If, as a result of geographical or climatic accidents, a small group of individuals becomes completely isolated from its native population, its genetic diversity will ipso facto be reduced. By simple random sampling, its average composition may be very different from that of the mother population. This will of course could be a handicap for the adaptation of this small isolated group and it will most likely disappear. But in the event of a favourable environment, this small group could be able to prosper and even, in the long term, to increase to a significant number. Due to its particular genetic composition, combined with new variations and natural selection, differences with the original population will increase and may become such that any crossing will have become impossible, due to differences in behaviour (e. g. marital parade) or other factors. A “reproductive barrier” will be put in place, creating a new species, a phenomenon that further increases the share of chance in Neodarwinian theory. This process must have occurred ‘in cascade’ with the “Darwin finches” on the Galapagos archipelago. 2 to 3 million years ago, a few individuals accidentally arrived from the American continent (in a single wave it seems). They found a favourable environment there, including abundant food and few predators, so they were able to survive, multiply and diversify and gradually colonize all the islands of the archipelago. These successive geographical isolations, leading to serial reproductive barriers, have led to the creation of 13 different species, spread over some 15 islands, which is a good example of what is known as “adaptive radiation” (see Figure).

  如果由于地理或气候事件，一小部分个体与其本地种群完全隔离，其基因多样性事实上将会减少。通过简单随机抽样，这一小部分个体所组成的种群平均构成可能与母种群有很大的不同。当然，这一特点可能会成为这个孤立小群体适应环境的障碍，并且很可能会使小种群消失。但在有利的环境下，这个小群体可能会繁荣起来，甚至从长远来看，会增加到相当大的数量。由于其特殊的基因组成，再加上新的变异和自然选择，新种群与原始种群的差异将增加，并可能由于行为差异（例如婚姻游行）或其他因素而变得不可能进行交配。这样一来就会形成“生殖屏障”，创造一个新物种，这一现象进一步增加了新达尔文理论中机会的份额。这个过程一定是与加拉帕戈斯群岛上的“达尔文雀”“级联”发生的。200到300万年前，一部分达尔文雀意外地从美洲大陆抵达（似乎是同一波）。他们在那里找到了有利的环境，包括丰富的食物和很少的捕食者，因此他们能够在此地生存、繁衍和多样化，并逐渐在群岛的所有岛屿上殖民。这些连续的地理隔离导致了一系列的生殖隔离，导致产生了13个不同的物种，分布在大约15个岛屿上，这是所谓的“适应性辐射”的一个很好的例子（见图）。