Those that survive can reproduce and pass those beneficial mutations to their offspring, progressively turning the species into a different one, with various isolated population of the same original species progressively drifting away until they differ enough to be incapable of cross-breeding. That mechanism is called speciation.
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Paul say: A natural process of gradual change through which living organisms remain well adapted to an ever-changing environment.
Christian biologist
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ming say: Evolution is the process of mass state matter being converted from energy state matter [matter include mass state matter (sometimes people call it matter) and energy state matter (sometimes people call it energy)] and driven by energy dissipation the matter become more and more complicated and ordered in its structure and movement. Life is living matter. life evolution is part of the mass state matter evolution process --- a higher stage of mass state matter evolution though it is caused by the same fundamental reason --- energy dissipation and bear the same effects --- the complication and ordering of its structure and movement.
Fittest survival, in its general meaning, can be seen as matter will fall into the (thermodynamically and behaviourally) more stable existence state of that moment. For instances: two hydrogen will fuse into one helium and release some energy under the conditions in the Sun; when disturbed, a rock will fall from hill to the valley; when prey see a predator it will run away from it to a safer place; people will tend to immigrate from a resources depleted and population saturated area to a relatively resources rich and population less dense area for a better live…
Evolution is the term we describe the long and steady process in normal conditions the mass state matter be converted from energy state matter and complicating and ordering its structure and movement. Though for the conversion process of mass state matter into energy state matter we also can use this term --- evolution to describe it. though that is a short and simpler process and only happen in extreme conditions --- mass state matter fall into the gravitational trap of the central black holes in the galaxies and under the extreme conditions (extreme pressure, temperature and magnetic field), the last structure of mass is crushed to release all its energy and jet it out far and wide into space through the two magnetic poles of the black holes. And there, the long process of energy dissipation --- entropy growing begin and mass state matter evolution start.