Gone with the Birds

ebaumsworld.com

Another great post by Salvador Herrando-Pérez.

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Through each new species, evolution assembles a unique combination of genes. Ever since living forms have populated our planet (> 3 billion years), the number of combinations is incalculable. That is why evolution resembles a cocktail shaker. Contemporaneous biogeographers look for order in that shaker to explain the history of life, as much as historians look for monarchs and revolutions in a library to explain the history of humanity.

The ethnic diversity of our suburb, village or city obeys factors of different temporal extent. Recent factors such as wealth, politics (war, segregation), culture (tradition, religion), and technology (airplanes, bridges, tunnels) determine racial migration, mixing and extinction. On the other hand, pre-historical factors express the expansion of the earliest hominids from Africa to the other continents – what makes a bantu ‘bantu’, or an inuit ‘inuit’.

Present ecological conditions and the macro-evolutionary past stock the elements by which biogeography attempts to understand the mechanisms shaping the spatial distribution of species, e.g., why kangaroos are restricted to Oceania, or why you could believe you were in Spain while strolling through a Greek forest.

Along those lines, Magnus Popp and colleagues¹ have recently examined the biogeography of crowberries [Ericales], a group of dwarf shrubs whose dispersal relies on fruit-eating animals. Crowberries show a curious bipolar distribution² with a complex of species occurring along the subarctic ranges of America and Eurasia, and one single species (the red crowberry Empetrum rubrum) endemic to the southern-most region of the Andes to Tierra del Fuego and several Atlantic islands nearby³. By means of a molecular clock, the Norwegian team led by Popp has established that the red crowberry has existed as a genetically distinct species from the end of the Pliocene, and potentially evolved from the black crowberry (E. nigrum) in north-western America¹ (see Figure below).

Molecular clock based on DNA of three crowberry species¹: black crowberry (Empetrum nigrum), purple crowberry (E. eamesii), and red crowberry (E. rubrum), from 41 locations in America, Europe, Greenland, Japan, Russia, and Gough, Malvinas and Tristan da Cunha islands [Landscape photo: Jaakko Hyvönen, Empetrum rubrum-Sphagnum magellanicum shrub in Tierra del Fuego]. Branch bifurcations show genetic splits between species in millions of years (Myr), and triangle width is proportional to number of locations considered by species and geographical region. Rates of genetic change were calibrated with DNA from fossil species, i.e., Paleoenkianthus sayreville (90 Myr), Rhododendron newburyanum (55 Myr), Vaccinium creedensis (27 Myr) y Leucothoe nevadensis (14 Myr). By their colour, we expect E. rubrum to have originated from E. eamesii (red fruits). However, genetics support that a North American population of E. nigrum dispersed some ~1 Myr (red arrow) to South America where it evolved to E. rubrum, fruit colour mutating from black to red as a result. [Shrub close-ups: Michael Jones (E. eamesii, E. rubrum), Corey Raimon (E. nigrum, Northwestern America) and Atli Arnason (E. nigrum, Northern Hemisphere)].

Species in space and time

Although Alfred Wallace and Charles Darwin founded ‘biogeography’ as an evolution-fueled discipline, the first biogeographers had the task of regionalising the planet according to current features of the distribution of species, such as areas of endemism, predominant vegetation or climate (ecological biogeography). In the 1980s, the already-accepted theory of continental drift (a dynamic fracture of a unique continent some ~ 250 million years ago), and the then-awakening technology to sequence and amplify DNA, gradually equipped biologists with a much better theoretical perspective and novel tools to explain species distributions also in time (historical biogeography⁴).

Thus, Popp and colleagues¹ clarify for crowberries one of the classical quandaries in biogeography⁵. Speciation often takes place because some individuals, or their spores, travel and found populations that evolves far from the ancestral distribution (dispersal hypothesis); or a geographical barrier fragments the ancestral species into several populations thereafter evolving independently (vicariance hypothesis).

