Darwin's Beagle Voyage

The Galapagos Finches and the notion of adaptive radiation The Galápagos finches are a famous example of the process of radiation of new species from a common ancestor. Darwin collected specimens of these birds during his 1835 visit to the remote Galápagos Islands, 900 kilometers (km) off the Pacific coast of South America.

Figure 6. Descent with modification: adaptive radiation of finches on the Galápagos Islands. The bills of these species are adapted to their diverse feeding habits

Darwin found that nearby islands in the Galápagos

had similar but non-identical species of finches

living on them. Moreover, he noted that each finch

species was well-suited for its environment and

role. For instance, species that ate large seeds

tended to have large, tough beaks, while those that

ate insects had thin, sharp beaks. Finally, he

observed that the finches (and other animals) found

on the Galápagos Islands were similar to species on

the nearby mainland of Ecuador, but different from

those found elsewhere in the world.

The Galápagos finches are thought to have

descended from an ancestral finch species that

reached the archipelago from South America or the

Caribbean. Over time, the Galapagos finches

diversified from their ancestor as populations

became adapted to different food sources on their

particular islands [Fig. 6].

For further details, refer the video on “The Origin of

Species: The Beak of the Finch” in the reference

section.

In 1837, a year after he returned from the voyage, Darwin sketched

a branching diagram representing the idea of descent from common

ancestors [Fig. 7]. By the time he published On the Origin of

Species, he could write, “I doubt not that the theory of descent with

modification embraces all the members of the same class.”

Figure 7. Darwin’s first speculative diagram of a phylogenetic tree, in an 1837 notebook. The numeral 1 represented the ancestor of groups A–D.

Extending the logic, he went on: “Analogy would lead me one step further, namely to the belief that all animals and plants have descended from some one prototype.” And finally, in one of the most daring thoughts anyone has ever had: “I should infer from analogy that probably all the organic beings which have ever lived on this earth have descended from some one primordial form.”

Extending his early sketch into a great metaphor, Darwin proposed

that all species, extant and extinct, form a great “Tree of Life,” or

phylogenetic tree. Closely adjacent twigs represent living sprived only recently from their common ancestors (shared

Darwinian Theory of Evolution

Charles Darwin based on his observations during Beagle voyage proposed the theory of evolution which states that all species have evolved over time from one or few common ancestors through the process of natural selection.

It consisted of two hypotheses: -

i) All organisms have descended with modification from common ancestral forms of life.

ii)A major agent of modification is natural selection.

Darwin’s theory of evolution includes five distinct components:

1. Perpetual change

It states that the living world is neither constant nor perpetually cycling, but rather steadily changing and that organisms are transformed with time.

2. Common descent

All forms of life descended from a common ancestor through a

branching of lineages. Life's history has the structure of a share relatively recent common ancestry have more similar features at all levels than do species that have only an ancient

common ancestry.

Figure 8. Darwin’s theory of descent with modification, represented by a phylogenetic tree

3. Multiplication of species

It explains the origin of enormous organic diversity. It states that the evolutionary process produces new species by the splitting and transformation of older ones. Species are now generally viewed as reproductively distinct populations of organisms that usually but not always differ from each other in organismal form. Once species are fully formed, interbreeding among members of different species does not occur.

4. Gradualism

It states that the large differences in anatomical traits that characterize different species originate through the accumulation of many small incremental changes over very long periods of time. This theory has been challenged, however, as an explanation of the evolution of structures like vertebrate scales, feathers and hair from a common ancestral structure.

All forms of life descended from a common ancestor through a branching of lineages. Life's history has the structure of a branching evolutionary tree, known as a phylogeny. Species that share relatively recent common ancestry have more similar features at all levels than do species that have only an ancient common ancestry.

Figure 9. Gradualism provides a plausible explanation for the origin of different bill shapes in the Hawaiian honeycreepers shown here.

Author: Rajarshi Mondal