184
J. HESLOP-HARRISON
expressivity , low temperatures promoting the formation of wax when
the genotype allows it. The correlation he deems most significant is that
with temperature along the altitudinal transect, increasing frost being
correlated with increasing glaucousness. Again, the physiological
significance of glaucousness in relation to the factor which is presumed
to be selective is not apparent.
Two ratio clines have been described in the corn field weed, Spergula
akensis, by New (1958, 1959). The characters concerned were the papillation of the seed coat, and the incidence of pubescence on stems and
leaves. The inheritance of the seed coat character was found to be
dependent upon a pair of alleles, there being no dominance. The inheritance of hairiness was less simple, but no dominance was found, and
New concluded that since two classes “medium hairy” and “densely
hairy” were usually distinguishable in field populations, the phenotypic
ratio must represent gene frequencies. There is no genetic association
between the two characters. Sampling and scoring were done directly in
the field. I n both characters clines were detected passing across the
British Isles from NNW-SSE, the proportions of densely hairy and of
non-papillate plants increasing towards NNW. Superimposed upon
these regional clines, local clines in the same characters were found
related to altitude, the.proportion of non-papillate and of dense-hairy
plants increasing with elevation. New carried out experiments under
controlled environments to determine whether the characters were
associated with any obvious selective factor, The direction of the geographical clines in the British Isles is aIso that of a gradient from a
warmer, somewhat drier climate towards a cooler, wetter one, and it is
therefore of interest that New found non-papillate, densely hairy plants
intolerant of high temperatures and low humidity, suggesting that they
would be at a selective disadvantage in the SSW of the country. Nonpapillate plants also show a lower fertility at high temperatures and low
humidities, and their seeds germinate better a t low temperatures than
high. All of this certainly suggests that the clines are selectively maintained by climate - those related to altitude as well as the larger geographical ones. Again, the physiological basis of the observed differences
in reactions of the different genotypes is not clear; as New points out,
there is even a certain perversity in the hairy types being less resistant
to low humidities, when pubescence is often considered to be an adaptation to reduce water loss.
4. Small Scale Qenecological Differentiation
Local patterns of genecological differentiation have now been studied
in several genera. Bradshaw (1959, 1960) has investigated the common
grass species, Agrostis tenuis, over a small area of central Wales. Thirty-
J. HESLOP-HARRISON
expressivity , low temperatures promoting the formation of wax when
the genotype allows it. The correlation he deems most significant is that
with temperature along the altitudinal transect, increasing frost being
correlated with increasing glaucousness. Again, the physiological
significance of glaucousness in relation to the factor which is presumed
to be selective is not apparent.
Two ratio clines have been described in the corn field weed, Spergula
akensis, by New (1958, 1959). The characters concerned were the papillation of the seed coat, and the incidence of pubescence on stems and
leaves. The inheritance of the seed coat character was found to be
dependent upon a pair of alleles, there being no dominance. The inheritance of hairiness was less simple, but no dominance was found, and
New concluded that since two classes “medium hairy” and “densely
hairy” were usually distinguishable in field populations, the phenotypic
ratio must represent gene frequencies. There is no genetic association
between the two characters. Sampling and scoring were done directly in
the field. I n both characters clines were detected passing across the
British Isles from NNW-SSE, the proportions of densely hairy and of
non-papillate plants increasing towards NNW. Superimposed upon
these regional clines, local clines in the same characters were found
related to altitude, the.proportion of non-papillate and of dense-hairy
plants increasing with elevation. New carried out experiments under
controlled environments to determine whether the characters were
associated with any obvious selective factor, The direction of the geographical clines in the British Isles is aIso that of a gradient from a
warmer, somewhat drier climate towards a cooler, wetter one, and it is
therefore of interest that New found non-papillate, densely hairy plants
intolerant of high temperatures and low humidity, suggesting that they
would be at a selective disadvantage in the SSW of the country. Nonpapillate plants also show a lower fertility at high temperatures and low
humidities, and their seeds germinate better a t low temperatures than
high. All of this certainly suggests that the clines are selectively maintained by climate - those related to altitude as well as the larger geographical ones. Again, the physiological basis of the observed differences
in reactions of the different genotypes is not clear; as New points out,
there is even a certain perversity in the hairy types being less resistant
to low humidities, when pubescence is often considered to be an adaptation to reduce water loss.
4. Small Scale Qenecological Differentiation
Local patterns of genecological differentiation have now been studied
in several genera. Bradshaw (1959, 1960) has investigated the common
grass species, Agrostis tenuis, over a small area of central Wales. Thirty-