Aims Elucidation of the molecular mechanism of local adaptation to changing environments represents a central goal in evolutionary biology and ecology. Formation of adaptive populations probably involves activation or repression of pre-existing tolerant genes by transcription factors (TFs) and retrotransposons under stress. Research in this area can lead toward to a unified theory of evolution.

Methods The consensus sequence of drought responsive element binding protein (DREB) from the wild soybean (Glycine soja) in Yellow River delta has been cloned, sequenced, and called GsDREB1. An integrase gene of Gypsy-like retrotransposon called GsINT was also cloned from G. soja. Southern hybridization indicated that GsINT consisted of multiple copies and showed restriction fragment length polymorphism among individuals in salinity populations. From both sequences of GsINT and GsDREB1, other pairs of primers were designed for amplification of 5'upstream of GsDREB1 in order to see if partial sequence of GsINT was inserted. Results indicated multiple copies of space sequence between INT and DREB and its diversity among individuals of plants within each population. Gene cloning and sequencing of amplified products showed possible existence of GsINT sequence in 5' upstream of GsDREB1.

Important findings Results led to a hypothesis on the origin of tolerant populations, i.e., a molecular mechanism on adaptive evolution of plants. In normal conditions, plant populations mainly contain non-tolerant individuals. Under stress conditions, retrotransposons increase their frequency of transposition. Most mutants are neutral without phenotypical change. Few insertions located into 5' upstream of transcriptional factors will change intensity of TFs' expression. Then a series of pre-existed stress responsive genes controlled by TFs will simultaneously change their rates of gene expression, resulting in formation of more tolerant or sensitive individuals. Individuals with high stress tolerance may appear by accumulation of several tolerant inserts step by step. As a result of complicated interaction among genes, plants with different levels of stress tolerance occur quickly under stress. By means of natural selection, stress-sensitive plants are lost under stress conditions; however, stress itself continuously creates new sensitive individuals. Few of them with a short life cycle may survive by using a short period time of weakened stress. This hypothesis explains that tolerant populations quickly form because transposition frequency under stress becomes very high. It also explains why tolerant populations contain higher genetic diversity and why a few stress-sensitive individuals exist under stress conditions.