What is released as a by-product of on-going radioactive decay?

HEAT is released as a by-product of on-going radioactive decay.

Decay heat (energy)

Explanation:

The decay heat is defined as the heat energy that is liberated due to the decay of radioactive elements. This heat is generated as an effect of radiation on the substances, where the energy released in the forms of α, β, and gamma radiation. These radiations forces the atoms to move due to the high heat energy.

The long-lived, unstable, radioactive isotope elements that are present on and under the earth's surface, continuously decay, as a result of which the daughter elements are produced, reducing the initial total amount, thereby releasing the heat energy.

The radioactive elements have a half-life, which differs from one element to the other.

not necessarily. higher fitness requires the organism to be reproductive successful. for example, an organism at age 10 with 15 offspring is much more fit than an organism at age 15 with only 5 offspring.