Species Mutations Mutations and evolution Speciation Case history Fossil evidence

Organisms within any one kingdom of living things are further classified into groups showing increasing structural similarity.

Phylum Class Order Family Genus Species

greater diversity greater similarity

Species
A species is a population of organisms which closely resemble each other structurally and physiologically, and which can interbreed to produce live fertile offspring.

They are therefore said to share the same gene pool, because the genes (inherited information) of the different individuals can mix through mating.

In contrast, if a horse and a donkey do breed, the offspring (mule) is sterile and therefore cannot breed itself, so the gene pools of the two different species remain separate.

Mutations
There is variation between all individuals in populations, as is obvious in the human population. Some of these differences are "acquired" such as tattoos and spiked hair, and these will not be inherited by the offspring. For inherited features though, the original source of the variation is due to changes in the genetic code (called mutations) which occur during the formation of the gametes (sex cells). These changes are randomly passed on to offspring leading to variation between individuals. A mutation in a sperm or ovum changes the genetic information, coding for a different mutant feature in the resulting offspring. Mutations occur naturally, but their frequency can be increased by factors such as radiation (e.g. X rays or nuclear fallout) and certain chemicals.

Mutations and evolution
Some mutations are fatal and the offspring never develop. If, however, the mutation creates a feature that aids survival (i.e. it is an adaptation), then that organism is more likely to survive and reproduce successfully, leaving more offspring which carry that mutation also. If the environment changes, then different features will be advantageous.

The British naturalist, Charles Darwin, travelled to South America in 1835 and his observations led him to suggest that species changed over time by a process referred to as "evolution". He hypothesised that this occurred by the process of Natural Selection or "survival of the fittest". By this he meant that there is variation between individuals in a population and those individuals best adapted in a certain environment will survive best and pass on those traits to their offspring. Therefore, over the next generations, the number of well-adapted individuals increases until most of the population carries the trait.

Speciation
Two separate species can develop from one ancestral species if the two groups are isolated over a long period of time, during which the environmental conditions change and natural selection operates.

Darwin's theory of evolution through natural selection was in contrast to the theory of Jean Lamarck (France, early 1800s) which suggested that organisms could aquire new characteristics during a lifetime and these could be passed to the next generation, whereas Darwin observed that the variation already existed in the population.

Case history
In Pre-Industrial England, light-coloured pepper moths were common, escaping predation by birds because of their camouflage against the light-coloured lichens on the trees. Dark moths were easy prey.

Industrialisation created air pollution and tree-trunks became darkened. Darker moths seemed to appear and become more numerous. These mutant forms now had a distinct survival advantage because light-coloured moths were easily seen on the darker trees and were eaten by birds. The mutant dark ones survived and had many dark offspring with the result that the population changed through time. A considerable time later, and with further environmental change, a new species may well evolve.

Fossil evidence
Fossil evidence suggests that life forms have changed significantly from the very simple organisms found in the earliest fossil layers. Similarities in embryonic forms and DNA (genetic information) also point to common ancestry.

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