The earliest crowberry species differentiated from the other two Empetraceae genera (Ceratiola, Corema) some ~ 24 million years ago in the Northern Hemisphere¹, much earlier than the continental drift left the continents as we see them today (250-240 million years ago). Given that the red crowberry is 1 million years old, this species could only settle in the Southern Hemisphere by pollen or fruit dispersal of black crowberries. The absence of crowberry pollen or plants (modern or fossil) over the ~8000 km  separating the current populations in both hemispheres, strongly suggests that a migrant bird ate blackberry fruits in Alaska or British Columbia and transported them in its digestive tract to Patagonia¹ – species such as the whimbrel Numenius phaeopus forage on these berries before undertaking that migration.

Future mirrors past

Molecular clocks and the growing fossil record pinpoint which species show contracting (e.g., gymnosperms) or expanding (e.g., birds) diversification⁶. The former will become extinct in the near geological future, so conservation actions might be limited to preserving individuals in captivity or refugia; the latter will represent the source of most of tomorrow’s genetic diversity, thus arguing for even greater conservation investments⁶.

Such a conservation approach perceives ecosystems, not so much by the occurrence of species A or B, but as an amalgamation of ecological and evolutionary processes^7-9. As a result, new strategies such as ‘conservation palaeobiology’ have emerged^10,11, and ongoing restoration ecology really becomes a matter of how far we want to look into the past¹². In the Earth’s past, factors such as climate change, seal level shifts or agriculture determined the evolution, migration, sovereignty and extinction of many plants and animals, and they continue to exert massive effects on demography.

If we understand how the machinery of evolution works in space and time, we might be in a better position to conserve the biodiversity and ecosystem services that our grandchildren are to enjoy in their future and much beyond.

References

1 Popp, M., Mirré, V. & Brochmann, C. A single Mid-Pleistocene long-distance dispersal by a bird can explain the extreme bipolar disjunction in crowberries (Empetrum). Proc Natl Acad Sci USA 108, 6520-6525, doi:10.1073/pnas.1012249108 (2011)

2 Distribution. Empetraceae, http://www.eol.org/pages/4267 (2011)

3 Li, J., Alexander Iii, J., Ward, T., Del Tredici, P. & Nicholson, R. Phylogenetic relationships of Empetraceae inferred from sequences of chloroplast gene matK and nuclear ribosomal DNA ITS region. Mol Phylogenet Evol 25, 306-315, doi:10.1016/s1055-7903(02)00241-5 (2002)

4 Crisci, J. V. The voice of historical biogeography. J Biogeogr 28, 157-168, doi:10.1046/j.1365-2699.2001.00523.x (2001)

5 Crisci, J. V. & Katinas, L. Darwin, historical biogeography, and the importance of overcoming binary opposites. J Biogeogr 36, 1027-1032, doi:10.1111/j.1365-2699.2009.02111.x (2009)

6 Willis, K. J. et al. How can a knowledge of the past help to conserve the future? Biodiversity conservation and the relevance of long-term ecological studies. Phil Trans R Soc B 362, 175-187, doi:10.1098/rstb.2006.1977 (2007)

7 Spector, S. Biogeographic crossroads as priority areas for biodiversity conservation. Conserv Biol 16, 1480-1487, doi:10.1046/j.1523-1739.2002.00573.x (2002)

8 Klein, C. et al. Incorporating ecological and evolutionary processes into continental-scale conservation planning. Ecol Appl 19, 206-217, doi:10.1890/07-1684.1 (2009)

9 Carvalho, S. B., Brito, J. C., Crespo, E. J. & Possingham, H. P. Incorporating evolutionary processes into conservation planning using species distribution data: a case study with the western Mediterranean herpetofauna. Divers Distrib 17, 408-421, doi:10.1111/j.1472-4642.2011.00752.x (2011)

10 Dietl, G. P. & Flessa, K. W. Conservation paleobiology: putting the dead to work. Trends Ecol Evol 26, 30-37, doi:10.1016/j.tree.2010.09.010 (2011)

11 Swetnam, T. W., Allen, C. D. & Betancourt, J. L. Applied historical ecology: Using the past to manage for the future. Ecol Appl 9, 1189-1206, doi:10.1890/1051-0761(1999)009[1189:AHEUTP]2.0.CO;2 (1999)

12 Jackson, S. T. & Hobbs, R. J. Ecological restoration in the light of ecological history. Science 325, 567-569, doi:10.1126/science.1172977 (2009)

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