Evolution
What has brought about the existence of so many different life-forms on Earth?
Evolution is the process by which new species are formed from pre-existing ones over very long periods of time. A species is a group of individuals that share very similar characteristics that can interbreed to prduce fertile offspring (a new species can be formed through allopatric or sympatric speciation: see VARIATION + EVOLUTION booklet). In biology the term EVOLUTION is used specifically for the processes that have transformed life on Earth from its early beginnings to the vast diversity of fossilised and living forms that are known today. The theory of evolution was first put forward by Charles Darwin. During his visit to the Galapagos Islands Darwin accumulated geological and fossil evidence that supported the idea that life changes with time. In 1859 he proposed natural selection as the force (mechanism) that causes changes in populations driving evolutionatry change.
What has brought about the existence of so many different life-forms on Earth?
Evolution is the process by which new species are formed from pre-existing ones over very long periods of time. A species is a group of individuals that share very similar characteristics that can interbreed to prduce fertile offspring (a new species can be formed through allopatric or sympatric speciation: see VARIATION + EVOLUTION booklet). In biology the term EVOLUTION is used specifically for the processes that have transformed life on Earth from its early beginnings to the vast diversity of fossilised and living forms that are known today. The theory of evolution was first put forward by Charles Darwin. During his visit to the Galapagos Islands Darwin accumulated geological and fossil evidence that supported the idea that life changes with time. In 1859 he proposed natural selection as the force (mechanism) that causes changes in populations driving evolutionatry change.
Adaptive Radiation in The Finches of The Galapagos Islands
In 1832, Darwin, then aged 22, travelled to South America to carry out a scientific survey. He studied the flora (the plants of a particular area) and fauna (animals of a particular area) of mainland South America and of some surrounding islands, including the Galapagos Islands. These islands were formed in recent geological time, the result of volcanic activity. His task was to observe, describe and classify the plants and animals that he found. He also collected fossils in the rocks and these showed him that different life forms had gone through many changes in the past.
Among the many animals that Darwin studied on the Galapagos Islands were the finches of which he observed fourteen different species. In geological terms, the islands were recently formed and any animals found there must have reached the islands from the mainland, some 600 miles away. Finches are unable to fly long distances and Darwin suggested that one ancestral species of finch had reached the islands with the help of the prevailing winds. As there were no other bird species inhabiting the islands there was a variety of food available to the colonising finches. He noticed how individual finches differed from one island to the next. The main differences were in the size and shape of their beaks and these were related to the different type of food eaten, e.g. insects, seeds, fruit.
It seemed that on each island the characteristics that best suited a particular finch to its environment were inherited by the offspring. Darwin suggested that the finches had developed from a common ancestor and that the type of beak had developed over time as it provided a selective advantage to these individuals and become specialised to feed on a particular food source. This is an example of adaptive radiation.
In 1832, Darwin, then aged 22, travelled to South America to carry out a scientific survey. He studied the flora (the plants of a particular area) and fauna (animals of a particular area) of mainland South America and of some surrounding islands, including the Galapagos Islands. These islands were formed in recent geological time, the result of volcanic activity. His task was to observe, describe and classify the plants and animals that he found. He also collected fossils in the rocks and these showed him that different life forms had gone through many changes in the past.
Among the many animals that Darwin studied on the Galapagos Islands were the finches of which he observed fourteen different species. In geological terms, the islands were recently formed and any animals found there must have reached the islands from the mainland, some 600 miles away. Finches are unable to fly long distances and Darwin suggested that one ancestral species of finch had reached the islands with the help of the prevailing winds. As there were no other bird species inhabiting the islands there was a variety of food available to the colonising finches. He noticed how individual finches differed from one island to the next. The main differences were in the size and shape of their beaks and these were related to the different type of food eaten, e.g. insects, seeds, fruit.
It seemed that on each island the characteristics that best suited a particular finch to its environment were inherited by the offspring. Darwin suggested that the finches had developed from a common ancestor and that the type of beak had developed over time as it provided a selective advantage to these individuals and become specialised to feed on a particular food source. This is an example of adaptive radiation.
Charles Darwin made many observations of the wildlife and plants during his voyage to the Galapagos islands. One of these observations were that different species of finch colonised different islands. The finches on one island were perfectly adapted to feed from one main type of food source (e.g. insects). He proposed this occured as a resultof adaptive radiation. The generation of several new species from a single common ancestral species (he identified 14 different species from the 14 islands) . Natural selection is thought to be the mechanism through which evolution occurs. Here, genetic variation may result in favourable or unfavourable characteristics. If the characteristics allow the organism to survive more effectively to reproductive age they are likely to outcompete other members of that population. Beneficial genes are selected for and get passed onto the offspring. Unfavourable genes are selected against and may become lost from the gene pool.
Draw the different types of finch beaks and describe how the beaks are adapted to different diets.
Natural selection
Darwin’s observations of variation within a population led him to propose the idea of natural selection:
A species may be defined as a group of organisms which share a large number of common characteristics and which can interbreed to produce fertile offspring. If a group of organisms becomes separated into two more isolated populations, these may become sufficiently different over time to prevent members of one group breeding with individuals from the other group (i.e. the two groups can no longer interbreed to produce fertile offspring). They have become two distinct species.
Darwin’s observations of variation within a population led him to propose the idea of natural selection:
- There is over-population of a particular species
- Competition causes a struggle for resources to occur between individuals (space, shelter, food, water, a mate)
- Individuals that are better adapted to the environment are more likely to outcompete others in the population
- The better adapted individuals are said to have a selective advantage (towards a particular selection pressure)
- These individuals are more likely to survive and are therefore more likely to reach reproductive age
- These individuals then reproduce and pass on the favourable characteristics (genes) to their offspring
- The favourable (beneficial) allele(s) get passed on from generation to generation and become established in the gene pool over time;
A species may be defined as a group of organisms which share a large number of common characteristics and which can interbreed to produce fertile offspring. If a group of organisms becomes separated into two more isolated populations, these may become sufficiently different over time to prevent members of one group breeding with individuals from the other group (i.e. the two groups can no longer interbreed to produce fertile offspring). They have become two distinct species.