The joint lecture on the Evolution of Camellias by Ted Irving and Richard Hebda was brilliantly reported by Jim Hofmann, Margaret deWeese and Bill McMillan, whose notes, taken in the utter gloom of the slide presentations, must have been supplemented by an unusual group osmosis, to produce a highly literate accounting of the evening.
First, then, from Jim’s notes, a précis of Ted Irving’s presentation: “Ted piqued our regional pride by noting that all Magnolias descended from their ‘Proto-’ ancestor found in the sandstone fossil record on Vancouver Island. Over millions of years, population isolation and genetic drift occurred as a result of many influences, resulting in the various species known today. What is known as ‘disjoint centres’ – in eastern North America and Tibet/Yunnan – are unusual but not unknown: they occur for example in Liriodendron – the tulip trees.
Geographical separations in plant populations were principally due to periodic climate changes: the elemental forces of heat, water and CO2 varying as the Earth’s geographical configurations have shifted. Three hypothetical states of climate were modelled: 1) the present ‘Icehouse Earth’ with polar icecaps separated by distinct equatorial and mid-latitude regional climates; 2) The ‘Greenhouse Earth’ with no polar icecaps and a melding of equatorial and mid-latitudes in an overall higher global temperature; and 3) ‘Snowball Earth’ with ice present at all latitudes. Ted Irving mentioned some of the causes of these differing models:- variations in atmospheric transparency/reflectivity; variations in the Earth’s orbit around the Sun including the angle of the elliptic which oscillates in a 40,000 year cycle, the precession of equinoxes when Sun and Earth are closest – shifting round the calendar in an average 20,000 year cycle, and the eccentricity of the Earth’s orbit varying the flatness of the ellipse over perhaps a 90,000 year period. Total solar radiation is constant, but its distribution over Earth is variable over time. Non-periodic influences include continental drift and crustal deformation resulting in geographic redistribution of land and sea masses. Ocean currents are well recognized in redirecting heat from the sun (think of the Gulf Stream), and over time, such currents have been diverted by land shifts. Variable distribution of ice over the Earth’s regions and crustal uplift to form mountains, are the other non-periodic influences.
Original plant populations – Magnolias, and others – may have been suppressed by extreme temperature variation or isolated by land separation, and then through genetic drift and mutation, evolved into separate, unique species. Jim Hofmann happily visualizes the original species in the floor of a valley, gradually moving upwards on the mountainside to escape the rising heat below, and then as extreme cold is introduced into the region, descending again to the welcoming warmth of the valley. Over eons, of course. And gradually all the while diversifying to new species.
Moving on to the specifics, Bill McMillan reports on the heresy that ‘protomagnolias originated in the West’. The fossils reveal the story: in the 80 million year-old Nanaimo sandstone of this Island, in rocks of that age in Southern USA, in the London clays and the brown coals of Eastern Europe. And no fossil records in Asia, the current home of so many Magnolia species. According to Richard Hebda, current distribution patterns include Southeast Asia, Southern USA, and Central and South America. DNA analyses reveal that most Asian and New World magnolias evolved separately. Most Asian examples in tropical and subtropical zones are evergreen although in northern regions, some deciduous species exist. All but one of the subgenus Yulania grow only in Asia. The exception is the deciduous acuminata, the ‘cucumber tree’, remnant of an ancient population. How did these South America. distributions evolve?
About 80 million years ago, magnolias only occurred in North America, and at relatively high latitudes. Ocean areas were large and there were no polar ice caps. About 60 million years ago, landmasses were more abundant and Ted and Richard speculate that a North Atlantic land bridge existed. Magnolias formed along a band at relatively high latitude but none grew in Asia. During cooling from 45 to 35 million years, land bridges were few but magnolias still flourished further north than they do today. About 35 million years ago, however, the Antarctic and Australian land masses separated. As Australia moved away, changing ocean currents led globally to strong temperature gradients and development of Icehouse conditions. Before that time, Magnolias were happy to about 60 degrees north; subsequently they could only survive at latitudes to about 45 degrees. Glaciation related to the cooling from 35 to 15 million years ago pushed the magnolias still further south, but not yet into Asia. Fossils in rocks in this age range are found in Europe and North America. From 10 to 5 million years ago until now, magnolias moved southward into Asia and to Central and South America and likely the Caribbean. Now only a few plants with ancient roots remained in North America, China and Japan. Magnolias below 20 degrees north evolved relatively recently. There are no native magnolias in Europe today, but they were abundant 50 to 40 million years ago. Perhaps they disappeared because they could not migrate southward as the climate cooled but were blocked by the Mediterranean Sea and the too dry Saharan region
Our third reporter, Margaret deWeese, notes that M. tripetala does exhibit a link between Asia and N.America, based on DNA identification. The connection is speculated to be via the ‘Bering Land Bridge’. Land bridges are considered significant to the Caribbean and South American spreads, and also as mentioned, the North Atlantic counterpart linking Europe with America. Magnolias of the Southern US, so vital to the romantic literature of a past century, include M. virginianum (Sweet Bay or Swamp magnolia) and three umbrella species. A fascinating fossil of Dawn Redwood (Metasequoia glypstroboides) has been found near Ellesmere Island; reflecting the ‘super-Greenhouse’ interval of 50 million years ago. Thinking of our current preoccupation with the scourge of CO2 vis à vis Kyoto, we might consider that without this gas, the mean average temperature on earth would be -6°C. Moderation is all, of course, but according to Richard Hebda, the potential of a rising temperature mean, could allow the more tender Michelias to grow widely in our temperature zone.