BIOGEOGRAPHY, ETC. Fall 2018
QUESTION SET 3

DUE: 15 November


1. Most higher taxa of animals (all major phyla, most major classes) are apparently of relatively ancient origin, their earliest common ancestors dating to the early Paleozoic (or even latest pre-Cambrian), not long (in the grand context) after multicellular life first appeared.
          A. Give a hypothetical explanation (or two if you can) for why no lineages of more recent origin have gained such status (consider what characteristics a lineage must have to be given phylum or class rank).  Is there a parallel here with adaptive radiations 'within' groups (like the diversification of mammalian orders in late Mesozoic/earliest Cenozoic)?
          B. Is there a way in which you might recognize an existing organism as the likely parent of a future adaptive radiation, or progenitor of a future higher taxon, if you were encounter such a creature now?

PART A: Phyla are generally differentiated by distinctiveness in 'basic' (often = 'ancient') traits or body forms, and by distinct ecological  habits or life-histories.  So: H1: it could just be that it takes a long time for sufficient divergence between lineages for sufficient 'distinctiveness' to develop, and for sufficient adaptive radiation within the lineage/clade for it to be visible in larger context (after all, all phyla have shared ancestors with other phyla, so they had to be not-too-different early on...); or H2: If phyla are distinguished by having distinctive adaptive traits or 'tricks', it may be that the range of possible ecological roles -- ways of making a living -- for metazoans were sufficiently fully occupied in early radiations of multi-cellular organisms (i.e., 'niche space' was filled quickly) that new tricks that might lead to phylum-level distinctiveness haven't been able to get a foot-hold, or H3: perhaps we need sufficient diversity of organisms in a group just to recognize their distinctiveness, and that takes time (this may be more about our subjectivity/perception than about reality...).  As some of you realized explicitly, there's no clear, objective criteria for recognizing a 'clade' as a phylum.  Could there be?
B.  For H1 and H3, it might not be possible; for H2, we'd have to recognize the organism as having some new trait that allows access to a whole, distinctive range of ways of making a living, a breakthrough into a new realm of 'niche space', or, alternatively, a new and much better way of doing something that's already being done that could allow a new adaptive radiation to replace an old one.  Possible in principle, but maybe very difficult to recognize until after the fact (would you have recognized mammals as the next big thing before they 'exploded' in the Cenozoic?)...

2. Here are two tree diagrams showing the relationships within a family of frogs and a family of midges; the members of each family are identified by the land mass on which they live; i.e., the frogs of Europe are most closely related to those of North America (instead of "australia" in each graph read 'the frog (or midge) that lives in australia'). One 'trichotomy' indicates that the sequence of divergence can't be fully resolved. You might find it useful to refer to the sequence of tectonic events in the splitting up of Pangaea.
    A. What's the significance of the similarity between these two trees?  What does it suggest to you about what caused (or allowed) the lineages that now live on these land masses to diverge (with regard to the alternative possibilities of initial allopatry/speciation being result of 'vicariance' or long-distance dispersal)?
    B. Identify where similarity of pattern breaks down, and offer a hypothesis as to what that means, how it might be interpreted.

HYLID FROGS                           MIDGES


In both phylogenies the Aust/NG taxon pair and the NA/Eur taxon pair are 'sibling taxa' -- share more recent common ancestors between them than with any other taxon.  Aus and NG are both Gondwanan fragments (and, in fact, quite recently joined when sea level dropped during glaciations), while NA and Eur are both parts of the northern supercontinent (Laurasia), suggesting that this pattern could be a result of vicariance -- geographical separation -- sequence due to the progressive breakup of Pangaea.  The placement of S.A. is problematic here, and differs for the two groups.  SA was part of Gondwana, so the midge story could be read as reflecting plate tectonic history through vicariance -- but the frogs are different.  SA remained connected to Aus/NG longer than any of these remained connected to the northern continents.  IF the general patterns are due to isolation of lineages by continental movement, THEN the frog story MUST involve either a secondary contact between SA and northern continents somehow, OR a cross-salt-water dispersal event somewhere in the background (but this seems unlikely for a frog). Possibly most parsimonious idea: initial branchings all tectonic vicariance, but hylid frogs went extinct in S.Am. and recolonized later when S.Am. connected to N.Am (problem with this: Eur. and N.Am. had already separated before Great American Exchange. However, N. Am. and Eur. have had various near-connections...). Alternatively, of course, could be any number of  cross-ocean dispersals leading to subsequent speciation and lineage branching in both groups, but these would 'feel' less parsimonious.  Some of you suggested there might be complex patterns of vicariance/isolation within Gondwana; yes, this could be made to work...

3. You now have a historical picture of the world alternating between a condition where most of the land is in a single large land-mass (with lower sea-levels and, therefore, narrower strips of shallow ocean around the continents), and a condition where there are multiple smaller land masses separated by ocean (with higher sea-levels, so with wide areas of flooded continental shelf). Combine this with what you understand about biological processes and patterns like evolution and species-area relationships (review species-area relationship in textbook if called for) to offer hypotheses addressing following questions. You can (re)view paleo-geography of continents and oceans at http://www.scotese.com/ if you want...
             A. How would you expect these changes to have affected diversity patterns and overall diversity in terrestrial biota?
             B. and how might they affect diversity patterns in marine biota (recognizing that much of the diversity in the oceans is in the shallow oceans – that is, on the continental shelves)?

