Effects of crop evolution under domestication and narrowing genetic bases of crop species

Crop improvement is very crucial to satisfy the world demand in the presence of different challenges like climate change, reducing arable land and increasing population growth. Crop improvement program is continuously striving to increase crop yield, enhance crop quality and  improve crop  tolerance to biotic and abiotic stresses. Domestication has a great role in increasing agricultural productivity through selecting suitable crop plants to human beings like high yielding varieties with resistance to biotic and abiotic stresses, improved nutritional quality, big seed and fruit size, non-shattering, reduction of seed dispersal mechanisms, a more compact growth habit, early matured crop plants. Domesticated food crops are derived from a phylogenetically diverse assemblage of wild ancestors through artifi cial selection for different traits. Plant breeding and domestication of crop plants have a profound impact on the genetic diversity through selecting the desirable crops by neglecting other crop plants with undesirable traits. Domestication of crop plants shifted from hunter-gatherer to agricultural societies which leaded the rise of modern civilization. The amount or quantum of genetic diversity available in a breeding population is referred to as genetic base of that population. In other words, genetic base represents spectrum of genetic variability in a plant breeding population. Depending upon the amount of genetic diversity present in a plant breeding population, genetic base is of two types, viz. broad genetic base and narrow genetic base. Plant populations that are composed of several pure lines, inbred lines or heterozygotes and homozygotes are said to have broad genetic base. Examples of such populations are mass selected varieties, multiline varieties, synthetics, composites and land races. Such genotypes have wider adaptation to environmental changes. Those plant populations that have been developed from single homozygote or heterozygote are-said to have narrow genetic base. Examples of such populations are pure line variety, varieties developed by backcross, pedigree method bulk method, and single seed decent method, clonal variety and hybrids between two inbred lines or pure lines. Genetic loss is directly connected with the reduction of genetic diversity including the loss of individual genes and as well as the loss of particular combinations of genes such as those manifested in locally adapted landraces. Eventually, genetic erosion is the depletion in population variation because of inbreeding and genetic drift which is largely causes the endangerment of small isolated populations. Narrowing of genetic diversity might result the complete loss of crop plants. Review Article Effects of crop evolution under domestication and narrowing genetic bases of crop species


Introduction
Agriculture was started before 10,000 years ago during the fi rst crops were domesticated in the Fertile Crescent [1]. Plant breeding is relying on the genetic variation and selection of desirable crop plant through identifying phenotypes. Some of the fi rst domesticated plants are thought to be the cereals wheat, barley, millet and emmer [2]. During domestication, Domestication is the transition of the living standard of human being from hunting, gathering to modern agriculture [5].
Domestication is a routine activity beginning with the wild species, then formation of the cultivated species, then selection of improved genotypes of the species by growers (called landraces), then to modern cultivars from scientifi c breeding.
Biodiversity is broad tem that is defi ned as the quantity of species and the variety of environments in which species or genes are present [6]. Plant genetic diversity is playing a key role in the continuation of agricultural development with signifi cant improvement in different morphological and agronomical characteristics [7]. Crop species with narrow genetic diversity are susceptible to emerging pathogens or other constraints leading to loss of productivity and this may lead to a serious decline in the areas of adaptation [8]. Crop plant genetic improvement is the changing of plants to satisfy human needs. The process of domestication involved in the continues identifi cation of certain desirable wild species combined with a process of selection of high yielding varieties with resistance to biotic and abiotic stresses, improved nutritional quality, big seed and fruit size, non-shattering, reduction of seed dispersal mechanisms, a more compact growth habit, early matured crop plants.
Conventional plant breeding refers to the selection of superior genotypes from genetically diverse populations derived from sexual recombination. Plant breeding is primarily depends on presence of substantial genetic variation to address the maximum genetic yield potential of the crops and exploitation of this variation through effective selection for improvement [9]. There is substantial genetic improvement A landrace represents the equilibrium between heterogeneous and heterozygous genotypes within a population of a crop that is maintained by continuous multiplication under a given set of climatic, soil and husbandry conditions. Landraces have substantial contribution for crop improvement in providing genetic materials. Despite its economic importance, land races were replaced by single dominant modern variety that promotes genetic erosion [10]. To meet the challenges in crop improvement, efforts were made to widen the genetic base by collecting and conserving germplasm across the world before it is lost forever, which led to the assembly of large collections at the national and international gene banks [11].
Agriculture is faced for many challenges such as human population growth, climate change, malnutrition, poverty, hunger and other stressors. Overcoming these diffi cult challenges will be harder in the absence plant genetic improvement to increase agricultural productivity through addressing the problem of yield reduction and its links with pest management and climate change [12]. However, agriculture  [3]. There is genetic alteration of morphological and agronomical characters in domestication processes for adaptation of crop plants [16]. McCouch [17] indicated domestication is about adapting the high yielding varieties with resistance to biotic and abiotic stresses, improved nutritional quality, big seed and fruit size, non-shattering,   Table 1. as an alleviation in evenness [27]. Genetic loss as depletion in evenness originates from the variability indicators used in population genetics, such as Shannon's index [28] and Nei's gene diversity index [29]. Genetic diversity is measured using