According to Darwin's theory, what are the three forces responsible for evolution?

Answer 1

1.Overproduction- all living beings are endowed with great fecundity.
2.Struggle for existence because of shortage of food and shelter
3.Survival of the fittest on the basis of advantageous variation as variation is the rule and no two individuals are alike.

Answer 2

1. Variation -- The fact that individuals are not all identical.

2. Inheritance -- The fact that traits are inherited.

3. Competition -- The fact that more individuals are born than can possibly reproduce ... so there is competition for resources and mating opportunities.

Given these three elements, those individuals that have traits that provide some advantages in survival or mating, will tend, *on average* to pass those very traits on to their offspring in bigger numbers. So over time, those traits increase in frequency in the population, and the population changes over generations (evolution).

Answer 3

You have to have:
Variance in a trait
The ability to pass on that trait (Heritability)
Advantage or disadvantage of having that trait

[Ex]
1) some giraffes have long necks some have short
2)neck length is genertic and can be passed on
3)Giraffes with short necks can not obtain as much food ,therefore, do not pass on as many genes

Result giraffes have longer necks to feed.
(In reality probably not the case but a decent example)