There is so much diversity among living organisms, and those that are similar in their structures and behaviors are grouped together into a hierarchy of Kingdom, Phylum, Class, Order, Family, Genus and species. Diversity occurs due to evolution of species where genetic mutations, and non random reproductive success cause organisms to change and become reproductively segregated from their species group. If this change gives the new organism an advantage within its environment it will cause the new species to survive and reproduce more effectively. This can sometimes create additional competition and lead to the extinction of the original species.
To what degree is an organism complex? Larger organisms can be said to be so because they contain many layers of cellular sophistication creating complex biological tissue systems, whereas organisms such as bacteria which are unicellular appear to adopt a simpler existence. Scientists often refer to a type of species as either primitive’, meaning the order form that is close to the ancestral species or advanced’, meaning a newer form that is less like the ancestral species (Keeton, 1980). Although the word advanced’ can be linked to being more complex and superior, these words used in evolution only represent on organism in relation to a time period.
Species are changing all the time and therefore some have existed on the earth longer than others. For example, the Horseshoe crabs (Limulus polyphemus) have not changed structurally for millions of years and are living fossils’ of their Chelicerate class (Keeton, 1980). Their species have been evolving and remained the same form for a long period of time, it seems the horseshoe crab has found its perfection, and such dedication makes us wonder why it doesn’t want’ to improve itself. All of life came from common ancestors, for example the first multi-cellular animals were around 680 million years ago, and from them all forms of animals evolved, therefore those of us who have not yet gone extinct all must have reached the same level of sophistication.
Life does not strive to be more complex, it strives to survive in its given environment, evolution provides loop holes’ for species to create alternative ways of life, they may become more complex in the process but complexity is not its goal and not always its path. It can be beneficial for an organism to evolve the loss of a complex part of their anatomy in order to survive. For example snakes are closely related to lizards, however they have lost their legs and move by contracting muscles which pull their scales in different directions. This could be viewed that the snake has lost a complex structure, however given that snakes are a numerous species it is clearly not a threat to their kind.
Evolution can create situations where the structure of an organism changes dramatically, usually from a mutation of the Hox gene which is responsible for the orientation of major body parts, for example in sea spiders (Pycnogonida), they can be found with leg duplications (Brusca. et al.2003). Mutations can also occur more subtly over time and cause gradual changes in a species, helped along by environmental changes, such as the weather changes on the Galapagos Islands, which caused the changes to the beaks of Darwin’s’ finches. However, an organism may appear to have evolved a more complex structure, but it might not be more complex genetically, for example much of are DNA is terribly similar among animals,. On a molecular level all life is essentially made from the same products, for example carbon and water are two of the main elements found in life-forms, the base pairs of DNA are the same in all organisms, it is only base sequence changes that change the phenotypes of organisms. We can see organisms as different recipes, creating different meals, but with the same ingredients, and ultimately the favorite meal is down to taste.
It is not always best to be a complex life form in this world, in some cases the smaller an organism is, the more likely it will survive as a species, for example bacteria can survive in extreme conditions that more structurally complex organisms would not, for example the archae bacteria can live at temperatures above 100 degrees Celsius, such as Crenarchaeota hyperthermophiles. Bacteria also has a high rate of reproduction, which would suggest a higher rate of evolution, however some modern forms are almost identical to the fossil bacteria of around 3.5 billion years ago (Taylor, 1983). They have survived and maintained this form because it is selectively appropriate for their existence in their environment, and essentially keeps then alive efficiently, so because it works well there is no need for it to change.
Organisms do not always evolve into more complex forms because evolution is a random process governed by many influences, such as environmental conditions, radiation speeding up mutations, random chromosome cross-over, and random gamete combinations. Evolution will lead an organism to become more structurally complex only if it will aid the organisms survival, otherwise an organism can continue to evolve and remain at an optimum level of complexity which best suits its survival.
>Biological Science,3rd edition, by william T. Keeton, Cornell university, published by W. W Norton and company, Inc( 1980), pg 814-815 and 1031.
>The Great Evolution Mystery, by Gordon Rattray Taylor, Published by Martin Secker and Warburg Limited (1983), pg 27.
>Invertebrates, 2nd edition, by Richard C.Brusca and Gary J. Brusca, published by Sinauer associates Inc (2003), pg 18.