From: arlin@ac.dal.ca
Newsgroups: sci.bio
Subject: re: reproductive success, fitness, and predictive theories
Date: 11 Apr 92 04:03:22 GMT
Organization: Dalhousie University, Halifax, Nova Scotia, Canada

"Fitness" denotes a concept of reproductive success that applies
within a population of organisms to classes of individuals that share
a trait.  The absolute Darwinian fitness of green-eyed individuals is
the expected average number of offspring of green-eyed individuals
that live to reproduce in the next generation.  Contrary to an early
suggestion in this thread, fitness may be used whether or not we have any
notion of the genetic basis of the trait, and even if the trait has no
genetic basis-- since we wouldn't want to exclude the possibility that
non-heritable traits affect fitness (of course they do!).

However, fitness is a much more fun and useful concept if the mode of
heredity of the trait is understood, and especially if the trait is
itself a gene.  In such cases, it is possible to associate the fitness
values with genes or genotypes, rather than individuals.  Precise
rules of genic inheritance then allow one to model what will happen
through many generations of organismal reproduction.  The units of
fitness and of predictions based on fitness could theoretically be
either biomass of genes/offspring or total number of genes/offspring
(obviously, this wouldn't normally make much difference within a
population of organisms that have a roughly constant biomass per gene
or per adult organism), but the tradition is to measure fitness with
number.

One participant in this thread has suggested that fitness should not
be counted in terms of the number of offspring that live to reproduce
in the next generation, since (for instance) the grandchildren might
be sterile.  This is a good point.  However, be warned that making
fitness the ultimate principle of evolution and modifying its
definition so that it always coincides with whatever properties are
observed _a posteriori_ to bring eventual success is guaranteed to
lead you straight into a logically circular theory of evolution.  It
is better to decide on a precise definition of fitness, and then to be
prepared to accept that evolution may not always favor the organism
with higher "fitness" so defined in the long run.  It is only in
religion that theories must explain absolutely everything.

The comparative "success" of two different species of organisms is not
the same as fitness.  Comparing growth rate, total numbers or biomass
of nematodes versus mammals might be an interesting excercise, but
ultimately most of their properties are incommensurable.  When one is
making a comparison WITHIN a population, one does not have to worry
about comparing apples and oranges.  We can expect two alleles in the
same species to come into direct competition to make contributions to
the next generation of genes, so that there is some scientific
usefulness to "success" (as defined by number or biomass or
longevity): it allows us to explain or predict which of two alleles
will increase in number or eventually prevail (by number, biomass, or
longevity) in a population.

By contrast, consider a comparison between different types of
organisms.  Suppose that I am told that cyanobacteria are more
"successful" than vascular plants, because they have more biomass, or
more species, or more cells, etc.  There is no inherent logical flaw
in such a concept, and this type of "success" is a valid experimental
tool, since (if precisely defined) it could be measured.  However,
this logical and measurable property will tend to be impoverished as
the basis of a theory for biosphere evolution. What scientific thing
can be done with this notion; how will it help to understand the world
or make predictions about the future?  It is clear that vascular
plants evolved for life on land, and the fact that they haven't
returned to the waters and kicked out the cyanobacteria suggests that
cyanobacteria are superior there; cyanobacteria have invaded all or
nearly all terrestrial habitats but have not replaced the vascular
plants (even though they could greatly increase their resource base by
taking over the rainforests), suggesting that vascular plants are
superior on land. [Curiously, the metabolism of vascular plants is
actually evolutionarily based on cyanobacteria (chloroplasts), so that
there is a very real sense in which cyanobacteria *have* indeed
thrived in the terrestrial environment, where chloroplasts actually
outnumber plant cells, though they do not exceed them in biomass.]

I personally am completely confused about what the "success" of
the cyanobacteria might mean, and I have little idea what it would
predict for the future of the biosphere.  Unlike individuals in the
same species, cyanobacteria and vascular plants are not really
competing very directly for the privilege of leaving offspring in the
next generation, so there isn't much that can be said in the way of
predictions.  I am not trying to say that all comparisons between
cyanobacteria and vascular plants are useless or invalid.  There is
nothing wrong with suggesting that cyanobacteria are more likely than
vascular plants to survive a global catastrophe: this can be
tentatively concluded from noting that viable cyanobacteria have been
recovered from decades-old frozen material, from areas within a mile
of a nuclear blast site, and from places so dry that no rain has been
recorded for over a hundred years.  However, it will be difficult to
employ cyanobacterial superiority in "catastrophe survival" or some
other measure of "success" in a general predictive theory of biosphere
evolution (unlike the case for using fitness to predict the fate of
alleles).

Arlin Stoltzfus
Department of Biochemistry
Dalhousie University

arlin@ac.dal.ca
Newsgroups: sci.bio