All processes that lead to either changes in the allele frequencies in the gene pool of a population

Population

A population is all the organisms that both belong to the same group or species and live in the same geographical area. The area that is used to define a sexual population is such that inter-breeding is possible between any pair within the area and more probable than cross-breeding with individuals...

 or a new combination of allele

Allele

An allele is one of two or more forms of a gene or a genetic locus . "Allel" is an abbreviation of allelomorph. Sometimes, different alleles can result in different observable phenotypic traits, such as different pigmentation...

s on the chromosome

Chromosome

A chromosome is an organized structure of DNA and protein found in cells. It is a single piece of coiled DNA containing many genes, regulatory elements and other nucleotide sequences. Chromosomes also contain DNA-bound proteins, which serve to package the DNA and control its functions.Chromosomes...

. According to modern evolutionary synthesis

Modern evolutionary synthesis

The modern evolutionary synthesis is a union of ideas from several biological specialties which provides a widely accepted account of evolution...

 all those processes are the root cause of evolutionary change.

• recombination,
• mutation
  Mutation
  In molecular biology and genetics, mutations are changes in a genomic sequence: the DNA sequence of a cell's genome or the DNA or RNA sequence of a virus. They can be defined as sudden and spontaneous changes in the cell. Mutations are caused by radiation, viruses, transposons and mutagenic...
  
  ,
• natural selection
  Natural selection
  Natural selection is the nonrandom process by which biologic traits become either more or less common in a population as a function of differential reproduction of their bearers. It is a key mechanism of evolution....
  
   and
• gene drift.

• migration
  Population genetics
  Population genetics is the study of allele frequency distribution and change under the influence of the four main evolutionary processes: natural selection, genetic drift, mutation and gene flow. It also takes into account the factors of recombination, population subdivision and population...
  
  ,
• gene flow
  Gene flow
  In population genetics, gene flow is the transfer of alleles of genes from one population to another.Migration into or out of a population may be responsible for a marked change in allele frequencies...
  
  ,
• isolation
  Reproductive isolation
  The mechanisms of reproductive isolation or hybridization barriers are a collection of mechanisms, behaviors and physiological processes that prevent the members of two different species that cross or mate from producing offspring, or which ensure that any offspring that may be produced is not...
  
  ,
• horizontal
  Horizontal gene transfer
  Horizontal gene transfer , also lateral gene transfer , is any process in which an organism incorporates genetic material from another organism without being the offspring of that organism...
  
   and
• vertical gene transfer, as well as
• hybridisation
  Hybridisation
  Hybridisation or hybridization may refer to:*The process of combining different varieties or species of organisms to create a hybrid *Nucleic acid hybridisation, the process of joining two complementary strands of DNA...