1835 was no ordinary year for Charles Darwin. Aboard the famous ship Beagle, Darwin embarked on a journey as a young naturalist, surveying the lands and its plethora of organisms. While heading to the Galapagos Islands, Darwin passed by a small island, marked distinctly for its volcanic cindric features. Although not as well known as the Galapagos, this island the Daphne Major is similarly home to finches of all sizes and shapes.
Since human arrival, the Daphne Major has shown resilience to invasive species and human influence. No invasive species, in other words, have intruded the lands, and no native species have turned extinct.
Because of this particular nature, a British couple the Grants traveled to the island in the early 1900s to observe the natural selection of finches in action. They also observed that there was little emigration and immigration of the birds, making Daphne Major a more ideal place to measure natural selection.
What did the Grants survey?
The Grants surveyed various phenotypic variance, including beak size and length and seed size. One key aspect of their historic study was that the bigger the finch parent’s beak was, the bigger the finch offspring’s beak was too.
–> There is a positive correlation between depth of parent’s beak and depth of offspring’s beak.
Another key observation that confirmed Darwin’s revolutionary study was that within the finches species, there was a wide range of seed size that individuals consumed. The finches with larger beaks tended to eat the bigger seeds, while the finches with smaller beaks tended to eat the smaller seeds.
The differences in consumed seed size depended on their beak size.
Directional Selection from 1977 to 1978
In 1977, the Daphne Major suffered from severe drought, causing many plants to die. Many of these dying plants produced small seeds, causing finches with smaller beaks to face trouble with eating the larger remaining seeds from thriving plants such as the caltrop. The Grants discovered those with a higher beak size survived more from the drought, thus leading to a natural selection for slightly larger beak sizes in the population of medium ground finches.
–> Remember: Beak size is heritable, and heritability is a key criteria for natural selection to occur.
Three Major Lessons
1) Natural selection may be variable.
Basically, it depends. If the seasons were wet and produced abnormally thriving conditions for plants with smaller seeds, then perhaps, natural selection would favor small beaks. Directional selection would shift to the left, on the smaller end.
2) Evolution can occur quickly or slowly.
As observed by the Grants, the directional selection that was observed with the finches in 1977 occurred at a rapid pace. The population evolved within a few generations. This observed speed suggests that evolution may not always be slow – like the gradual changes in fossil records.
3) Natural selection fluctuates over time.
Big beaks, small beaks, long beaks, short beaks. Natural selection may favor big beaks one month and small beaks the next month.
- Evolution: Making Sense of Life by Zimmer and Emlen