The ideas behind evolution, on the face of it, seem so implausible as to be ridiculous. Indeed only 51% of the UK population think that evidence for evolution exists (see this survey from the British Council/Ipsos-Mori). This clearly has a lot to do with the incredible lengths of time involved. It is very hard to mentally envisage periods of thousands of years, and yet evolution has taken place over a scale of billions of years (It's worth adding a side note at this point that evolution takes place over varying timescales depending on the type of organism and it's environment. It's not worth getting tied up in specifics for this brief overview. To learn more about this have a look at some of the suggested reading at the bottom of the page).
There is a second complication, and that is a kind of human bigotry. It is standard learning in human culture all around the world that homo sapiens is the dominant organism on this planet. Genesis 1:26 states:
Then God said, "Let us make man in our image, in our likeness, and let them rule over the fish of the sea and the birds of the air, over the livestock, over all the earth, and over all the creatures that move along the ground." (biblegateway.com)
This is hardly the sort of background we need before learning that all living organisms are quite literally our distant cousins!
To get an idea of how evolution works, we need to ask what exactly we mean by evolution. The ask dictionary offers 7 separate definitions for evolution. The one that we are interested in is this;
"Change in the genetic composition of a population during successive generations, as a result of natural selection acting on the genetic variation among individuals, and resulting in the development of new species."
Many people I think will accept the idea of there being genetic variation between individuals. The fact that there are people taller and shorter than yourself is evidence of the genetic difference between you and others. The colour of your eyes and hair along with all manner of other features that you posess are determined by your genes, the difference in two people's genetic make-up is the reason that these attributes differ. Indeed the differences that exist between organisms of different species are also due to the difference in their genetic composition. All animals are to a great extent made up of the same chemicals and compounds, the difference being that the genes possess instructions to organise those compounds in a different way.
The quote above states that this variation is acted upon by natural selection. So what exactly is natural selection?
An oft used interpretation of natural selection is "survival of the fittest" which is a nice basic interpretation, but the use of the word 'fittest' could be taken to infer the strongest, or fastest, or something similarly specific. Natural selection is actually better stated as the 'survival of the genes that offer the organism in which they reside the best chance of reproducing.' Of course if that is the accurate phraseology, it's no wonder that 'survival of the fittest' caught on!
The less verbose way of stating the above, is that natural selection is the selection of traits within members of a species that have successfully reproduced over and above those that have not. It is easiest to illustrate this with an example.
Imagine if you will a herbivore that grazes on bushes in a temperate part of the earth. For a period of perhaps many hundreds of years the rains are regular, the plant-life always healthy, and the grazing good. In these conditions both the plants and the animal in question proliferate. Now imagine that slowly the climate of the area becomes more arid. Many centuries pass and the rains become more and more rare. In these circumstances the plants become fewer meaning that there is suddenly more competition for food in the herbivore's life. Now those grazers that have an advantage of some description will find it more likely to obtain food, and therefore survive for long enough to find a mate.
We will say for the sake of our scenario that one of the advantages is that certain individuals of our unnammed species has a longer neck than the others. For those lucky few it will be possible to graze on the taller plants that are out of reach to the majority of the population. In successive generations, those longer necked individuals will survive to reproduce, meaning the 'long-necked' trait will be passed on. It could be said that nature, in this way, is selecting those individuals with longer necks. As the scenario hopefully makes clear, there is no conscious selection, however some individuals (or more specifically, some traits within individuals) have nonetheless been selected for survival over others.
The more astute among you will have noticed that this will only work up to a point. If there is a gene that provides a neck of length x (it really isn't as simple as that, but such things are beyond the scope of a blog post) surely over time those individuals who have a gene for any shorter neck length will be selected out of existence? The advantage that the 'long-necked gene' conferred would suddenly disappear!
Our analysis of the definition above isn't yet complete, and it provides the answer. The first 5 words are "Change in the genetic composition..." Clearly there is something occurring that means that the genes, and therefore the traits that are available for selection (for or against) are changing over time. How might this happen? Well a simple explanation that would give an idea, but not necessarily the detail would go as follows.
Our genes are coded in an alphabet of four letters. When cells split, for example in the production of sex cells for reproduction, the information for creating an organism is copied from one 'book' to another. Very, very occasionally, this copying is not 100% precise. This is known as a mutation. When this happens, it may create a change in the physical traits of the organism.
The vast majority of mutations are very very small, and do not confer any specific advantage (or even a noticeable difference) to the organism in question. Occasionally, however, they do. To see how let's return to our scenario from earlier.
Over many thousands of years, the selection towards longer necked herbivores has been progressing. There is still a fair mix of long necked and short necked individuals, and now the tallest of the animals is finding his advantage much diminished. Over these thousands of years, there have been various mutations. Some have been benign, such as to provide a slightly longer tail. Some have been downright cruel, like being born with much diminished sight for example. In one case however a genuinely positive mutation has taken place, one individual has been born with a longer neck than the pre-existing gene pool would allow!
This organism has a distinct survival advantage over his peers, and therefore (bad luck notwithstanding) will be likely to live long enough to pass on his genes to a future generation. This advantage will then spread slowly through the gene pool of this species until, over perhaps another few thousand years, a similar mutation occurs offering it's possessor even greater advantage! You can see how with sufficient time, a creature could evolve to be much much taller and longer-necked than its ancient forebears.
Now to the last part of our definition; the "..development of new species". Occasionally it will occur that one population of a species will become divided geographically, through migration or other methods, and become subject to different environmental pressures. This would result in different traits being 'rewarded' by natural selection and may make previous ones less favourable, creating a divergence in the two evolutionary paths.
If we return to our example, we shall suppose that in the spread of our herbivores throughout their habitat, one part of the population speads into an area that is also home to a predator. Now although previously being tall and long-necked would serve well in competition against other members of your species, it would not serve you as well if you wished to avoid being dinner!
In this separate population, it is useful to grow tall to a point, however it as also necessary to have strong legs in order to run away more quickly. Also the tallest members of the species may be punished by being easier to spot and pick out as prey. Over the generations these differences in the selection pressures would lead to different mutations surviving to reproduce. When the genetic distance between two populations over time grows to such a point that they can no longer interbreed, they are considered different species.
Hopefully you now have a good idea about how the evolutionary process works. Below are some sponsored links to books that I enjoyed reading when learning this topic. If I've piqued your interest, go have a look. They are highly recommended.