A. Here are the components that might go into a discussion here:  Larger land masses (reasoning from species-area relationships) should support more species than smaller ones.  BUT separation of land masses would allow for differentiation between isolated lineages; IF multiple small continents each had completely distinct biotas (i.e., if they stayed isolated long enough for speciation between isolated sibling lineages), there might be higher global diversity, even though each small land mass would have lower diversity than the single large one from which they all split..  BUT (again), when smaller continents come back into contact, competition between their biotas is likely to lead to competition between ecologically similar species from the two continents and some consequent extinction (even though there might be some coexistence where roles/niches have diverged sufficiently from anybody on the other continent) (the Great Biotic Exchange between North and South America would be a case of this).  Thus, putting a bunch of small continents  together into one large land-mass is likely to reduce diversity overall, even though the big mass would have higher diversity than any single small mass.  (This is, by the way, consistent with the fossil record.)
B. The continental shelf areas can be thought of as isolated areas -- continents or islands for most marine biota -- because shallow-water biotas are pretty distinct from deep-water biotas.  But there might be more effective dispersal through open ocean between shelves than between terrestrial continents.  In any case, much the same dynamic might follow; loss of diversity when separate biotas are rejoined, enhanced global diversity when multiple isolated continents.  Also, when continents join, there's less total 'edge', so less total area => less total diversity? (BUT AN ADDED SUBTLETY: during super-continent periods, sea-floor spreading stops -- think about it -- leading to cooling and sinking of sub-oceanic crust, leading to deeper ocean basins, leading to sea level falling at continent margins, leading to overall decreased area of shallow/shelf seas -- so even more loss of diversity because of reduced area).

4. Historian Alfred Crosby, in his book "Ecological Imperialism", refers to the effects of the age of exploration and subsequent economic globalization as 'reuniting the seams of Pangaea' -- he draws a parallel between the effects of human activity and plate tectonic cycles, specifically with respect to biological distributions.  What do you suppose he means by this metaphor?  How might it inform our thinking about approaches to conserving global biodiversity?

This could link pretty directly to previous question.  Making dispersal across oceans (at least for some groups) very frequent/likely is equivalent to making the oceans 'go away' - just like the fusion of previously separate continents -- reducing opportunities for divergence and setting up situations for competition and potential competitive exclusion.  (However, this does NOT make the individual land masses bigger, so species-area effects may not be changed, and the smaller continents might not be able to sustain the diversity of an 'actual' Pangaea.).  Of couse, the story lines are different in time-frame as well; geological mergers are slow and gradual and not all simultaneous as supercontinents form.  In terms of thinking about conserving biodiversity (question is NOT asking whether that's a good agenda or not), this might mean it would be useful to look at effects on diversity of past mergers of land-masses.  It might argue for more attention/regulation directed towards reducing (unintentional) transport of organisms.  Or some research trying to determine what groups/types of organisms are more prone to human transport and so should be more carefully attended to...

5.  Caro-Beth Steward, as described in Dawkins and discussed in class, has used the distribution of hominoid fossils as well as distributions of living apes to postulate that an ape that was  common ancestor of chimps, gorillas, and humans, but a sister group to the Asian great apes, dispersed out of Asia and into Africa (the 'out-and-in-and-out-of-Africa hypothesis').  This theory is based primarily on a parsimony argument regarding the number of independent movements between continents required (see the figure in  'The Orang Utan's Tale' -- or the original article (with color figure) here, if you wish).  
A) How might this hypothesis be further tested?  What kinds of evidence would be needed?
B) How would the argument be affected if, say, a fossil apparently related to the orangutan-Sivapithecus clade were found in Africa?


A) The phylogenetic placement of the extinct lineages is probably the most speculative part of this; more rigorous placement (maybe with DNA from some of the more recently extinct ones) would help.  Better understanding of intervening habitats/conditions (i.e., of opportunities for or barriers to dispersal) would be helpful.  More fossil evidence, of course, showing distribution of the putatively Asia-only lineages (looking where they're not currently known!). Are there other groups of organisms with similar patterns? But in any case, be specific about how lines of evidence might be used to demonstrate (in)consistency with hypothesis (e.g., Finding more 'basal' fossils in the orang lineage IN ASIA would be consistent; finding more recent ones IN AFRICA would not...)
B) It would be a monkey-wrench, though not necessarily fatal; it would REQUIRE a less parsimonious expansion model -- there'd have to be AT LEAST one more movement between Africa and Asia.  Could be as simple as one more 'back-to-Africa' expansion by ancestor of this fossil (whose lineage died out in Africa).  Could be an EARLIER back-to-Africa by common ancestor of orang-human-chimp-gorilla and one or two 'back-to-Asia' expansions to produce Dryopithecus and Ouranopithecus...  And so on.  It makes for more models that are less different in parsimony terms...