Macroevolution: Alive and Well
by Dr. James E. Platt
Guest Editorial in The American Biology Teacher of
Kindly provided by the author
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Creationists remind us often about the alleged inability of
evolutionary biologists to directly demonstrate macroevolution.
They grant that microevolutionary change might be possible, but
moths are still moths, dogs are still dogs, frogs are still frogs,
etc. Now they'll have to add a new example to their tiresome
litany the threespine stickleback. Only this time, they
will be confronted with an example of macroevolution
in action that they cannot easily dismiss.
So what's the basis of my enthusiasm? Three papers, one in Science,
one in Nature, and one in the Proceedings of the National
Academy of Science (Colosimo et al., 2005; Shapiro
et al., 2004; and Cresko et al., 2004) appeared
in 2004 and 2005 showing that dramatic morphological changes
have appeared independently in a number of populations
of freshwater threespine sticklebacks that have all evolved since
the end of the last glaciation (i.e., between 9,000 and 14,000
years ago). Some (perhaps even most) of these populations are
thought to be new species. This phenomenon, known as parallel
speciation (Rundel et al., 2000), is remarkable in
its own right, but is not the subject here.
The subject is the rapid morphological change that has accompanied
the probable speciation events. The marine ancestors of the freshwater
populations of sticklebacks have: 1) a series of bony dermal
plates (i.e., body armor) that extend the lateral length of the
trunk all the way to the tail fin; and 2) pelvic spines that
are part of the pelvic appendages of these fish (homologous to
tetrapod hindlmbs). The remarkable observation is that these
structures, both body armor and pelvic spines, have been repeatedly
lost in multiple populations of freshwater sticklebacks.
Even more surprising, some of these populations are in locations
as distant from each other as Alaska and Iceland (Shapiro et
al., 2004). There is no question about what we have here.
This is major morphological change that has taken place in about
10,000 years, not just once, but a number of times! If this isn't
macroevolution by any reasonable definition of the term, I'll
eat my evolutionist's hat.
But the story gets better, much better in fact. As this work
has gone forward, the investigators have combined the techniques
of classical genetics and modern molecular genetics to identify
the genes involved in these changes and to elucidate the probable
ways in which gene function has been altered to produce loss
of body armor and loss of the pelvic spines. The genes are inherited
in a pattern that is largely consistent with classical Mendelian
rules. Furthermore, both the gene that controls body armor and
the gene that controls the pelvic spines turn out to be developmental
regulatory genes. This is precisely what the emerging field
called "evo-devo" (i.e., the evolution of development)
has predicted we would find to be the genetic basis of macroevolutionary
change (e.g., see Goodman and Coughlin, 2000; Pennisi, 2002a,
In the case of the body armor, the relevant gene encodes Ectodysplasin,
a cell signaling protein known to be involved in regulating the
development of dermal scales and a variety of ectodermal structures
(Colosimo et al., 2005). The alleles of this gene that
are involved in armor loss appear to have altered regulatory
regions. These alleles are already present at very low frequencies
in marine populations, but become fixed due to positive selection
in the freshwater populations (Colosimo et al., 2005).
In the case of the pelvic appendages, the primary gene involved
is called Pitx1 (Shapiro et al., 2004). As these
authors point out, this same gene is involved in hindlimb development
in tetrapods (e.g., mice that carry a double recessive lethal
form of the gene show greatly reduced hindlimbs and a pronounced
left-right asymmetry in this reduction; the same left-right asymmetry
is seen in the stickleback pelvic appendage). In sticklebacks
with reduced or absent pelvic appendages, the Pitx1 protein
is not altered; rather, the expression of the gene is
drastically reduced, but only in the pelvic appendages and the
tail fin. The observation strongly suggests that this is a regulatory
Were it not for the pressure that is being applied to biology
teachers by creationists, this article might be one of mainly
academic interest. But that pressure is increasing significantly
because to the recent surge in activity of "intelligent
design" creationists who cry out for us to "teach the
controversy" (e.g., see Scott and Branch, 2003). A central
part of this alleged "controversy" is the supposed
impossibility of macroevolution. The direct experimental
demonstration of the specific genetic basis of a documented macroevolutionary
change thus becomes very significant to biology teachers as well
as to research scientists. By the way, this example also shows
clearly that macroevolutionary changes are accomplished using
classic microevolutionary mechanisms to alter the frequencies
of developmental regulatory alleles.
