Bengt
PH411 Philosophy of Biological and Cognitive Sciences
LSE/KCL
The notion of random
drift
1)
Kimura’s ‘neutral theory of molecular
evolution’
- The neutral theory of molecular evolution is “the
theory that at the molecular level evolutionary changes and polymorphisms are
mainly due to mutations that are nearly enough neutral with respect to natural
selection that their behaviour and fate are mainly determined by mutation and
random drift” (Kimura, 1983, p. xii).
2)
Kimura’s notion of random drift
- “By random drift I mean random fluctuation of gene
frequencies in a population caused by random sampling of gametes in
reproduction. In any sexually reproducing species, the total number of
individuals is not only finite, but also can be regarded as a random sample
chosen from much larger collection of male and female gametes (or ‘gene pool’)
produced by the parental generation. The amount of fluctuation is expected to
be larger, the smaller the population.” (Kimura, 1983, pp. 36-37)
- Kimura’s definition includes both ‘process’ (i.e.
random sampling of gametes) and ‘outcome’ (i.e. the random fluctuation of gene
frequencies).
- Sober (1984) and Brandon/Carson (1996) offer similar
‘mixed’ definitions of random drift.
-Questions: Are all instances of random sampling
instances of drift? Are all random fluctuations in gene frequencies instances
of drift?
3)
‘Outcome’ based definitions of drift
- Wright (1955): All random fluctuations in gene
frequencies are instances of drift.
- Problem: How to distinguish random drift from
selection in changing environments?
4)
‘Process’ based definitions of drift
- Millstein (2002): Drift is a process in which
heritable physical differences between organisms (at any stage, including
gametic) are causally irrelevant to differences in reproductive success.
Selection is a process in which heritable physical differences are causally
relevant to differences in reproductive success.
- For Millstein, Beatty’s ‘moths’ example (Beatty,
1984) is an instance of selection, not drift.
- Problem: Millstein’s notion of causal relevance is
ambiguous.
5)
- Rosenberg (1994): a) Every instance of drift can be
spelled out in terms of selection and b) an omniscient observer would not need
the theory of random drift to explain evolution.
- Millstein: a) and b) are false! Instances of drift
cannot be spelled out in terms of selection and even an omniscient observer
needs the theory of random drift to give a full
explanation of evolution.
- Millstein’s ‘snail’ example (Millstein, 2002)
- Individual-level vs. population-level explanations
6)
A brief comment on Sober
- Neither Sober’s view on drift in Sober (1984) nor
his view in Sober (2005) seems very helpful to make sense of the
neutralist/selectionist debate. In Sober (1984) it is denied that one can make
sense of a statement about the relative significance of drift and selection
(Sober, 1984, p. 117). In Sober (2005) he investigates whether given certain
data the, what he calls, ‘pure drift’ hypothesis is more likely than the ‘drift
plus selection’ thesis.
Literature:
- Beatty, J. (1984): ‘Chance and Natural Selection’,
in Philosophy of Science 51, pp.
183-211.
-
- Kimura, M. (1983): The Neutral Theory of Molecular Evolution.
- Millstein, R. (2002): ‘Are Random Drift and Natural
Selection Conceptually Distinct?’, in Biology
and Philosophy 17, pp. 33-53.
- Rosenberg, A. (1994): Instrumental Biology or the Disunity of Science.
- Sober, E. (1984): The Nature of Selection.
- Sober, E. (2005): ‘Is Drift a Serious Alternative to
Natural Selection When it Comes to Explaining Adaptive Complexity?’, in A.
O’Hear (ed.): Philosophy, Biology and
Life.
- Wright, S. (1955): ‘Classification of the Factors of
Evolution’,