke pugettensis living either free or commensal with
the starfishes Patiria miniata or Luidia foliata has been referred to
already in the section on commensalism and will not be discussed
further here. The final example is of the isopod Asellus communis
which lives in ponds among herbage and vegetation and also in streams
and rivers. Animals collected from streams are consistently more
rheopositive than animals from ponds (Allee, 1912).
We consider this particular field of habitat selection very important, and the few references we have been able to find suggest that
populations of a species that live in contrasting habitats do show
preferences in accordance with these contrasts. It is not known how
the populations might have become initially separated.
The
separation might have been a result of the development of a geographical
barrier, or might have been caused by mutant behavioural changes in
the population or by the colonization of different habitats by a few
aberrant individuals. Some of these processes at least are likely to
be taking place as populations sporadically split off from a parent
stock and move into new habitats. Indication of the way the initial
separation may have arisen might come from a study of intertidal
animals which have an extended vertical distribution on the shore and
which, therefore, may be considered as living in different habitats when
at either extreme of their range. The behaviour of individuals of three
species of Littorina collected from extremes of their ranges accords
with their habitats (Gowanloch and Hayes, 1927 ; Hayes, 1927) ; for
example, those collected from higher levels of the shore were more
photopositive and geonegative than those from lower levels.
The successful colonization of habitats that are new to a species
cannot be very common, otherwise a large proportion of the range of
habitats within a particular environment would eventually be colonized