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
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?