It is determined that the homozygous recessive genotype for a gene with two alleles has a relative fitness of 0.8. Can't be determined from only this information. Descent with modification occurs as favored genetic variations accumulate over time. Only those variations that help organisms to survive and reproduce are passed on from parent to offspring. In nature, organisms produce many more offspring than survive to reproduce.
Organisms display heritable variations among themselves.
All of the above were major points made by Darwin. Fossils in younger rocks are more similar to living organisms than are fossils in older rocks. Humans and chimpanzees share no more of their genetic information than either shares with a panda bear. Fossils in older rocks are more similar to living organisms than are fossils in younger rocks. One of the points made was that ALL of these forces consistently operate to increase the extent to which populations are adapted to their environment.