Neo-Darwinism explains variation through Mendel's laws of inheritance, sexual reproduction allowing for gene recombination during meiosis, and mutation causing rapid gene changes. It views the population, not the individual, as the unit of natural selection. The synthetic theory of evolution combines Darwin's ideas with modern genetics and the understanding of gene frequencies. The selfish gene theory proposes that genes are the true units of evolution and compete to increase their own frequencies within a population. The endosymbiotic theory suggests that eukaryotic cells originated from the fusion of bacteria, with some bacteria becoming cellular organelles through symbiotic relationships.