[Paleopsych] SW: On the Great Chain of Being
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Evolution: On the Great Chain of Being
Subject: EVOLUTION: ON THE GREAT CHAIN OF BEING
http://scienceweek.com/2005/sw050624-3.htm
The following points are made by Sean Nee (Nature 2005 435:429):
1) For centuries the "great chain of being" held a central place in
Western thought. This view saw the Universe as ordered in a linear
sequence starting from the inanimate world of rocks. Plants came next,
then animals, men, angels and, finally, God. It was very detailed
with, for example, a ranking of human races; humans themselves ranked
above apes above reptiles above amphibians above fish. This view even
predicted a world of invisible life in between the inanimate and the
visible, living world, long before Antonie van Leeuwenhoek's
discoveries. Although advocates of evolution may have stripped it of
its supernatural summit, this view is with us still.
2) Common presentations of evolution mirror the great chain by viewing
the process as progressive. For example, in their book THE MAJOR
TRANSITIONS IN EVOLUTION, John Maynard Smith and Eors Szathmáry take
us from the origin of life, through to the origin of eukaryotic cells,
multicellularity, human societies and, finally, of language. They
explicitly point out that evolution does not necessarily lead to
progress, and even refer to the great chain by its Latin name, scala
naturae. But it is impossible to overlook the fact that the "major"
evolutionary transitions lead inexorably, step by step, to us.
Similarly, in their recent essay in Nature, "Climbing the co-evolution
ladder" (Nature 431, 913:2004), Lenton and colleagues illustrate their
summary of life-environment interactions through the ages with a
ladder whose rungs progress through microbes, plants, and, at the top,
large animals.
3) In his recent book THE ANCESTOR'S TALE, Richard Dawkins reverses
the usual temporal perspective and looks progressively further back in
time to find our ancestors. Like Maynard Smith and Szathmáry, he
cautions us against thinking that evolution is progressive,
culminating with us. He emphasizes that with whatever organism we
begin the pilgrimage back through time, we all are reunited at the
origin of life. But by beginning the journey with us and looking
backwards along our ancestry, Dawkins generates a sequence of chapter
titles that would read like a typical chain to a medieval theologian,
albeit with some novelties and the startling omission of God.
4) By starting with us, Dawkins regenerates the chain because species
that are more closely related to us are more similar as well, and such
similarity was an important criterion in determining the rankings in
the classical chain. But there is nothing about the world that compels
us to think about it in this way, suggesting, instead, that we have
some deep psychological need to see ourselves as the culmination of
creation. Illustrating this, when we represent the relationships
between species, including ourselves, in a family tree, we
automatically construct it so that the column of species' names forms
a chain with us as the top, as in the first of the trees pictured. But
the other construction is equally valid.
References:
1. Lovejoy, A. O. The Great Chain of Being (Harper and Row, New York,
1965)
2. Gee, H. Nature 420, 611 (2002)
3. Maynard Smith, J. & Szathmáry, E. The Major Transitions of
Evolution (W. H. Freeman & Co., Oxford, 1995)
4. Dawkins, R. The Ancestor's Tale (Weidenfeld & Nicolson, New York,
2004)
5. Nee, S. Nature 429, 804-805 (2004).
Nature http://www.nature.com/nature
--------------------------------
Related Material:
EVOLUTIONARY BIOLOGY: ON THE SCHEME OF ANIMAL PHYLA
The following points are made by M. Jones and M. Blaxter (Nature 2005
434:1076):
1) Despite the comforting certainty of textbooks and 150 years of
argument, the true relationships of the major groups (phyla) of
animals remain contentious. In the late 1990s, a series of
controversial papers used molecular evidence to propose a radical
rearrangement of animal phyla [1-3]. Subsequently, analyses of
whole-genome sequences from a few species showed strong, apparently
conclusive, support for an older view[4-6]. New work [7] now provides
evidence from expanded data sets that supports the newer evolutionary
tree, and also shows why whole-genome data sets can lead
phylogeneticists seriously astray.
2) Traditional trees group together phyla of bilaterally symmetrical
animals that possess a body cavity lined with mesodermal tissue, the
"coelom" (for example, the human pleural cavity), as Coelomata. Those
without a true coelom are classified as Acoelomata (no coelom) and
Pseudocoelomata (a body cavity not lined by mesoderm). We call this
tree the A-P-C hypothesis. Under A-P-C, humans are more closely
related to the fruitfly Drosophila melanogaster than either is to the
nematode roundworm Caenorhabditis elegans[5,6].
3) In contrast, the new trees [1-3,7] suggest that the basic division
in animals is between the Protostomia and Deuterostomia (a distinction
based on the origin of the mouth during embryo formation). Humans are
deuterostomes, but because flies and nematodes are both protostomes
they are more closely related to each other than either is to humans.
The Protostomia can be divided into two "superphyla": Ecdysozoa
(animals that undergo ecdysis or moulting, including flies and
nematodes) and Lophotrochozoa (animals with a feeding structure called
the lophophore, including snails and earthworms). We call this tree
the L-E-D hypothesis. In this new tree, the coelom must have arisen
more than once, or have been lost from some phyla.
