The struggle for existence (fight for survival?) is not
chance exactly. There may be some happy/unhappy circumstances
that throw a curve ball in, but it's all about how well
one critter's abilities match their habitat.
A critter's ability to survive is a strong variable in calculating
its chance of reproduction.
As to the rest ... if you can calculate a probablity then
the event can happen ... that's what porbabilities are for.
e.g.
The chance of throwing a 1 on each of 100 dice is 6.5 * 10^77
or thereabouts ... doesn't mean you can't do it, but it's
unlikely it'll happen every time.
The chance of throwing a seven on a single standard die is
ZERO ... it cannot happen no matter how often yoy try.
For abiogenesis, for example, you only need it to happen once,
somewhere in the universe, sometime before approx. 3.5 billion years
ago. Creationists revel in generating finite probabilites for
this ... claiming it makes it impossible, when in fact
being able to apply a probability (no matter how low) actually
prooves that it is possible, if unlikely.