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When Charles Darwin visited the Galapagos Islands in the 1830s, thirteen species of finches inhabited the place. Finches show a variety of shapes and sizes of beaks, all of which are suitable for their different types of food and lifestyles. Darwin gave this phenomenon the following explanation: they are all descendants of the original pair of finches, and that natural selection is responsible for their differences. The Grants studied the infamous finches, the same ones that Darwin studied two centuries ago. Darwin believed that it would take thousands of years to witness evolution, but Grants have proven that it can take only several years. Grants, in their investigation, generally focused on the medium ground finch, Geospiza fortis, which has a stubby beak and mostly eats seeds.
What affected the food supply, and consequently triggered the natural selection of the finches, was the event that the Grants witnessed in 1977. The directional form of natural selection begins to act in a changing environment, in the Grants study, drought. During that event, there was no rain for almost 18 months, and the plants withered, producing practically no seeds. Due to the lack of food supply, medium ground finches with larger beaks could eat alternative foods, because they could crack open bigger seeds.
However, the medium ground finches with smaller beaks had difficulties with finding alternative food sources; therefore, they died of starvation. When the Grants returned to the island to document the changes in the finch population, they realized that the beak size had become larger in comparison to the pre-drought generation. Thus, a directional form of natural selection occurred.
Evolution by genetic drift took place when immigrant members of large ground finches started breeding on the island. The new population experienced a genetic bottleneck (microsatellite allelic diversity fell), and inbreeding depression occurred (Grant R. & Grant P., 2003, p.968). The results were shown by how poorly the 1991 cohort survived. In this example, changes in the beak structure of the species were caused by genetic drift.
Daphne Major was a perfect example for studying the pre-zygotic and post-zygotic reproductive isolation in middle ground finches and the cactus finches because of their moderate degree of isolation. The Grants have found it out by capturing and measuring the species to determine phenotypic variation, comparing offspring with their parents to determine inheritance, and following their fates across years to detect selection (Grant R. & Grant P., 2003, p.966). There was a definite transformation, since there was a significant heritable change in beak size and body size among the species.
Hybridization is the process of the formation or production of hybrids, which is based on the union of the genetic material of different cells in one cell. The causes of hybridization include extra-pair mating, the interspecific takeover of nests with eggs, and the dominant singing of a close neighbor (Grant R. & Grant P., 2003, p.970). In the 1990s, the flow of genes from the medium ground finch to the cactus finch population contributed to a decrease in mean body size and a blunter beak morphology of cactus finches (Grant R. & Grant P., 2003, p.970). In this case, the barrier in song differences between the two species was broken, and the environmental change happened to have been the most crucial factor in this process.
As it was previously stated, song divergence plays a significant role in keeping the species distant from each other. The song is a fascinating attribute as it is culturally and not genetically inherited. This conclusion was supported by field observations of the songs of offspring, parents, and even grandparents by the Grants. The song is acquired through learning early in life, in a process that resembles imprinting; it is generally received from fathers during the period of parental dependence, in association with parental morphology (Grant R. & Grant P., 2003, p.970). Therefore, cultural inheritance occurred within the species studied by Grants, independent from their genealogy.
References
Grant, B. R., & Grant, P. R. (2003). What Darwins finches can teach us about the evolutionary origin and regulation of biodiversity. BioScience, 53(10), p. 965. Web.
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