Drosera evolution (part 3)

Fernando Rivadavia Lopes (ferndriv@cat.cce.usp.br)
Tue, 6 Jun 1995 15:55:26 -0300 (EST)


Let's imagine Australia a few million years ago, when the
climate began to get drier and the Drosera habitats began to shrink.
Let's assume the ancestral Drosera there had no adaptations for dry
habitats, living in perennially wet soils. At one point, chance
mutations which resulted in higher resistance to drier soils around
these shrinking "ideal" habitats began to be fixed in one or more
species. Over millions of years, these mutations accumulated and
resulted in the 3 types of dormancy observed in the 3 main groups
of Drosera in Australia (tuberous, pygmy, and petiolaris-complex
I believe the pressure to evolve these new characteristics
would be on those plants living on the edges of their ideal habitats,
in the transition zones, where the soil is drier and the plants don't
grow as well. These are the plants that would be suffering the "lack-of-
water-stress" which we're supposing is responsible for the "boom" of
Drosera speciation in Australia and the adaptation of the species to
life in drier habitats.
Now imagine Drosera living on mountain tops in Brazil, next to
a stream or seepage that doesn't dry up during the dry season in the
winter. Around these wet habitats, you'll see extremely dry areas which
drain very quickly after rains as a result of the sandy/gravelly soil.
These are especially dry in the winter and the only plants capable of
resisting in these habitats during the dry season are cacti,
Velloziaceae, tuberous orchids, etc. There is obviously a transitional
area between these wet and dry sites, where you'll always find weaker
D.montana, D.graminifolia, D.villosa, or whatever.
Now wouldn't the water stress on these plants be the same as that
on the primitive Drosera in Australia? The only difference is that in
Australian the habitats were shrinking while on these mountain tops in
Brazil the habitats have been apparently stable (especially if you
consider you "buffer" hypothesis). The ancestral Drosera in Australia
surely didn't know their habitats were shrinking and that they faced
almost certain extinction if they didn't adapt to the new conditions.
There obviously is a water stress which has been acting on the
Brazilian species for millions of years, which resulted in the evolution
of the various annual species found here. Now why didn't tubers, fleshy
roots, or compact stipule buds evolve to protect the plants in dry
habitats during the winter, allowing the plants to grow during the summer
wet season? Tuberous Drosera in Brazil would be a sure hit on these
mountain habitats! Why are there few annuals in Australia compared to the
tuberous and pygmy Drosera? I believe we're still missing something and
that the lack of water can't be entirely responsible for the speciation of
Drosera in Australia.

(to be continued)

Sao Paulo, Brazil