[From somewhere along the `Instinct' thread]:
AFAIK the most parsimonious (unit of reproduction and trait -- germ line? -- replication) and hence preferred view is still individual selection. ravi
Yes, I agree there is honest debate about this; however, I consider the data that supports the idea of group selection compelling. miles
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(I decided I would post this for general information rather than address any particular post on the instinct thread. I think I'll post another item on the relationship between genotype and phenotype, because like the individual v. species selection argument, it comes up a lot.)
There are some critical points missing in the discussions on natural selection as the basis of evolutionary change. First, this is not an either/or debate. Second, natural selection is not a single horizontal class of phenomenon. And third, natural selection is not the only mode of evolutionary change.
Natural selection can be argued to show effects at several different hierarchal levels from individual organisms on up. The reason is that natural selection is not a single horizontal class, say that only effects individuals, but a hierarchical family of selection classes related together by their effects on different levels.
Gould's term Hierarchical Selection, or those selection phenomenon which effect groupings, in this case species. above the individual organism level are associated with Macroevolution. The natural selection class that shows effects at the individual level and below are termed Microevolution.
>From Gould's The Structure of Evolutionary Theory:
``I have long regarded species selection as the most challenging and interesting of macroevolutionary phenomena, and the most promising centerpiece for macroevolutionary theory. While I continue to espouse this view, my rethinking for this chapter has led me to appreciate the significant power of two other species-level processes: drives of directional speciation as just discussed ... and species drift, the higher-level analog of genetic drift. I would now argue that the interaction of these three processes sets the distinctive character of macroevolution.
As for natural selection at the organismic level, the two major modes of species selection operate by different rates of generating daughter species (the analog of birth biases in natural selection) and differential geological longevity before extinction (the analog of death biases in natural selection). At the species level, however, the difference between these two modes does not rest upon the same basis that distinguishes their analogs at the organismic level.
At the organismal level, natural selection by birth bias works mainly upon such `internal' traits as reproductive rate and brood size, and often doesn't increase adaption in the conventional sense of phenotypic molding to better biomechanical design for local environments. For example, an organism gains large selective advantage merely by breeding a bit earlier, though nothing else about the phenotype need alter...(..referred to..as `selection without adaptation'). But natural selection by death bias among organisms usually yields phenotypic adaptation for better fit to the ambient environment.
At the species level, however, our main concern moves to an interesting difference in causal locus. Most cases of selection by differential speciation operate by the interaction of an irreducible species-level character---some feature of population structure--with environment, and therefore represents genuine species selection. After all...organisms don't speciate; only populations do. But for selection by differential extinction, a higher frequency of cases can probably be explained as the simple summation of organismal deaths...for both organisms and species die. ...
However, the most interesting of all differences between organismal and species selection may lie not in the phenomena themselves, but rather in the character of their interaction with the other primary modes of evolutionary change: drives, and drift...''
(731p, Chp 8 Species as Individuals in the Hierarchical Theory of Selection, The Structure of Evolutionary Theory)
The term natural selection was originally used by Darwin to distinguish a natural or non-human cause from human controlled selective breeding and culling. In that context, natural selection was limited to naturally occurring environmental causes that effected birth rates and death death rates among individuals in a species.
CG