This episode discusses Chapter XIV where Darwin applies his ideas of evolution and descent with modification to explain the developing "natural system" of classification, the unity of embryos and why organisms have rudimentary or vestigial organs.
Classification
From the most remote period in the history of the world organic beings have been found to resemble each other in descending degrees, so that they can be classed in groups under groups. This classification is not arbitrary like the grouping of the stars in constellations. The existence of groups would have been of simple significance, if one group had been exclusively fitted to inhabit the land, and another the water; one to feed on flesh, another on vegetable matter, and so on; but the case is widely different, for it is notorious how commonly members of even the same sub-group have different habits.
Cuvier in 1817 proposed a system of classification that recognized animals as belonging to one of four forms - Vertebrata, Mollusca, Articulata (arthropods) and radially shaped animals (Radiata).
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| image from http://www.buffalolib.org/content/milestones-science/georges-cuvier |
A nice example is the phylogeny of placental mammals, those are mammals different from marsupial mammals (kangaroos, opossums, etc.) and monotremes (egg laying mammals like platypus) in that they retain the embryo internally in a placental sac where they feed and protect the developing embryo. Placental mammals represent the majority of extant species of mammals today and seem to have evolved from three major geographical locations - Africa, Laurasia and South America. This is a a beautiful artistic representation of the placental mammal phylogenetic tree.
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| http://eurwentala.deviantart.com/art/Eutheria-345487389 |
A comparison of early development of placental mammals from the Afrotheria clade (left most branch) shows similarities and divergences in the different types of mammals.
[A.. Tenrec [tenrecoidea], B. Golden mole [Chrysochloridae] C. Elephant shrew [Macroscelididae] D. aardvark [Tubulidentata] E. Bush elephant [Proboscidae] F. dugong [Sirenia] G. hyrax [Hyracoidea] Image from Hautier, Lionel, et al. "Patterns of ossification in southern versus northern placental mammals." Evolution 67.7 (2013): 1994-2010.
Modern phylogeny and classification is based on Darwin's ideas of descent with modification and we now use DNA, RNA, and protein sequences to expand and improve our understanding of the relatedness of organisms
Embryology
We have seen that the members of the same class, independently of their habits of life, resemble each other in the general plan of their organisation. This resemblance is often expressed by the term "unity of type;" or by saying that the several parts and organs in the different species of the class are homologous. The whole subject is included under the general term of Morphology. This is one of the most interesting departments of natural history, and may almost be said to be its very soul. What can 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 similar bones, in the same relative positions?
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| Homologous bone structure of tetrapod limbs |
James attempted to differentiate homologous structures from analogous structures but showed how complicated it can be when discussing flying adaptations in mammals. As you can see in the figure above, the limbs of birds, bats, humans, seals and turtles contain the same bones in the same configuration making them homologous in morphology. What changes in the relative size to each other not their relative position. In vertebrates, the evolution of powered flight occurred independently three times - in Pterosaurs (reptiles), bats (mammals), and birds (again reptiles) so their wings are functionally analogoussince they are wings constructed of different specific materials. James erroneously said that the pterodactyl wing was from a super elongated index finger but in fact it is the 4th digit what we associate with the pinky finger. We colored the figure below of a pterodactyl wing to conform to the color legend in the figure above.
Embryology
Earnst Haeckel was a famous scientist in Darwin's time who applied Darwin's idea of the evolution of organisms in his studies of the embryonic stages of chordates
As Sarah mentioned this set of illustrations were updated but the overall conclusion does not differ. Follow this link to read a wonderful summary of the Haeckel embryo controversy with modern drawings and interpretation. Overall embryo development does show that chordates exhibit very similar and distinct stages of development whereas the adult forms can be quite different in form.
Rudimentary Organs
Humans exhibit a number of traits, that we see in other mammals, but are degraded or rudimentary in their form. Some classic examples we discussed were the coccyx (tail bone), wisdom teeth and appendix but failed to mention the degrading nictating membrane in the corner of our eyes. In other chordates it is semitransparent film that can over the eye to clean and protect it. Ours is reduced to a little nubbin in the corner of our eye.
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| http://www.bio.miami.edu/dana/pix/nictitans.jpg |
The opening and closing theme to Discovering Darwin is "May" by Jared C. Balogh. http://freemusicarchive.org/music/Balogh/Revitalized_Eyes/MAY
interlude music is Otrov by Black Bear Combo http://freemusicarchive.org/music/Black_Bear_Combo/
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