Movements of land masses

Long ago, the curious distributions of species between continents, seemingly inexplicable in terms of dispersal over vast distances, led biologists, especially Wegener (1915), to suggest that the continents themselves must have moved. This was vigorously denied by geologists, until geomagnetic measurements required the same, apparently wildly improbable explanation. The discovery that the tectonic plates of the earth's crust move and carry with them the migrating continents, reconciles geologist and biologist (Figure 1.11b-e). Thus, whilst major evolutionary developments were occurring in the plant and animal kingdoms, populations were being split and separated, and land areas were moving across climatic zones.

Figure 1.12 shows just one example large flightless birds of a major group of organisms (the large flightless birds), whose distributions begin to make sense only in the light of the movement of land masses. It would be

adiastola group 5 -

planitidia group (17-33)

glabriapex group

punalua 58 group

35 37

51 50

60 61

grimshawi group 74 (66-101)

62 68

84 85

77 81

45 46 i 56

73 78

93 96 100

C^Niil

Niihau

Kauai

Oahu

Molokai

Maui

Lanai s>

Kahoolawe

Hawai

Hawai

Figure 1.10 An evolutionary tree linking the picture-winged Drosophila of Hawaii, traced by the analysis of chromosomal banding patterns. The most ancient species are D. primaeva (species 1) and D. attigua (species 2), found only on the island of Kauai. Other species are represented by solid circles; hypothetical species, needed to link the present day ones, are represented by open circles. Each species has been placed above the island or islands on which it is found (although Molokai, Lanai and Maui are grouped together). Niihau and Kahoolawe support no Drosophila. (After Carson & Kaneshiro, 1976; Williamson, 1981.)

/ \

Paleo-^er^e

Eocene i i i i

Oligocene

Miocene i i i

Pl

65 60 55 50 45 40 35 30 25 20 15 10 5 0 Millions of years ago

(b) 150 Myr ago

V\

(d) 32 Myr ago
(e) 10 Myr ago

Figure 1.11 (a) Changes in temperature in the North Sea over the past 60 million years. During this period there were large changes in sea level (arrows) that allowed dispersal of both plants and animals between land masses. (b-e) Continental drift. (b) The ancient supercontinent of Gondwanaland began to break up about 150 million years ago. (c) About 50 million years ago (early Middle Eocene) recognizable bands of distinctive vegetation had developed, and (d) by 32 million years ago (early Oligocene) these had become more sharply defined. (e) By 10 million years ago (early Miocene) much of the present geography of the continents had become established but with dramatically different climates and vegetation from today; the position of the Antarctic ice cap is highly schematic. (Adapted from Norton & Sclater, 1979; Janis, 1993; and other sources).

Tropical forest

■ Grassland/open savanna

Paratropical forest (with dry season)

Subtropical woodland/ woodland savanna (broad-leaved evergreen)

Mediterranean-type woodland/thorn scrub/ chaparral

Polar broad-leaved deciduous forest

Temperate woodland (broad-leaved deciduous)

Tundra

Temperate woodland (mixed coniferous and deciduous)

□ Ice

| Woody savanna

Figure 1.11 (a) Changes in temperature in the North Sea over the past 60 million years. During this period there were large changes in sea level (arrows) that allowed dispersal of both plants and animals between land masses. (b-e) Continental drift. (b) The ancient supercontinent of Gondwanaland began to break up about 150 million years ago. (c) About 50 million years ago (early Middle Eocene) recognizable bands of distinctive vegetation had developed, and (d) by 32 million years ago (early Oligocene) these had become more sharply defined. (e) By 10 million years ago (early Miocene) much of the present geography of the continents had become established but with dramatically different climates and vegetation from today; the position of the Antarctic ice cap is highly schematic. (Adapted from Norton & Sclater, 1979; Janis, 1993; and other sources).

Figure 1.12 (a) The distribution of terrestrial flightless birds. (b) The phylogenetic tree of the flightless birds and the estimated times (million years, Myr) of their divergence. (After Diamond, 1983; from data of Sibley & Ahlquist.)

Ostrich

Cassowary

Ostrich

Emu Flightless Birds Continental Drift

Figure 1.12 (a) The distribution of terrestrial flightless birds. (b) The phylogenetic tree of the flightless birds and the estimated times (million years, Myr) of their divergence. (After Diamond, 1983; from data of Sibley & Ahlquist.)

Cassowary

Brown kiwis (North Island)

Brown kiwis (South Island)

Greater spotted kiwis

Little spotted kiwis

Cassowaries

Emus

Tinamous Ostriches Rheas

Brown kiwis (North Island)

Brown kiwis (South Island)

Greater spotted kiwis

Little spotted kiwis

Cassowaries

Emus

Myr unwarranted to say that the emus and cassowaries are where they are because they represent the best match to Australian environments, whereas the rheas and tinamous are where they are because they represent the best match to South American environments. Rather, their disparate distributions are essentially determined by the prehistoric movements of the continents, and the subsequent impossibility of geographically isolated evolutionary lines reaching into each others' environment. Indeed, molecular techniques make it possible to analyze the time at which the various flightless birds started their evolutionary divergence (Figure 1.12). The tinamous seem to have been the first to diverge and became evolutionarily separate from the rest, the ratites. Australasia next split away from the other southern continents, and from the latter, the ancestral stocks of ostriches and rheas were subsequently separated when the Atlantic opened up between Africa and South America. Back in Australasia, the Tasman Sea opened up about 80 million years ago and ancestors of the kiwi are thought to have made their way, by island hopping, about 40 million years ago across to New Zealand, where divergence into the present species happened relatively recently. An account of the evolutionary trends amongst mammals over much the same period is given by Janis (1993).

Was this article helpful?

0 0
Lawn Care

Lawn Care

The Secret of A Great Lawn Without Needing a Professional You Can Do It And I Can Show You How! A Great Looking Lawn Doesnt Have To Cost Hundreds Of Dollars Or Require The Use Of A Professional Lawn Care Service. All You Need Is This Incredible Book!

Get My Free Ebook


Post a comment