One of the most striking evolutionary patterns observed is called adaptive radiation. To radiate means to spread outward; not in the sense of speading out physically, but referring to a species that diversifies (“spreads out”) and generates multiple daughter species.
From Biology Online: When Charles Darwin was in the Galapagos islands, one of the first things he noticed is the variety of finches that existed on each of the islands. All in all, there were many different species of finch that differed in beak shape and overall size. This is adaptive radiation and natural selection at work.
These finches, better known as ‘Darwin’s Finches’ illustrated adaptive radiation. This is where species all deriving from a common ancestor have over time successfully adapted to their environment via natural selection.
Previously, the finches occupied the South American mainland, but somehow managed to occupy the Galapagos islands, over 600 miles away. They occupied an ecological niche with little competition.
As the population began to flourish in these advantageous conditions, intraspecific competition became a factor, and resources on the islands were squeezed and could not sustain the population of the finches for long.
Due to the mechanisms of natural selection, and changes in the gene pool, the finches became more adapted to the environment, illustrated by the diagram below.
As competition grew, the finches managed to find new ecological niches, that would present less competition and allow them, and their genome to be continued.
As indicated by the diagram above, the finches adapted to take advantage of the various food sources available on the island, which were being used by other species. Over the long term, the original finch species may have disappeared, but by diversifying, would stand a better chance of survival.
All in all, the finches had adapted to their environment via natural selection, which in turn, has allowed the species to survive in the longer term, the prime directive of any species.
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Follow this link to read more about rapid adaptive radiation in the species rich Heliconius butterflies (longwings or passion-flower butterflies).