Histories and homologies (tree thinking I) Life is incredibly diverse! ut the diversity is organized hierarchically (as groups within groups). Groups ( clades ) are united by exclusively shared ancestors. phylogenetic tree represents an inferred or hypothesized history of shared descent. Freeman & Herron s hypothetical example showing how population splitting ( cladogenesis ) and characterstate change ( anagenesis ) are represented on evolutionary trees It compactly summarizes much knowledge, and implicitly makes many testable predictions. iology 3410, 23 January 2009 Fig 2-14, p. 51, 4 th edn Darwin s hypothetical phylogeny (Fig. 1-and-only) Homology versus analogy Structures are homologous if they descend from a common ancestor, but analogous if independently derived. Subtlety: The same structure can simultaneously be homologous and analogous at different levels of organization. Example: wings of bat and bird. They are homologous as forelimbs, but analogous as wings. 1
Homologies provide compelling evidence of evolution (and useful information about relatedness) Quiz: Is character homologous or analogous? What could be more curious than that the hand of a man, formed for grasping, that of a mole for digging, the leg of the horse, the paddle of the porpoise, and the wing of the bat, should all be constructed on the same pattern, and should include the same bones, in the same relative positions? Darwin (1859) p. 434 (1) (2) Homologies of this kind ( shared derived characters ) are the strongest evidence of shared ancestry. (3) (4) Vestigial structures: Spectacular evidence of descent with modification Genomes are packed with (homologous) molecular vestiges! 2
Fossils meet vestiges: How whales lost their legs (and their teeth) (bowhead, alaena mysticetus) nd how snakes lost their legs (a process not as far advanced 95 million years ago as it is today) nd yes, Virginia, there are many missing links! Fossils meet biogeography: The law of succession (of organic forms) 3
ut when did these fossils live? sk the long-lived radioisotopes! John Maynard Smith (1920-2004) C reporter: Professor Maynard Smith, is there any discovery that could overturn the theory of evolution? JMS: Yes, a Devonian rabbit. Methodological Naturalism versus Ontological Naturalism MN: n acceptance of the rule that while wearing their scientist hats, scientists can consider only hypotheses involving natural causes. If we allow ourselves the option of appealing to supernatural explanations, then we have no way of knowing when we should keep struggling with a hard problem and when we should simply give up and declare the phenomenon in question a miracle. Through the discipline of never giving up, scientists have made great progress in solving mysteries that previous generations deemed beyond the reach of rational understanding. Darwin s triumph in explaining the origin of species is just one example among many. (Freeman & Herron, p. 62) famous biologist who is an Ontological Naturalist: We are on our own in the universe. Humanity can expect no help from outside, so our help, such as it is, must come from our own resources. s individuals we should make the most of the short time we have, for it is a privilege to be here. We should seize the opportunity presented by our good fortune and fill our brief minds, before we die, with understanding of why, and where, we exist. (Richard Dawkins) ON: commitment to the belief that the natural world is all there is (as opposed to the methodological assumption, for the sake of inquiry and argument, that nothing else exists). 4
famous biologist who is a Methodological Naturalist: nonbeliever, of course, puts his or her trust in science and finds no value in faith. nd I certainly agree that science allows believer and nonbeliever alike to investigate the natural world through a common lens of observation, experiment, and theory. The ability of science to transcend cultural, political, and even religious differences is part of its genius, part of its value as a way of knowing. What science cannot do is assign either meaning or purpose to the world it explores. This leads some to conclude that the world as seen by science is devoid of meaning and absent of purpose. It is not. What [this limitation of science] does mean, I would suggest, is that our human tendency to assign meaning and value must transcend science and, ultimately, must come from outside it. (Kenneth Miller) 5