The Oligocene to the Quaternary: climate and biology - Climate Change: Biological and Human Aspects - page 177

been noted that the lodgepole pine's seeds at the northern end of their range had
smaller wing loadings (lower weight/wing area ratio) than those at the southern end
(Cwynar and Macdonald, 1987).
From intra-species genetic variation it can be seen that the LGM-Holocene trans-
ition also facilitated the speciation of animals. Indeed, many species would have
to change and sometimes in ways fundamental to their life cycle. For example, in
a warmer world species' migration patterns would change. Three North American
bird species - the common flicker ( Colaptes auratus ), the yellow-rumped warbler
( Dendroica coronata ) and the dark-eyed junco ( Junco hyemalis ) - each have their
own eastern and western North American race or subspecies. These races have their
present-day geographical distribution either side of a line that runs from Canada's
north-western British Columbia to New Orleans, Louisiana, in the USA. That each of
these species has two subspecies with the same geographical range is not accidental
but reflects that during the glacial the two populations migrated south to two different
parts of the USA separated by a large dry desert, so enabling allopatric speciation,
before migrating back at the glacial's end.
Other pairs of bird species are more distinct (but still owe their evolution to the
last glacial-interglacial cycle). These include the northern shrike ( Lanius excubitor )
and loggerhead shrike ( Lanius ludovicianus ); the Bohemian wax-wing ( Bombycilla
garrulous ) and the cedar wax-wing ( Bombycilla cedrorum ); and the northern three-
toed woodpecker ( Picoides tridactylus ) and the black-backed three-toed woodpecker
( Picoides arcticus ). It is thought that for each of these pairs there was a common
ancestor but that during the LGM one population rode out the worst of the glacial in
Eurasia and the other in America. Then after the glacial the two species moved north
to overlap their distribution in North America.
One adaptation that plants can make is to adjust to the different day lengths
throughout the year at different latitudes. Photoperiodism (the phenomenon by which
plants flower at a specific time of the year and day) needs to change when species
migrate latitudinally. Many high-latitude plants are long-day plants: they cannot
flower until spring has become summer. Conversely, many temperate-zone plants are
short-day plants. Change in photoperiodism manifests itself in species at the level
of different plant races. So, the glacial-interglacial climate change was the impetus
for the creation of numerous new races of plants. But it was not all new races and
speciation. The end-glacial climate change brought extinctions too.
We briefly looked at the Earth's major extinction events in the previous chapter
and saw that many of these were climate-related: even if the immediate cause was
not climatic (such as an asteroidal or bolide impact), climate change often followed.
So, not surprisingly the Quaternary series of glacials and interglacials have been
associated with extinctions. In the main these extinctions happened at the end of the
glacials. One of the biggest of these took place at the end of the last glacial.
True, North America had the largest non-polar ice sheet during the last glacial, but
the extinction at the end of the last glacial was not confined to that continent: there
were extinction events in South America, Africa, Asia, Australia and Europe. Not all
species extinctions take place at the glacial's end, but there are more species threatened
during this time if only because that is the part of the glacial-interglacial cycle that