What does the phrase "survival of the fittest" mean in terms of life science?

What is usually (but not always) related to the metabolic processes of living organisms in its organic form?

14) whenever diploid populations are in hardy-weinberg equilibrium at a particular locus a) the allele's frequency should not change from one generation to the next, but its representation in homozygous and heterozygous genotypes may change. b) natural selection, gene flow, and genetic drift are acting equally to change an allele's frequency. c) this means that, at this locus, two alleles are present in equal proportions. d) the population itself is not evolving, but individuals within the population may be evolving.

What is the function of the nucleus in the euglena cells you observed?