Unit of selection
There is debate among evolutionary biologists about the extent to which evolution has been shaped by selective pressures acting at these different levels.
[7] Two useful introductions to the fundamental theory underlying the unit of selection issue and debate, which also present examples of multi-level selection from the entire range of the biological hierarchy (typically with entities at level N-1 competing for increased representation, i.e., higher frequency, at the immediately higher level N, e.g., organisms in populations or cell lineages in organisms), are Richard Lewontin's classic piece The Units of Selection[8] and John Maynard-Smith and Eörs Szathmáry's co-authored book, The Major Transitions in Evolution.
...the principles can be applied equally to genes, organisms, populations, species, and at opposite ends of the scale, prebiotic molecules and ecosystems."
1-2)Elisabeth Lloyd's book The Structure and Confirmation of Evolutionary Theory provides a basic philosophical introduction to the debate.
In both of these cases, gene sequences increase their relative frequency in a population without necessarily providing benefits at other levels of organization.
Such insertions can be very mutagenic and thus reduce drastically individual fitness, so that there is strong selection against elements that are very active.
Meiotic-drive alleles have also been shown strongly to reduce individual fitness, clearly exemplifying the potential conflict between selection at different levels.
Since the proliferation of specific cells of the vertebrate immune system to fight off infecting pathogens is a case of programmed and exquisitely contained cellular proliferation, it represents a case of the individual manipulating selection at the level of the cell to enhance its own fitness.
Indeed, the proper, directly selected group property is that of "not killing the rabbit too early" rather than individual virulence.
[15] Proponents of species selection include R. A. Fisher (1929);[15] Sewall Wright (1956);[15] Richard Lewontin (1970);[15] Niles Eldredge & Stephen Jay Gould (1972); Steven M. Stanley (1975).
[16][15] Gould proposed that there exist macroevolutionary processes which shape evolution, not driven by the microevolutionary mechanisms of the Modern Synthesis.
Such properties include, for example, population structure, their propensity to speciate, extinction rates, and geological persistence.
While the fossil record shows differential persistence of species, examples of species-intrinsic properties subject to natural selection have been much harder to document.
[11] In the microbial realm, it has been interpreted that the unit of selection is a blend of ecological and functional behaviors, or guilds, beyond the species-level.
