In this episode we decided to break from the standard form of the podcast and discuss two chapters at once instead of the traditional single chapter per episode. Episode 7 covers Chapter VI - Difficulties with the Theory and Chapter VII - Miscellaneous Objections to the Theory of Natural Selection. We note that Chapter VII is really more of a vanity press project where Darwin dedicates a whole chapter to personally rebuke the concerns and critiques of Mr. St. George Mivart, a fellow biologist who published criticisms against Origin of Species.
 |
| St. George Mivart, as Charles Darwin saw him |
"First, why, if species have descended from other species by fine gradations, do we not everywhere see innumerable transitional forms? Why is not all nature in confusion, instead of the species being, as we see them, well defined?” pg 212
James used the imagery of a rainbow to argue that we often ignore the transitions or force transitional forms into the more distinct categories.
The biggest challenge of a transitional form is that it is a challenge that increases exponentially with each attempt to address the lack of transitions. We discussed that if you see Species A and Species C and believe they are closely related then the criticism of lack of transition can be leveled at the evolutionist. If the transitional form Species B is found in the fossil record then the challenge for transitional forms doubles because now you must find the transition between A & B and B & C. Each new transitional form increases the demands for more transitional forms. Sarah made the argument that the transitional challenge is often the case of moving the goal posts.
The other challenge of transitional forms is that speciation can occur in two major patterns - anagenesis and cladogenesis. This figure represents the two forms of speciation.
 |
| figure from http://krupp.wcc.hawaii.edu/ |
In anagenesis the species goes through transformation such that the original form is replaced by the new form and both are recognized as unique species. This shift in form can often occur quickly so that there is little opportunity for fossil evidence of the forms or the transitional forms are quickly replaced by the new forms.
Old photographs of common dog breeds show how quickly we have modified the breeds in just 100 years. A summary of this can be found at Science and Dogs website where we collected this intriguing comparison of how the bull terrier has changed over the past 100 years. Of course these are not true species but the premise is the same, shifting from one form to another can occur quickly and no transitional forms remain for comparison. This is where my rainbow metaphor comes to play.
The other form of speciation is called cladogenesis which involves a subset of the species that diverge from the ancestral form. Josh spoke about bat bugs and bed bugs and how they share an ancestry but it would be difficult to find the specific transitional individual between bat and bed bugs.
 | |
| image from http://ohioline.osu.edu/hyg-fact/2000/2105a.html |
Saltations
"It has been asked by the opponents of such views as I hold, how, for instance, could a land carnivorous animal have been converted into one with aquatic habits; for how could the animal in its transitional state have subsisted? [220]"
The evolution of whales, at one time a challenge to Darwin, have now become one of the best examples of evolution. The fossil record for whale evolution is robust because the ancestors of whales were semi-aquatic or aquatic mammals and their remains would often sink to the silty bottom of the ocean or bay where they lived. Buried in the mud the remains would quickly become entombed so their skeletons remained intact. Our current understanding of whale evolution is beautifully represented by this graphic from Berkeley website.
James talked about the flying squirrel and how cute it is. Here is his photographic proof of its cuteness.
 |
| flying squirrel in Kentucky, cutest animal on earth? |
Complex Structures
The eye to this day gives me a cold shudder, but when I think of the fine known gradations, my reason tells me I ought to conquer the cold shudder. - Charles Darwin in a letter to Asa Gray (February 1860).
The complexity of the eye with all of its intricate interconnecting parts seem, at first blush, to be too complex to have been formed by natural selection. This has historically been a difficult problem to solve because the eyeball does not fossilize so we are left to look for examples of eye evolution by looking at extant species. However, modern biology using molecular techniques and studying the genes involved in the production of the eye have beautifully reconstructed how a camera-like eye of vertebrates could have evolved from a simple light sensing structure seen in primitive chordates like the hagfish.
 |
| hagfish with slime - photo from ecouterre |
There a nice TED talk with awesome graphics that summarizes our current state of knowledge of how the eye evolved.
The opening and closing theme to Discovering Darwin is "May" by Jared C. Balogh. http://freemusicarchive.org/music/Balogh/Revitalized_Eyes/MAY
Copyright: Attribution-NonCommercial-ShareAlike:
http://creativecommons.org/licenses/by-nc-sa/3.0/
