Darwins Finches

Darwin in the voyage of the H.M.S Beagle

From 1831 to 1836, Charles Darwin embarked on a voyage on the H.M.S Beagle, a ship that departed from Plymouth, in England and travelled around the world for five years. The mission of the H.M.S Beagle was to survey the South American coastline. Although he was hired as a geologist, Darwin spent a lot of time observing and collecting animals, plants, and fossils.

Fig. 1. Blue-footed booby from the Galapagos Islands. Source: Unsplash

After the five years of the voyage, the Beagle stopped over at the Galapagos Islands, a group of volcanic islands 900 km west of South America. The Galapagos Islands are home to diverse, endemic animal life, including blue-footed boobies, giant land tortoises, and the Galapagos finches. The differences between the finches led him to question the idea that species were static, as was the belief in his time. In other words, they planted the seed that would lead to the theory of evolution.

During his travels and research, he kept a diary of his observations. Later he would revise his notes and write The Origin of Species where he details his famous theory of evolution by natural selection.

Darwin’s finches and their beaks

While in the Galapagos Islands, Darwin noticed that organisms were similar from island to island but had certain features that distinguished them from each other. Finches, in particular, caught Darwin’s attention. He found that over a dozen species of finches inhabited the islands. In particular, Darwin noticed that the beak shape of the finches varied among the different species on the islands.

Darwin attributed the unique beak shapes to the food sources available in their specific environment. Looking at the diagram below, you can see four species that are part of the group of Darwin's finches.

Each of these species' beak has a different shape and size:

• Geospiza magnirostris has a large curved beak that helps in breaking and holding large, hard seeds.
• Geospiza fortis has a curved beak too, but it is smaller in size because this species feeds from smaller and softer seeds.
• Geospiza parvula and Certhidea olivacea have shorter and smaller beaks that form a straighter line than the beaks from the species above. They are adapted to catch and eat insects.

As you can see, the distinct beak shape and size give each species a higher chance of survival in their niche. The connection between the food source and the physical characteristics of the birds (their phenotype) was the first clue for Darwin that species can indeed adapt, and that the individuals with the best adaptations will be most likely to survive and pass these traits onto their offspring.

Traits and behaviours that help organisms survive and reproduce are called adaptations. Darwin noted that the finches’ beak shapes were adaptations that helped them obtain food in their habitat. By explaining how these unique finch species came to be, Darwin was able to formulate his theory of evolution by natural selection.

Darwin's finches: a short activity

Here's a short activity you can do to understand how different beak shapes (like the ones in the different species of Darwin's finches) are adapted to collect specific food types.

You will need the following materials:

• Food sources, like sunflower seeds, grapes, hard-boiled eggs, and yoghurt

• Tools: a toothpick, a pair of tweezers, a spatula, and a fork

• Paper plates

Methodology:

1. Fill a plate with sunflower seeds. This plate will be called Plate A.

2. Leave Plate B empty.

3. For 30 seconds, try to transfer as much content from Plate A to Plate B using each tool.

4. Weigh Plate B and write in the table below how much you were able to transfer.

5. Repeat the process for each of the food sources and use every tool for each one.

Table 1. Weight of Plate B (g).

Answer the following questions:

1. What tool was the best for collecting sunflower seeds? And what about the grapes, hard-boiled eggs, and yoghurt? Why?

2. What tool was the worst for collecting each of the food sources? Why?

3. How would you relate this activity to the finches' beak shapes as adaptations?

   Can you imagine having to pick tiny seeds, like chia seeds, with a big beak like that of a tucan?

4. What do you think would happen to finches with beak shapes that are unsuitable for the available food source? How about finches with the most suitable beak shapes?

   Are you able to compete at your full potential when you haven't had a proper breakfast?

