Polyploidy is defined as possession of more than two sets of chromosomes of an organism. It is known to play a major role in evolution of organisms, but few studies are available on Sahelian trees. In the case of Acacia senegal (distributed across the Sahel), it is important to clarify the potential role of polyploidy in adaptation to dry growth conditions. This thesis therefore aims at increasing the understanding of polyploidization in adaptation of A. senegal by the means of population genetics and quantitative genetic tools. An assessment of the distribution of polyploids across ten natural sites with different rainfall and salinity showed no simple geographical pattern in the frequency of polyploids. However, salinity was found to be positively correlated with frequency of polyploids. Analysis of population differentiation between cytotypes compared to genetic relationship among populations within cytotypes revealed that the studied polyploid populations were more differentiated than diploid ones. The analysis of genetic relationships further suggest multiple origins of polyploid A. senegal and provide novel information for understanding the evolutionary history of the recently revealed polyploidy in A. senegal. Estimation of the frequency of polyploids were also made among four natural populations based on trees planted in a progeny trial located in dry and hot region of Senegal; Dahra: annual rainfall < 300 mm and temperature 35-45 ºC. Here also, polyploid trees were observed in all tested populations. In this study the growth rate of each tree could be estimated very precisely, because the sampled trees were part of a research trial, and the comparison between cytotypes in the progeny trial showed significantly higher growth rate of polyploids compared to diploid at this test site. A drought stress trial under controlled conditions (growth chamber) showed that diploid and tetraploids had similar growth rate when grown under well watered conditions, but tetraploids grew faster than diploids when subjected to drought stress. The findings suggest that presence of polyploids play an important role for the species ability to adapt to harsh growing conditions. The level of genetic variation for growth and gum yield for diploids and polyploids was estimated from quantitative genetic analysis based on assessments in the progeny trial during two years (2012 and 2013). Sibling relationship among and between trees from the different open pollinated progenies was tested by application of genetic markers to support the quantitative genetic analysis. The results suggested different mating systems in diploid and polyploids, and this complicated the quantitative genetic analysis and reduced the precision of the estimated quantitative genetic parameter. Still, the results from the quantitative genetic traits analysis showed strong genetic variation in growth and gum production between provenances and families for each level of ploidy. Ploidy level appeared to have a consistent effect on growth rate of the trees, but not on the economically important gum yield in the investigated years. Still, estimates of genetic parameters showed that gum yield is a heritable trait (based on analysis of diploids). The results therefore suggest that breeding for high gum yield could be effective. These results represent a good start-up for a future program of conservation, reforestation and restoration of A. senegal in the Sahel region.