The increasing order of evolution in plants is Zosterophyllum-Ferns-Ginkgo-Gnetales.

These term are used to classify organisms according to their evolutionary history. Simple organisms refer to those organisms that have simple structural and functional organisation and are considered primitives, In contrast, complex organisms refer to those organisms that have higher and complex level of structural and functional organisation. These are more advanced and said to have arisen from simple organisms.

Nascent oxygen is highly reactive and can react with any biomolecule of the cell.
If it reacts with DNA, it can lead to mutations and defective proteins, structural or functional.
If it reacts with proteins, and enzymes are degraded, many metabolic pathways may be impaired.

Migration may bring in more such alleles, that bestow upon the individuals of the population, such adaptations/traits, which are selected by nature.
similarly, emigration may lead to removal of such alleles that confer better adaptations.
Immigration can also bring in those alleles which confer those traits, which are not suitable to the given environment, i.e. they are not selected by nature.

Survival of the fittest means that those individuals which survive and reproduce successfully in their given environment or habitat, are called fit.
Such individuals produce more progeny (with more fit individuals), than the others who are less-adapted to survive and reproduce in that environment.