4) Molecular analyses have been divided in their support for these
competing hypotheses. Trees built using single genes from many species
tend to support L-E-D, but analyses using many genes from a few
complete genomes support A-P-C [5,6]. The number of species
represented in a phylogenetic study can have two effects on tree
reconstruction. First, without genomes to represent most animal phyla,
genome-based trees provide no information on the placement of the
missing taxonomic groups. Current genome studies do not include any
members of the Lophotrochozoa. More notably, if a species' genome is
evolving rapidly, tree reconstruction programs can be misled by a
phenomenon known as long-branch attraction.
5) In long-branch attraction, independent but convergent changes
(homoplasies) on long branches are misconstrued as "shared derived"
changes, causing artefactual clustering of species with long branches.
Because these artefacts are systematic, confidence in them grows as
more data are included, and thus genome-scale analyses are especially
sensitive to long-branch attraction. Long branches can arise in two
ways. One is when a distantly related organism is used as an
"outgroup" to root the tree of the organisms of interest. The other is
when one organism of interest has a very different, accelerated
pattern of evolution compared with the rest.
References (abridged):
1. Aguinaldo, A. M. A. et al. Nature 387, 489-493 (1997)
2. Winnepenninckx, B. et al. Mol. Biol. Evol. 12, 1132-1137 (1995)
3. Adoutte, A., Balavoine, G., Lartillot, N. & de Rosa, R. Trends
Genet. 15, 104-108 (1999)
4. Mushegian, A. R., Garey, J. R., Martin, J. & Liu, L. X. Genome Res.
8, 590-598 (1998)
5. Blair, J. E., Ikeo, K., Gojobori, T. & Hedges, S. B. BMC Evol.
Biol. 2, 7 (2002)
6. Wolf, Y. I., Rogozin, I. B. & Koonin, E. V. Genome Res. 14, 29-36
(2004)
7. Philippe, H., Lartillot, N. & Brinkmann, H. Mol. Biol. Evol. 22,
1246-1253 (2005)
Nature http://www.nature.com/nature
--------------------------------
Related Material:
EVOLUTION: GENOMES AND THE TREE OF LIFE
The following points are made by K.A. Crandall and J.E. Buhay (Science
2004 306:1144):
1) Although we have not yet counted the total number of species on our
planet, biologists in the field of systematics are assembling the
"Tree of Life" (1,2). The Tree of Life aims to define the phylogenetic
relationships of all organisms on Earth. Driskell et al (3) recently
proposed a computational method for assembling this phylogenetic tree.
These investigators probed the phylogenetic potential of ~300,000
protein sequences sampled from the GenBank and Swiss-Prot genetic
databases. From these data, they generated "supermatrices" and then
super-trees.
2) Supermatrices are extremely large data sets of amino acid or
nucleotide sequences (columns in the matrix) for many different taxa
(rows in the matrix). Driskell et al (3) constructed a supermatrix of
185,000 protein sequences for more than 16,000 green plant taxa and
one of 120,000 sequences for nearly 7500 metazoan taxa. This compares
with a typical systematics study of, on a good day, four to six
partial gene sequences for 100 or so taxa. Thus, the potential data
enrichment that comes with carefully mining genetic databases is
large. However, this enrichment comes at a cost. Traditional
phylogenetic studies sequence the same gene regions for all the taxa
of interest while minimizing the overall amount of missing data. With
the database supermatrix method, the data overlap is sparse, resulting
in many empty cells in the supermatrix, but the total data set is
massive.
3) To solve the problem of sparseness, the authors built a
"super-tree" (4). The supertree approach estimates phylogenies for
subsets of data with good overlap, then combines these subtree
estimates into a supertree. Driskell et al (3) took individual gene
clusters and assembled them into subtrees, and then looked for
sufficient taxonomic overlap to allow construction of a supertree. For
example, using 254 genes (2777 sequences and 96,584 sites), the
authors reduced the green plant supermatrix to 69 taxa from 16,000
taxa, with an average of 40 genes per taxon and 84% missing sequences!
This represents one of the largest data sets for phylogeny estimation
in terms of total nucleotide information; but it is the sparsest in
terms of the percentage of overlapping data.
4) Yet even with such sparseness, the authors are still able to
estimate robust phylogenetic relationships that are congruent with
those reported using more traditional methods. Computer simulation
studies (5) recently showed that, contrary to the prevailing view,
phylogenetic accuracy depends more on having sufficient characters
(such as amino acids) than on whether data are missing. Clearly,
building a super-tree allows for an abundance of characters even
though there are many missing entries in the resulting matrix.
References (abridged):
1. M. Pagel, Nature 401, 877 (1999)
2. A new NSF program funds computational approaches for "assembling
the Tree of Life" (AToL). Total AToL program funding is $13 million
for fiscal year 2004. NSF, Assembling the Tree of Life: Program
Solicitation NSF 04-526 (www.nsf.gov/pubs/2004/nsf04526/nsf04526.pdf)
3. A. C. Driskell et al., Science 306, 1172 (2004)
4. M. J. Sanderson et al., Trends Ecol. Evol. 13, 105 (1998)
5. J. Wiens, Syst. Biol. 52, 528 (2003)
Science http://www.sciencemag.org
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