Flóra: tímarit um íslenzka grasafræði - 01.01.1963, Blaðsíða 144

fá tré hverrar tegundar komast fyrir í sama skógi, og tvílitna forfeður
núlifandi fjöllitninga hafa fyrir löngu liðið undir lok.
Því hefur oft verið haldið fram, að einkenni gamalla og lengi ein-
angraðra eylanda sé rnikill fjöldi af einlendum jurtum, sem orðið hafa
til við úrval síðan einangrunin hófst. Sumir hafa talið skort á einlend-
um jurtum á eyjum Atlantshafsins norðanverðs vera sönnun þess, að
jurtir þessara landa hljóti að liafa borizt þangað seint annars staðar frá.
Skýringin er þó sennilega sú, að fjöllitni, sem gerir tegundinni kleift
að auka fjölbreytnina svo, að hún getur lifað af hina erfiðustu tíma,
kemur líka í veg fyrir að náttúrlegt úrval geti valdið skjótum breyt-
ingum á ytra útliti hennar, vegna þess að fleiri kon geta valdið sama
eiginleika en ef tegundin er tvílitna, en þar sem tvílitna tegundir eru
algengari, getur úrvalið og vissir aðrir þróunarferlar valdið skjótum
skiptum. Þannig er ekki ósennilegt, að fjöllitnin, sem bjargaði flest-
um íslenzkum jurtum frá tortímingu á ísöldinni, hafi líka valdið því,
að íslenzkar jurtir eru í flestu eins og þeir jurtahópar nágrannaland-
anna, sem þær skildu við fyrir milljónum ára og hundruðum þúsunda
ættliða.
DISTRIBU l’lON OF POLYPLOIDS.
It has long been known that polyploids are more frequent in northern or al-
pine locations than they are in more moderate climates. Wlten this was first poin-
tecl out, by Hagerup (1932), it was supposed to be caused by a liigher frequency of
formation of polyploids under extreme conditions. Phytogeograpliical observations,
however, proved this to be a misconception, since the distribution of most northern
polyploids indicates that they are ancient and may have been well-established long
before the formation of the climatical conditions under which they live at present.
Several explanations of the increased frequency of polyploids have been ventured
in recent decades. An opinion expressed by Stebbins (1950) and supported by Reese
(1961) and others, suggests that the increased frequency of polyploids towards north
is caused by the supposedly greater ability of polyploids than of diploids to invade
areas newly laid bare, in this case the regions of northern Europe from which the
glaciers retreated some 10—15.000 years ago. This explanation seems, however, to
be amply contradicted by the fact that the time since the ice retreated from nor-
thernmost Scandinavia is more than sufficient for the dispersal to tltat region and
later stabilization of every species of higher plants met with in unglaciated parts
of Central Europe. It is also in conflict with the detailed studies of weeds recently
niade by Mulligan (1960). Above all, it is refuted by the fact that the frequency of
polyploids in the northlands is highest on subarctic or arctic islands with a high
percentage of glacial survivors, whereas more recent invaders to these countries
show a distinctly lower frequency of polyploids (cf. Löve 8c Löve, 1949, 1953, 1957,
1963; Löve, 1953, 1959). The only biogeographically and evolutionarily sound ex-
planation of this phenomenon seems to be that of the selective superiority of poly-
138 Flóra - tímarit um íslenzka grasafrædi