According to Darwin, in the struggle for survival, those individuals whose traits best enable them to survive and reproduce will, on average, leave the largest number of offspring that in turn survive to reproduce. This is the basic idea behind his theory of:

This is the basic idea behind his theory of Natural selection

Explanation:

The theory of Natural selection was demonstrate in Darwin's book known as 'Origin of species' which was written in 1859

In this book, the process of change of an organism over time is described and also mentioned about their physical as well as behavioral changes.

Natural selection means the survival and reproduction of individuals by changing the phenotypes.

Natural selection is a process by which the nature accepts those organisms that can change their phenotype in order to survive with changing environment and eliminate those phenotypes that are not favorable for their survival.

genetic variation can be caused by mutation (which can create entirely new alleles in a population), random mating, random fertilization, and recombination between homologous chromosomes during meiosis (which reshuffles alleles within an organism's offspring).

answer: grassland biomes are closer to the equator and receive more rain.

grasslands are biomes that are dominated by grasses rather shrubs and trees. savanna are biomes that exhibit grasses, shrubs and small tree as vegetation. grasslands are located near the equator of the earth. these are found in north america. savanna biomes are found in africa, south america, australia and india. grassland exhibit moderate climatic conditions, it receives   amount of rainfall higher than savanna.   therefore, savanna exhibit dry and hot climatic conditions.