1. Darwin's theory of evolution by natural selection:

- Variation: Variation exists among individuals in a population. This may be due to mutations, genetic recombination, or other mechanisms that give rise to new heritable traits.

- Inheritance: These variations in traits can be passed down from parents to offspring through genetic inheritance.

- Selection: The environment and natural conditions play a role in determining which individuals survive and reproduce. The differential survival and reproduction of individuals with advantageous traits, known as natural selection, leads to changes in the population's genetic makeup.

2. Mendelian genetics:

- Genes: Traits are determined by genes. Genetics, as understood through Mendelian principles, explains the patterns and mechanisms of inheritance.

Integration:

- The modern synthesis combined Darwin's theory of natural selection with Mendelian genetics and population genetics. It explains the genetic basis of evolution, demonstrating how genetic variations influence traits, how these variations are inherited, and how they affect an individual's chances of survival and reproduction.

- The integration of genetic concepts and evolutionary biology allowed for a more comprehensive understanding of how species diversify, adapt, and evolve over time.