Somatic vs Gametes
Gametes are special cells preserved in your body from birth. These cells have their genetic repair systems tuned to over-drive and at the end of several decades their DNA is still almost identical to the DNA they started with.
Somatic cells are every other type body cell, and for the most part they aren't as hot on DNA repair. Which is why most cancers occur in these cell lines. They are by no means push overs, most of the DNA sequences are still intact decades later, but there are describable differences between the DNA in your hand, and the DNA in your liver.
For the most part these energies are balanced, and any spikes are well within the cells ability to repair damage.
Excessive Light energy encourages genetic abnormalities, and particular protein complexes that are generally hyper-beneficial. The overall health, and function of the particular creature is increased. The powers begin to manifest shortly after birth, and continue to accrue as they age, causing older individuals to be quite god-like in comparison to their younger selves, while also allowing their own parents to raise them before they become too god-like. This hyper-beneficiality however is not without its downsides.
A fetus coming into existence with X+ years of demi-god like evolution is simply too risky for a mother to carry to term. If the father was the god-like individual the likely-hood is that the baby would drain the mother of all physical resources essentially killing her. Conversely a god-like mother carrying a god-like child would be able to carry the fetus into a much later term but would likely suffer serious injury when the fetus begins to kick. After all the mother is not protected from a herculean punch from within her. At best the baby would be born premature, the only thing helping it would be the god-like powers. Unfortunately though the baby would likely die due to lack of care. Anyone who attempted to render that care would likely be harmed, the bottles used to feed it snapped like twigs.
This is a problem for evolution, because all the women, or all the men wish to mate with a single demi-god (because they are by far the best), and demi-god like fetuses kill the mothers. This would be disastrous for a species. It would be better if those abilities had not been passed on. Hence biology suppresses these changes from occurring in the gamete lineage. The species that did not adapt this way have already died out.
Excess dark energy tends to afflict the gene responsible for body-layout and alters the production of certain protein complexes. For any specific individual these induced changes are relatively stable, and manifest at the developmentally appropriate age without further negative effect genetically. This has the effect of produce poisonous bile (even to its ownself), extra legs, no legs, and other abnormalities.
Generally speaking these mutations are horrible for the individual they are inflicted on. However from a species perspective these are a great chance, every now and then the mutation is successful, and repeatable. This encourages speciation: Snakes from lizards, whales from land predators, pegasi from horses.
Due to the advantage in the long run, most species do not overly suppress the genetic deformations caused by dark energy in the gamete lineage. Some may in fact encourage it, hoping to increase the rate of speciation.
Which is better
Obviously light is great for individuals, but horrible for the species. This tug of war ensures that individuals do not confer their god-like propensities.
Dark energy is better for the species giving them more baskets to store their proverbial eggs in. This allows entire families of similar species to handle changing environments, and extinction events - some of them are likely better suited to living and survive. The unfortunate downside is that many afflicted individuals experience short, brutish lives until normal evolution smooths out the issues. Imagine a horse with skinny protrusions from its back, that horse is going to have a bad life, but perhaps its descendants learn to use those protrusions to fly, now they are a gift allowing escape from many predators.