Biology Reference
In-Depth Information
theless important. If the temperature decline from the LPTM/EECL had
continued unabated into the glacial age, the extinctions and radiations, al-
though more gradual than those resulting from catastrophes, would likely
have produced a very different set of ecosystems within which we and
other organisms later evolved, from which we radiated, and on which we
currently depend.
Next came a resumption in the downward shift in temperatures begin-
ning in the middle Miocene—a trend toward cooler temperatures, less
evaporation of water from the ocean surface, drying climates, greater
seasonality, cooler marine currents, and the eventual appearance of local
desert conditions correlated with increasing continentality and the rise of
mountain systems. This was an important time in the evolution of lineages
and ecosystems because it was when deciduous forests became widespread
across the temperate latitudes of the Earth, and when individual elements
were coalescing into early versions of shrubland/chaparral-woodland-
savanna, grassland, and deserts, including the cerrado, caatingas, pam-
pas, monte, and dry tropical forests of South America. Lineages growing
in seasonally dry climates—sometimes on coarser, drying, acidic soils,
with reinforcing slope, exposure, and rain shadow—experienced their
greatest period of diversifi cation and radiation, favorable to the appear-
ance of many drier-habitat crown groups. Better-defi ned deserts, savan-
nas with fewer trees to constitute true grasslands, tropical dry forests,
pine-oak forests, and the beginnings of recognizable and more widespread
tundra, steppe, and páramo developed in and after the middle Miocene
around 15 Ma.
These refi nements in ecosystems continued into the Quaternary. That
epoch was once thought to extend back only to about 1 Ma and was gener-
ally considered too short an interval for the generation of many modern spe-
cies. However, it has now been extended back to 2.6 Ma, providing ample
generation time for trees, shrubs, and especially annuals. Furthermore, the
Milankovitch and fi ner variations pumped considerable climatic variability
into the system, causing extinctions, disjunctions, migrations, and wide-
spread contact between previously allopatric species in both the high and
low latitudes. As a result, the pulse of the redefi ned Quaternary can now be
viewed as a signifi cant generator of new species.
In sum, much of the existing biodiversity of the Earth is a product of
events and processes at specifi c intervals when environmental events com-
bined with evolutionary processes to increase the tempo of evolution in
particular clades. These events and processes include (1) some probable
enhancement of deciduousness as a result of the asteroid impact at the end
