> CB: Why does it have to be at the cost of one's own fitness ?
Because that’s what makes it altruistic.
^^^ CB; Could be helping the other's "fitness" and not at the cost of one's fitness. That would be altruism , too. ( As I say, I don't subscribe to individual fitness.)
We have covered this before, I am afraid. Darwin’s theory, as stated, and used upto the New Synthesis, applies to individual fitness among individuals within a species (or to be precise, phenotypes/traits manifested in individuals). New species emerge because of this fitness struggle, as differentiation between individuals leads to sexually incompatible groups (where applicable; similar and other mechanisms/considerations apply to asexual reproduction).
^^^^^ CB: I gotta go , but I don't think differentiation between individuals comes out of the fitness struggle. The result of a "fitness" struggle between individuals would be one individual survives to reproduce and the other individual doesn't. That wouldn't result in differentiated individuals co-existing and going on to become sexually incompatible. It would lead to only one individual and offspring continuing to exist. In Darwin's model, it is whole species that go extinct. Becoming sexually incompatible arises from geographic separation of populations, not some "fitness" struggle between individuals.
With the New Synthesis, a genetic understanding of the underlying process was added, as well as an effort to explain such “anomalies” as altruism via ideas like kin selection or “inclusive fitness" (Hamilton, Williams, Trivers, et al). Whether such behaviour counts as “group level selection” is much debated, but at any rate, the conventional view of selection is that it occurs primarily at the individual - the “unit of reproduction” - level.
^^^^^ CB: The New Synthesis includes Darwin's concepts....so. As I say , I gotta go, so I hope we can continue this tomorrow, but If two individuals are _of the same species_ the survival to reproduction of one and not the other cannot give rise to a new species. The one that survives is of the same original species, and its offspring are not a new species from that of the original two. That is not the process by which "species orginate" or it is not "the origin of species", which is what Darwin's theory is about. If a species exists, then part of its population separates off; and then there is genetic drift to the point that it can't reproduce with the old population; and the old population of the original species goes extinct, then a species orginates.