Darwin's finches and the theory of evolution

Darwin theorized that all of the different finch species on the Galapagos Islands came from one parent species (a common ancestor) that first colonized the islands millions of years ago. Darwin explained that, as populations of the parent species spread from one uninhabited island to the next, they adapted to different ecological niches and rapidly evolved into many descendant species. In other words, Darwin thought that the finches had adapted to their environment, increasing their chances of survival and reproduction.

How does natural selection cause evolution?

In any habitat, resources are limited, so organisms have to compete to survive. Organisms with better-suited traits have a higher chance of survival.

Because they have a higher chance of survival, organisms with favourable traits can also reproduce and pass on these traits. Darwin called this the process of natural selection, which is more popularly known as "the survival of the fittest.” Natural selection is the process where organisms with better traits tend to survive and reproduce at higher rates than other individuals because of those traits.

This means that organisms with favourable traits will outnumber those with less favourable traits in the succeeding generation. Over time, species with better adaptations to their habitat are formed. This is how natural selection leads to evolution. If enough differences accumulate between an original species and the members of that species after many generations exposed to natural selection, a new species can form.

The evolution of Darwin's finches

The number of different species of finches in the Galapagos Islands presents an interesting case because speciation tends to produce only one new species from a parent species over a long period. However, the Galapagos finches underwent a burst of evolution, producing a large number of different descendant species in a relatively short time. This is a type of speciation called adaptive radiation.

To illustrate how adaptive radiation might have taken place:

• Let’s say Finch Population A occupies one area.

• Over time, Finch Population A adapts to its new environment and becomes different from its parent species.

• Finch Population B from the same parent species enters the same area and competes with Finch Population A.

• There is pressure on both Populations A and B to adapt to different niches (different roles, like feeding from different nutrient sources or living in different types of trees).

• They pass on traits suited to each niche.

• When this process happens several times in one area, several new species may be formed from a single parent species in a short period.

This explains how over a dozen different finch species evolved from one parent species in a relatively short time in the Galapagos Islands (Fig. 2).

Fig. 3. A diagram showing how a parent species of finch rapidly formed several new species of finch with different beak shapes and feeding habits

What is the significance of Darwin's finches?

Darwin's discovery that different species of Galapagos finch had unique beak shapes adapted to the food sources available in their specific habitats led to the idea that life forms are not perfect and unchanging. Instead, they were changing over time in response to their environment, i.e. they were evolving. Such changes manifest in observable traits, such as the beak shapes of Galapagos finches.

Based on his findings during the Beagle Voyage, Darwin argued that traits change in a population because individuals that inherit traits that are better adapted to the environment have better chances of survival and reproduction; thus, these traits have better representation in succeeding generations. These insights became the foundation for Darwin's theory of evolution by natural selection.

Another key insight that Darwin had drawn from his observations is the process of adaptive radiation. Morphological similarities among the species of Galapagos finches led him to believe that they all descended from a common ancestor but rapidly diversified and speciated because each population adapted to a different ecological niche.

In summary, the discovery of the different beak shapes in Darwin's finches led Darwin to conceptualise the following key concepts in biology and environmental science today:

• The theory of evolution: species are not static, they change over time, they evolve. This in turn led to the refutation of creationism: Darwin contradicted the existing belief that God had created each species in its perfect and unchanging form. This was quite the statement in those days.
• Natural selection: Darwin proposed that species changed and new species formed because the individuals with traits that were beneficial for survival and reproduction had better chances of having offspring. Thus, beneficial traits were more likely to be passed on to the next generation, and the frequency of those traits would increase with succeeding generations, changing the species overall. The process by which beneficial traits are favoured in nature is called natural selection.
• Under certain circumstances, evolution and speciation can happen more rapidly than expected. In the Galapagos Islands, the competition between finches was so intense, that they rapidly adapted to different niches in the same islands. From a single common ancestor, more than a dozen species appeared, each with their particular traits (especially particular beak shape and size) that had been selected for by natural selection. This process of intense evolution is called adaptive radiation.

As you can see, Darwin's finches allowed the famous biologist to develop his groundbreaking theory of evolution, so their significance is incalculable.