Science 240

Adaptation and historical explanation

Lectures #38, #39, and #40: Friday, 4 December, Monday, 7 December, and Wednesday, 9 December

1. The idea of adaptation
   1. Recall Paley and natural theology: plants and animals show remarkable fitness to the circumstances in which they occur. An adequate evolutionary theory must explain that fitness.
   2. Evolution by natural selection is the evolutionary explanation for adaptation.
2. What constitutes an adaptation?
   1. Two characteristics of adaptations
      1. A characteristic is an adaptation if it is useful for some purpose.
      2. A characteristic is an adaptation if it arose through natural selection.
   2. Opposable thumb in humans
      1. Adaptation for grasping objeccts.
         1. It is useful to grasp objects, e.g., spears used in hunting, bowls used for cooking.
         2. Presumably individuals with an opposable thumb were more likely to survive than those that lacked it
      2. Not an adaptation for holding a baseball bat
         1. Baseball didn't exist when the opposable thumb evolved
   3. Refinement of our definition
      1. A characteristic is an adaptation if it is useful for some purpose.
      2. A characteristic is an adaptation if it arose through natural selection, and the purpose it serves is causally responsible for the increased fitness through which natural selection acted.
   4. Feathers in birds: are they an adaptation for flight?
   5. Claims about adaptations are historical claims, i.e., claims about particular events that happened in the past.
      1. Physical sciences seek to exclude particulars of time and place from their theories.
      2. Certain aspects of evolutionary biology are bound to these particulars.
      3. Questions about historical sciences:
         1. Are the methods of historical scientists so different from those of other scientists that they should not be called scientists?
         2. If historical scientists are called scientists, how do we distinguish what they do from historians do?
3. Dinosaur extinctions as an example of historical science
   1. Initial objective: determine sedimentation rates associated with a limestone deposit in northern Italy
      1. Certain rare elements, especially iridium, are much more common in asteroids than in earth's crust
      2. Approximately constant accumulation of asteroid impacts
      3. Use iridium concentration to determine rate of deposition
   2. Results
      1. Ir abundance 300 more concentrated in layer associated with boundary between Cretaceous and Tertiary periods (K/T boundary)
      2. No physical features suggesting slower rate of deposition.
      3. Therefore, impact of major comet or asteroid at K/T boundary. Impact of this comet is responsible for major extinction event at the K/T boundary, including extinction of dinosaurs
   3. Additional evidence for impact
      1. Iridium anomaly found in rocks at everywhere K/T boundary has been studied
      2. Similar anomalies for other "extraterrestrial" minerals: rhodium, Ir/Rh ratio characteristic of extraterrestrial origin
      3. Other features associated with known asteroid/comet impacts associated with K/T boundary
      4. Should be able to identify a site of impact
   4. Consequences of impact
      1. Air temperatures of about 2000K
      2. Wild fires ignited for hundreds or thousands of miles in all directions
      3. Rock dust and soot prevents virtually all light from reaching the earth's surface for a year or more
   5. Challenges to the impact hypothesis
      1. Impact at K/T boundary not the cause of extinctions
      2. The seven meter gap
      3. High-latitude dinosaurs
4. Explanation of historical events: general principles
   1. Components of an explanation (in general)
      1. Explanans from which the explanandum can be logically deduced.
      2. Explanans must contain general principles.
      3. Explanans whose general principles are well-supported and empirically justified
   2. Historical explanations in particular
      1. Explanandum: a particular historical fact or circumstance to be explained, e.g., extinction of the dinosaurs
      2. Explanans: general principles combined with particular facts about the circumstances from which the explanandum can be deduced, e.g., iridium anomaly implies impact of comet or asteroid
      3. Seem to fit the requirements, but how is explaining dinosaur extinction different from explaining the rise of Hitler?
   3. Question: Testability demarcates empirical knowledge from non-empirical knowledge. Does it demarcate science from non-science?