In the Light of Evolution Volume III: Two Centuries of Darwin (2009) / Chapter Skim
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15 Did Darwin Write *the Origin* Backwards?--Elliott Sober
15 Did Darwin Write *the Origin* Backwards?--Elliott Sober
Pages 307-328
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From page 307... ...
I argue that common ancestry has evidential priority. Argu ments about natural selection often make use of the assumption of common ancestry, whereas arguments for common ancestry do not require the assumption that natural selection has been at work.
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From page 308... ...
For these reasons, Darwin's theory is better described as "the origin of diversity by means of natural selection." Darwin's concept of natural selection has several noteworthy features. Although the Origin introduced the idea of natural selection by first describing artificial selection, Darwin hastened to emphasize that natural selection is not an agent who intentionally chooses.
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From page 309... ...
in the same manner the variations of each creature are determined by fixed and immutable laws; but these bear no relation to the living structure which is slowly built up through the power of natural selection, whether this be natural or artificial selection. Darwin (1868c, p.
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From page 310... ...
A tribe including many members who, from possessing in a high degree the spirit of patriotism, fidelity, obedience, courage, and sympathy, were always ready to aid one another, and to sacrifice themselves for the com mon good, would be victorious over most other tribes; and this would be natural selection. At all times throughout the world tribes have sup planted other tribes; and as morality is one important element in their success, the standard of morality and the number of well-endowed men will thus everywhere tend to rise and increase.
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From page 311... ...
The last facet of Darwin's concept of natural selection that i want to mention concerns his comment in the Origin (1859, p.
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From page 312... ...
COMMON ANCESTRY With these caveats about natural selection duly noted, is "evolution by natural selection" a good characterization of Darwin's theory?
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From page 313... ...
. instead of describing Darwin's theory as evolution by natural selection, the theory is better described as common ancestry plus natural selection.
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From page 314... ...
are separated from each other by walls. no one doubted, then or now, that natural selection FiGUre 15.3 The only diagram in the Origin.
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From page 315... ...
however, if we set natural selection aside and consider instead the idea of common ancestry, the picture changes. Darwin thought he had strong evidence for common ancestry.
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From page 316... ...
Contrast this with the torpedo shape that sharks and dolphins share; this similarity is useful in both groups. one might expect natural selection to cause this trait to evolve in large aquatic predators whether or not they have a common ancestor.
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From page 317... ...
The torpedo shape of sharks and dolphins involves a likelihood ratio that is close to 1; the tailbones of humans and monkeys and the gill slits of human fetuses and fish involve likelihood ratios that are much larger than unity. Darwin's Principle applies outside of biology, both in other sciences and in everyday life.
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From page 318... ...
let's take the second part first, the one about neutral or deleterious characters. if a drift process goes on long enough, the resulting character states of the descendants X and Y will have about the same probability, regardless of whether the common ancestry or the separate ancestry hypothesis is true.
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From page 319... ...
This point might underlie the thought that complex adaptations can provide substantial evidence of common ancestry even if simple ones do not. The second context in which Darwin is wrong to dismiss adaptive similarities is a bit less obvious. Consider the 2 fitness functions shown in Fig.
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From page 320... ...
The higher the peaks and the wider the valleys, the more strongly their similarity favors common ancestry over separate ancestry. These last two cases, in which adaptive similarities provide strong evidence for common ancestry, are not just abstract possibilities.
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From page 321... ...
And adaptive characters often provide little or no evidence for common ancestry. HOW COMMON ANCESTRY AND NATURAL SELECTION ARE RELATED IN DARWIN'S THEORY Darwin (1859, p.
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From page 322... ...
There are other explanations to consider that are more rooted in the details of what Darwin says in the Origin. Perhaps he placed natural selection at center stage because he thought that selection is more important than common ancestry.
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From page 323... ...
Arguably ancestral influence has had a bigger effect on trait evolution than natural selection has had because 50 is closer to 40 than it is to 100. Darwin is saying that even when ancestral influence has been very strong (as in the hypothetical case we are considering)
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From page 324... ...
Darwin's claim that selection is not the exclusive cause of evolution plays an essential role in allowing him to develop his evidence for common ancestry. his conjunction -- common ancestry and natural selection -- would be unknowable, according to Darwin's Principle, if the second conjunct described the only cause of trait evolution.
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From page 325... ...
. Darwin discusses this in chapter 6 of the Origin, which he called "Difficulties for the Theory." he begins by noting that if the different eye designs found in nature can be arrayed in a graded sequence, from simpler and cruder to more complex and more adaptive, this will be the beginning of an argument that the trait evolved by natural selection.
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From page 326... ...
Parsimony considerations, applied to an independently attested phylogeny, also play an important role in testing hypotheses about natural selection. Consider, for example, the hypothesis that land vertebrates evolved 4 limbs to help them walk on dry land.
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From page 327... ...
Darwin is explicit, here and elsewhere, that what makes a trait currently useful may differ from what made the trait initially evolve. it FiGUre 15.8 The use of parsimony to reconstruct the character states of ances tors in the lineage leading to modern land vertebrates (which is represented by the dashed line)
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From page 328... ...
you do not need to assume that natural selection has been at work to argue for common ancestry; in fact, what Darwin thinks you need to defend hypotheses of common ancestry are traits whose presence cannot be attributed to natural selection. This is the evidential asymmetry that separates common ancestry from natural selection in his theory.
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