Industries (markets) evolve and change over time. Many industries start out competitive (many businesses) but over time some begin to emerge as dominate ones in the industry (Economic Darwinism).
Their business practices (best practices, exclusive contracts, economies of scale, clever entrepreneurial skill, innovation, etc.) make them stand out over time and they are able to eliminate competitors.
Government protection also helps some to dominance and to remain dominant and protected (patents, favorable trade barriers, etc.).
However, Over time these dominant businesses may become less innovative and abusive and new innovative businesses challenger them (either in same industry or new industry). Through innovation and entrepreneurial skill they break down the barriers to entry and challenge the "big businesses (new products, new processes, business practices, etc.).
Pick an industry and trace the evolution of that industry from beginning to present (or a time frame long enough to address the changes) utilizing the above discussion as your guide to your discussion.