Furthermore, the example is pedagogically useful. While the three
referenced papers themselves are probably too difficult for most
high school students, there have been several popularized summaries
of the work aimed at less specialized readers (e.g., see Gibson,
2005; Pennisi, 2004). Some of the illustrations that contrast
the marine ancestors with the freshwater descendants lacking
body armor and pelvic appendages are quite spectacular. At least
in the case of the Cresko et al. (2004) article, these
full-color illustrations can be directly downloaded from the
P.N.A.S. website as PowerPoint slides for teaching: http://www.pnas.org/cgi/content/full/101/16/6050
. One of these slides is even arranged to show the Mendelian
pattern of inheritance of the traits. The cited URL will take
you directly to a full-text version of the Cresko et al.
paper so that you can scroll through and pick the illustrations
that might seem appropriate for your classroom.
The results that I have presented here are merely the tip of
a giant "evo-devo" iceberg that is providing remarkable
new evidence in support of the theory of evolution. The field
of evolutionary developmental genetics is emerging as a field
that is generating empirically testable hypotheses about molecular
pathways of evolution that organisms have followed to reach their
present state. Finding out about this evidence will be challenging
for the classroom teacher. Creationists will continue to "spin"
their own interpretations of the evidence. Nevertheless, it is
my hope that this article will be seen by "evo-devo"
researchers who will recognize the importance of sharing their
findings with teachers in future issues of The American Biology
Colosimo, P. F., K. E. Hosemann, S. Balabhadra, G. Villarreal
Jr., M. Dickson, J. Grimwood, J. Schmutz, R. M. Myers, D. Schluter,
and D.M. Kingsley. 2005. Widspread parallel evolution in sticklebacks
by repeated fixation of ectodysplasin alleles. Science
Cresko, W. A., A. Amores, C. Wilson, J. Murphy, M. Currey,
P. Phillips, M. A. Bell, C. B. Kimmel, and J. H. Postlethwait.
2004. Parallel genetic basis for repeated evolution of armor
loss in Alaskan threespine stickleback populations. Proc.
Nat. Acad. Sci. U.S.A. 101: 6050-6055.
Gibson, G. 2005. The synthesis and evolution of a supermodel.
Science 307: 1890-1891.
Goodman, C. S., and B. C. Coughlin. 2000. The evolution of
evo-devo biology. Proc. Nat. Acad. Sci. U.S.A. 97:
Pennisi, E. 2004. Changing a fish's bony armor in the wink
of a gene. Science 304: 1736.
Pennisi, E. 2002a. Evo-devo enthusiasts get down to details.
Science 298: 953-955.
Pennisi, E. 2002b. Evo-devo devotees eye ocular origins and
more. Science 296: 1010-1011.
Rundle, H. D., L. Nagel, J. W. Boughman, and D. Schluter.
2000. Natural selection and parallel speciation in sympatric
sticklebacks. Science 287: 306-308.
Scott, E. C., and G. Branch. 2003. Evolution: what's wrong
with "teaching the controversy". Trends in Ecology
and Evolution 18: 499-502.
Shapiro, M. D., M. E. Marks, C. L. Peichel, B. K. Blackman,
K. S. Nereng, B. Jonsson, D. Schluter, and D. M. Kingsley. 2004.
Genetic and developmental basis of evolutionary pelvic reduction
in threespine sticklebacks. Nature 428: 717-723.
Dr. James E. Platt
Department of Biological Sciences
University of Denver
Denver, CO 80210