Levels of Selection
Second important clarification of adaptation concept has to do with what is called "levels
of selection problem"
Key issue here is, What level of biological organization (gene, cell, organ, individual,
group, population, species, ecosystem, biosphere.
..) are adaptations generally designed
Darwin had no workable mechanism of inheritance, but the discovery of genes as the
primary mechanism of inheritance in organic evolution had many profound
consequences for evolutionary theory, including a clarification of the levels of selection
In particular, linkage of natural selection theory to Mendelian inheritance provided
answer to query "survival of the fittest
The ultimate answer is, the fittest gene or gene combination; but of course genes
express their fitness (i.e., relative rates of replication) thru their phenotypic effects
But what kind of phenotypic effects will be favored by natural selection?
In past, biologists often spoke of evolved traits as serving "the good of the species," or
even the stability of ecosystems; but there is no basis in evol. biology for supposing that
natural selection favors any trait for that reason
That's because selection acts via differential fitness of heritable variants, and this
process works much more rapidly and effectively on variants that affect individual
survival and reproduction than those that affect populations, species, or higher-level
Thus, in most cases individual-level selection outpaces any higher-level selection
To see why, consider example of population regulation discussed by Irons (pp 11-12)
If population increases beyond the sustainable limits, it will overharvest its food, and
perhaps crash to low levels or even go extinct
So wouldn't natural selection favor populations that limit size to sustainable level?
Well, population growth is function of individual reproduction; so this really means