Epigenetics is the study of changes in gene expression that occur via mechanisms such as DNA methylation, histone acetylation, and microRNA modification. When these epigenetic changes are heritable, they can influence evolution. Current research indicates that epigenetics has influenced evolution in a number of organisms, including plants and animals. In plants Overview DNA methylation is a process by which methyl groups are added to the DNA molecule. Methylation can change the activity of a DNA segment without changing the sequence. Histones are proteins found in cell nuclei that package and order the DNA into structural units called nucleosomes. DNA methylation and histone modification are two mechanisms used to regulate gene expression in plants. DNA methylation can be stable during cell division, allowing for methylation states to be passed to other orthologous genes in a genome. DNA methylation can be reversed via enzymes known as DNA de-methylases, while histone modifications can be reversed by removing histone acetyl groups with deacetylases. Interspecific differences due to environmental factors are shown to be associated with the difference between annual and perennial life cycles. There can be varying adaptive responses based on this. Arabidopsis thaliana Forms of histone methylation cause repression of certain genes that are stably inherited through mitosis but that can also be erased during meiosis or with the progression of time. The induction of flowering by exposure to low winter temperatures in Arabidopsis thaliana shows this effect. Histone methylation participates in repression of expression of an inhibitor of flowering during cold. In annual, semelparous species such as Arabidopsis thaliana, this histone methylation is stably inherited through mitosis after return from cold to warm temperatures giving the plant the opportunity to flower continuously during spring and summer until it senesces. However, in perennial, iteroparous relatives the histone modification rapidly disappears when temperatures rise, allowing expression of the floral inhibitor to increase and limiting flowering to a short interval. Epigenetic histone modifications control a key adaptive trait in Arabidopsis thaliana, and their pattern changes rapidly during evolution associated with reproductive strategy. Another study tested several epigenetic recombinant inbred lines (epiRILs) of Arabidopsis thaliana - lines with similar genomes but varying levels of DNA methylation - for their drought sensitivity and their sensitivity to nutritional stress. It was found that there was a significant amount of heritable variation in the lines in regards to traits important for survival from drought and nutrient stress. This study