Battle of the beaks: Evidence of natural selection in action
Updated: Mar 22, 2021
Competition is a strong driving force in nature. We all are, undoubtedly well acquainted with the idea of competition. We have experienced competition at every stage of our lives; with our siblings at home, in academics, in sports, and even in our careers. The outcome of such an interaction significantly impacts our personal and professional lives.
Now that we know that competition occurs between two or more groups of people sharing common interests, one may ponder as to how this affects species in the wild? Ecologically, when two or more species having a recent common ancestor depend on similar food resources, one species can be denied access to the resource due to heavy competition. Gradually, one species might specialise in and be better able to utilise the particular resource while the other may shift to a similar but different resource. This process can lead to changes in beaks and jaws of the species involved, which can aid in reducing the effect of competition. Such structural modifications in a species which can occur due to the competition are called ‘character displacement’.
But what is the proof that competition can lead to physical changes in a species? To better understand how this phenomenon works, let’s head to a small island of Daphne Major, situated in the Galapagos islands where researchers reported the first case of changes happening in the beak size of a bird due to competition. Several species of finches live on the Galapagos islands, which are collectively known as Darwin’s finches. Around the year 1977, Daphne Major was home to only one species of finch, the medium ground finch (Geospiza fortis). Around 1980, the average beak size of this finch increased, and in 2005, the beaks of these finches strikingly became smaller than what they were in 1977!
Daphne Major, which is just about 0.34 sq. km, is a part of the Galapagos group of islands in the South American continent.
Medium ground finch (Geospiza fortis) and how its beak changed size multiple times between 1975 and 2005
What was exactly going on?!
The 1977 drought
G. fortis, a seed eater, flourished on this island feeding on small seeds of plants. In 1977, however, the island was hit by a drought! The supply of small seeds became scarce and only larger, harder seeds survived the drought.
Following this, G. fortis had to shift their diet to these larger seeds. One such seed G. fortis fed on was that of Jamaican fever plant (Tribulus cistoides). Since these seeds were hard and large, only larger beaked individuals of the population of G. fortis were able to feed on them and hence reproduced better. The parents passed on their larger beak genes to their offspring, and as a consequence, in successive generations, the average beak size of the population increased! So now, beaks of G. fortis individuals were much larger in 1980 than what they were in 1977!
Medium ground finch (Geospiza fortis) with a small beak
Medium ground finch (Geospiza fortis) with a large beak
A large and hard seed of the Jamaican fever plant ((Tribulus cistoides)
1982: Storm and breeding of competitor
However, things began to change again, when in 1982, large ground finches (Geospiza magnirostris) which otherwise only visited this small island in the dry seasons, set up a breeding population for the first time. A strong storm towards the end of 1982, also brought abundant rainfall and hence, food. The primary source of food of the newly arrived finches (G. magnirostris) was the seeds of the Jamaican fever plant, the same seeds which were also fed upon by the large-beaked members of G.fortis. However, the larger, newly arrived finches were much better in breaking open these seeds than G. fortis. With strong competition, G. fortis soon found it unprofitable to attempt feeding on these larger seeds and had to shift their diet back to smaller seeds, which were by now, abundant on the island.
Large ground finch (Geospiza magnirostris) — the much larger competitor with a bigger beak
With the shift in the diet because of competition, the smaller beaked individuals of G. fortis were better able to find food and hence, reproduce. As before, this time, smaller beaked individuals had more young ones and passed on their genes of smaller beaks to the next generation. So in the forthcoming generations, the average beak size of G. fortis again started reducing till it was similar to the beak size in 1977! Till 2003, the two populations sustained happily on available seeds.
The 2003 drought!
Overconsumption of these large seeds and scanty rainfall in 2003-2004 led to severe depletion of food sources for the finches. A large number of both species of finches died due to starvation in these 2 years. In fact, only 83 individuals of G. fortis and 13 individuals of G. magnirostris were left on the island by 2005!
However, while most finches were dying because of no food, something exciting was happening too — G. fortis individuals with tiny beaks could easily pick up even smaller seeds which large finches never ate!
So, there was again a shift in diet. G. fortis with tiny beaks survived the drought and hence reproduced better. That is the reason why, in the 2005 generation of finches, the beaks of G. fortis were even smaller than those in 1977!
We would have imagined evolution to take hundreds of years to produce the necessary change in a species for its survival. You would be surprised to know that all that it took for the beaks to undergo a major change in the beak size was just 22 years, a few generations, strong competition, and harsh climatic changes!
Researchers involved in this study have spent over three decades to provide first-hand evidence for evolutionary changes as a result of competition and climatic conditions occurring in nature. Curiosity, persistence and determination were what drove the researchers in this study to witness this massive evolutionary response unfold within the lifetime of humanity. This observed phenomenal event was the greatest recorded in the thirty-three years of study and marks a significant contribution to evolutionary studies and the first ever recorded event of physical changes due to competition.
Anyone out there still questioning evolution?!
Original research paper: Grant, Peter R., and B. Rosemary Grant. "Evolution of character displacement in Darwin's finches." science 313.5784 (2006): 224-226.
Link to the original paper: https://science.sciencemag.org/content/313/5784/224.abstract
This article has been contribued by Nagarathna Balakrishnan, edited by Devica Ranade