Selection pressure in declining populations

[moreover]

"Furthermore evolution will push very hard not to get excluded of selection entirely. ". I heard this statement many times, yet i find no mathematical basis for it. For example - If for example mutation allows you to increase your successful offspring from 7 to 8, how come its inferior to mutation that increase it from 0 to 1?
Anyway if this is true, does it mean that in declining fertility rates - selection pressure increases , because for more and more members of the population it becomes matter of whether to have 0 or 1, not 7 or 8.

Does it differ in tournament and monogamous species?

Lastly i want to transfer humans. In countries and societies where the fertility rates are much below replaceable, does something differ in how reproductive success is redistributed in society(especially in males), compare to countries with high fertility? Is there such thing as gini coefficient in reproductive success in males - that is how much the top 20 % reproductively successful have more kids on average than the bottom 20 %?

Any references to any of the topics will be helpful, thanks!

Edited May 29, 2015 by moreover

[Roamer]

 

"Furthermore evolution will push very hard not to get excluded of selection entirely. ".

 

Evolution doesn't do things, it just happens.

Only organisms and sometimes communities/tribes/flocks will "do" and may or may not push for survival.

 

A mutation from 7-8 is only an increase of 1/7th birthrate, while going from 0 to 1 is a relative infinite increase.

(especially considering most if not all species age and die.)

But let's compare mutations from 1 to 2 and from 5 to 6 instead.

from 1 to 2 is still +100% vs 5 to 6 being +20%.

Competition, for the remaining habitat/food-supply etc, will also only be fiercer with more offspring,

generally speaking, if there are 100 organisms of a species, it means there's room for some 100 organisms,

making more offspring then 100 means the survival chances of the first 100 decreases,

while making LESS offspring enables the parents to put more time/effort into the individual offsprings, increasing their survival chances.

 

Does it differ in tournament and monogamous species?

 

Though the underlying mechanics differ a bit, they'll still get an average of 2 successfull offspring per mother.

 

Countries-differences will mostly depend on culture, government and is more of a sociology-question.