Evolution - A Brief Explanation

Here I want to go through the basic principles of the Theory, the mechanisms of Evolution and the history of the theory.

In 1859, Charles Darwin published his theory on Evolution. Here he made four key observations. Firstly, organisms produce more offspring than actually survive, for instance for every five offspring born only two will survive to maturity. He also noted that there is variation in the characteristics of members of the same species and that some of these characteristics can be passed on from one generation to the next. Finally, individuals that are best adapted to their environment are more likely to survive. Natural selection is just one process by which evolution occurs. 

Darwin's theory of evolution by natural selection explains his observations. Individuals within a population show variation in their characteristics, this is caused by mutations as a retaliation to predation, disease and competition which creates a struggle for survival. Individuals with better adaptations are more likely to survive, reproduce and pass on their advantageous adaptations to the next generation. Over time, the number of individuals with the advantageous adaptations increases. 

Evolution can lead to speciation, this is where a species evolves into a completely new species. A species is defined as a group of similar organisms that can reproduce to create fertile offspring. Species can exist as one or more populations, for instance there are different populations of American Black Bear in the USA and Canada. Speciation happens when populations of the same species evolve to become so different that they can't breed with members of the unevolved species to produce fertile offspring. 

Sketches of Darwin's Finches, the larger beaks are used for feeding on
 hard nuts whilst the small beaks are best adapted for
feeding on fruit. Sketches are by Charles Darwin himself.

Darwin's finches are the best example for the theory of evolution. The finches of the Galapagos Islands are adapted to feeding on different food sources on the islands. All the species of finch had a common ancestor. Different populations became isolated on different islands, and therefore each population has adapted to the varying environment on the islands. The populations evolved to become so different that they could not interbreed to form fertile offspring and thus created different species of finch. 

There is a lot of evidence to support the theory. Most of this evidence comes from the fossil record. By arranging fossils in chronological order, from oldest to youngest, it is possible to identify gradual changes in organisms that lead to the formation of different species. There is also DNA evidence. Evolution suggests that all organisms evolved from common ancestors, the more closely related two species are, the more recently the species diverged. This is a result of the gradual change in the base sequence in DNA. So, species that diverge more recently will have more similar DNA. A good example of this is that humans and chimpanzees share 94% of their DNA base sequences with each other. 

Different Forms of Evolution:
Convergent Evolution - this is where two species evolve similar features because they are living in the same environment. For example, bats and insects both have wings and can fly but they belong to different phyla.

Coevolution - here two species influence each other's evolution. For instance, flowers will emit a scent whilst being brightly coloured in order to attract insects that then act as pollinators. 

Adaptive Radiation - species rapidly evolve in order to take advantage of ecological niches, Darwin's finches in the Galapagos Islands are the best example here, some had large beaks for eating nuts whilst others used smaller beaks to eat fruits.