Jan. 17 Max Delbruck, “A Physicist Looks at Biology”
The nature of biology (at least through the eyes of a physicist!)
Differences between physics and biology
“Traditional” philosophy of science: physics as the exemplar
Two questions:
1. Should physics be the exemplar?
2. Which physics were philosophers emphasizing?
Newtonian? Relativity? Quantum theory?
Traditional views of “explanation” (Hempel)
Deductive-Nomological Model
With ‘L’ symbolizing a law (deterministic)
And ‘C’ symbolizing some aspect of the “initial conditions”
And ‘E’ symbolizing that which is to be explained:
1. L1, L2, L3 … LN
2. C1, C2, C3 … CN
----------------------- Deductively entails
E
Statistical Explanations
1. Probabilistic L1, L2, L3 … LN
2. C1, C2, C3 … CN
======================== Makes highly probable
E
Are there laws of either sort in biology?
Well, there’s the “Hardy-Weinberg Law” in population genetics
and it is probabilistic. It is used in population genetics to
determine if evolution (defined here as change in gene or allele
frequency) has occurred.
Time line for Delbruck’s talk: 1949, 4 years before Watson’s and
Crick’s discovery of the structure of DNA (what genes are and
how they are able to replicate)
Maybe the most notable “understatement” in science in Watson’s
and Crick’s article in Nature April 25, 1953:
“It has not escaped our notice that the specific pairing we have
postulated immediately suggests a possible copying mechanism
for the genetic material.”
Each of the 4 bases links equally to another of the 4: the amount
of Adenine (A) is always equal to that of Thymine (T); and the
amount of Cytosine (C) is always equal to that of Guanine (G).
On the 21st of February 1953, Watson saw that the length of the
two pairings was the same. If the bases were paired in this way,
each rung of the twisted ladder of the helix would be of equal
length.
The specific base-pairing (with its helical structure) underlies the
perfect copying of the DNA molecule that is essential for
hereditary.
Back to Delbruck…
What strikes the physicist – in terms of ontology, epistemology,
and methodology – in biology that is strange compared with what
physicists are used to?
Ontology – and its implications for epistemology and
methodology
 There are “no absolute phenomena in biology”
 Everything is time and space bound
 The entity one deals with (organism or microorganisms involved) is but a link in an evolutionary
chain, none of which has any permanent validity.
 Even the more “fundamental” (molecular species or
chemical reactions) are “fashions of the day”.
 Again, the organism being studied is not an “ideal,”
but one thread in an infinite web of living forms, all
interrelated and all interdependent.
 The “nature” of evolutionary theory itself
 Its implication: “Any living cell carries with it the
experiences of a billion years of experimentation by
its ancestors. You cannot expect to explain so wise
an old bird in a few simple sentences.”
Contingencies and history
 The mechanisms and processes that yield “so wise old birds”:
 Natural selection
 Variation (sexual reproduction, mutation, genetic drift
migration, developmental constraints … and perhaps
more)
 Selection pressures (environment [intra and interspecies
relationships; prey/predator relationships; climate;
available niches, etc.]; scarcity of resources, struggle for
existence)
 If an organism (or gene or group) has an advantage in
terms of a variation and the “environment” that
constitutes selection pressures, it tends to be more likely
to survive and tends to be more likely to reproduce, and
it tends to pass on the traits (genes, behavior repertoire,
capacity) that are advantageous to its offspring.
“Divert the comet and the dinosaurs survive, and our ancestral
mammalian species do not or do not thrive…” S.J. Gould
Specificity, chance rather than law-like processes
Are some features of living organisms “generalizable”?
Many are not. And if some are, does this reflect “law-like”
relationships or natural selection?
Unlike in physics, when generalizations in biology only fit “a middle
range of classes,” they are typically regarded as “artifacts” rather
than (limited) law-like generalizations.
If, unlike the universality of the physicist’s account, the biologist’s
account of any particular organism, micro or macro evolutionary
event, predominance of an allele (gene) in a population, etc., is the
result, not of laws, but rather of accumulation of experience through
generations, then
How does living matter manage to record and perpetuate its
experience?
Perpetuate (sort of…): inheritance (sexual or asexual, DNA or RNA)
Record? Inheritance of successful forms and their “information”?
Information?
Coded and transmitted genetically, and sorted/built via natural
selection.
What “level”?
That of traits: phenotypic (including physical and behavioral) of
individual organisms?
That of genes (that code for phenotypic – physical and/or behavioral
– traits that enable their carriers to successfully survive and
reproduce… and thus pass on the genes in question?
The phenotypic/genetic traits dominant in a group that enable it to
survive/reproduce more successfully than con-specifics of a distinct
and different (traits wise) group?
In what manner: sexually or asexually?
The “cost” of sexual reproduction
The benefits of sexual reproduction
The “cheapness” of asexual reproduction
The potential costs of asexual reproduction
More contingencies!