Introduction to Evolution

Evolution is inherited change in an individual, it is also the word we use to describe the process whereby living things differentiate into other living things as a result of the natural processes of selective pressure, competition and individual variation.

Charles Darwin brought together the individual concepts which support the theory of evolution after a lot of years of thought and personal research, and published them as, "On the Origen of Species by Means of Natural Selection, or The Preservation of Favored Races in the Stuggle for Life" on the 24 November 1859. Darwin's original work has been tested and modified since then by a large number of intelligent, thoughtful people, but it is still considered sound, there is a considerable body of literature published on the subject and you should be able to find some in your local library if you want to know more.

 

 

Personally I consider the theory of evolution to be the most important building block in modern understanding of the living world around us. A good grounding in the basic principles is essential to any ecological or animal behavouristic understanding of the natural world.

In a nutshell the Theory of Evolution through Natural Selection says that because all individuals are different and because some of these differences are inheritable, those inheritable differences which result in greater breeding success for their possesors will become more common.

Given a bit more meat it goes like this. :-
All the living creatures we see, including ourselves have a form that we perceive, this means not only their size and color, but also there perception of the world and how they live and relate to each other, and to us. Scientists now know that this form is created for us by our 'genes' working in two different ways, these can be called primary and secondary genetic expression and equate to the old terms of nature and nurture.

Primary Genetic Expression

Primary genetic expression is the aspect of genetics and evolution that most people are aware of. It is the explanation for the fact that a pair white mice will probably produce white mice babies and a pair of black mice will produce black mice babies. We know that the color of a baby mouse is controlled by a set of genes which act as a set of instructions saying make this mouse white, or grey or piebald.

Each person (and most other living things) contains a double set of genes (each single set consists of a number of groups of genes linked together in one giant moloecule called a chromosome), one from each parent. These act as a double set of instructions controlling what all the different bits of your body should look like; in other words you have two sets of each of the genes or instructions which contribute to telling your hand to have 4 fingers and a thumb or your hair what color to be.

A guy called Gregor Mendel who lived as an Augustinian monk in a monastery in what is now Czechoslovakia was the first person to discover the existence of simple genetical inheritance. From about 1875 onwards he did a series of experiments with pea plants, some of which were Tall and others of which were Dwarf. He developed strains that either grew all Tall generation after generation or all Dwarf generation after generation, regardless of the number of generations. He then discovered that if he cross-pollinated them he got all tall plants from the resulting seeds, this is called the F1 generation. He also showed that if he then self pollinated these F1 plants, each one produced 75% tall plants and 25% dwarf plants this is called the F2 generation. Further more he went on to show that if he self pollinated these F2 plants, the 25% Dwarf plants all produced Dwarf offspring. However of the 75% Tall plants 2 thirds or (50% of the F2 generation) produced 75% Tall plants and 25% Dwarf plants (note that this is the same ratio as the earlier F2 out of F1 self-pollinated generation) and 1 third or (25% of the F2 generation) produced all tall plants. These 25% of Tall plants always produced Tall offspring and the 25% Dwarf plants always produced Dwarf offspring. He had discovered simple genetics, what we now call Mendelian genetics, i.e. that in this example Dwarf plants crossed with Dwarf plants always produce Dwarf offspring but Tall plants crossed with Tall plants can produce either Tall or Dwarf offspring, and that they do so in predictable ratios. A trait of a living organism that is always inherited by the offspring is called Homozygous or true breeding, and one that is not always inherited is called Heterozygous or mixed. The table below explains what I have just said in more graphical form.

Human Eye Colour

Many people have heard some genetics in relationship to human eye colour, and a number seem confused, so here are a few basic notes. Remember the genes that tell your eyes what colour to be are inherited, one set you got (inherited) from your mother, and one set you inherited from your father. In reality there is more than one pair of genes that can work towards controlling eye colour just as there are more than just blue or brown eyes. Here I am going to pretend there is only one set genes involved (one pair) that effect eye colour, making them either blue or brown. In the most simple explanation they work like this; if the set of genes for eye colour from your mother says 'make this babies eyes blue' and the set from your father says the same thing, then you will have blue eyes. If the set of genes for eye colour from your mother says 'make this babies eyes brown' and the set from your father says 'make babies eyes brown' then you will have brown eyes.

The interesting bit is what happens when the genes from your father say 'make the eyes blue', and the genes from your mother say 'make the eyes brown', or visa-versa. In the case of eyes, what happens is that the two sets of genes have an argument and the genes for brown eyes win (why they do is a bit to complicated to explain, you would need to know alot of other background information even to understand the words). This means that you have brown eyes with one gene for brown eyes which is 'dominant' or active and one gene for blue eyes which is 'recessive' or inactive, if when you grow up you have children they will have one of your genes for eye colour and one of your partners. You can't know before hand which of your genes your child will inherit because the selection process is a random one, we know how it works but again it is a bit to complicated to explain here.

So you can see that your child could have one blue gene from you and one blue gene from your partner giving it blue eyes because you need two blue genes to get blue eyes. Or one blue gene from you and one brown gene from your partner giving it brown eyes, or one of each the other way around, also giving it brown eyes, or finally it could have two pairs of brown genes again resulting in brown eyes, now you know why more people have brown eyes. In general, 2 blue eyed people cannot produce a brown eyed child (grandparents being irrelevant) because the character 'blue eyes' is recessive. So a brown eyed baby in a family with two blue eyed parents makes one suspicious of the fidelity of the female. BUT there are many complications, and blue eyes / brown eyes is not a clear cut distinction. Particularly if one of the G-parents is brown eyed, and could have passed on the brown allele to his/her child, then one would be very chary about breaking up a relationship on eye colour alone. Brown eye may not be completely penetrant (ie not everyone with the gene may show the character) and there could be other genes that suppress the brown eye (i.e. a fundamentally genetically brown eyed person could for various reasons have the phenotype of a blue eyed one). Thus it is possible for brown eyes to reappear in the cross blue x blue. Paternity testing depends on rather better characteristics than eye-colour, these days! As I said before , anything at this level is a bit of a simplification because though it works like I have said most of the time there are other genes which can interact with the brown/blue genes and change how they work but you need to go to a proper genetics text if you want to find out more.

Not all things act the same as your eyes though, in some cases the two sets of instructions work together, thus in some plants if the mother has red flowers with two genes for red flowers and the father has white flowers with two genes for white flowers the offspring will all have one gene for each colour and because neither one is really dominant they will have pink fllowers.

Finally it is important to realise that many character traits in both plants and animals are subject to constraints from the environment, this means that if you have a two plants with the same genes for height are grown in different conditions, one in poor soil and one in rich soil, the one in rich soil will grow taller. This is the same as you getting bigger muscles if you do more work, though your genes determine what potential you have it is often your life experiences that have the final say in how you look and act.

Secondary Genetic Expression

Learning is a very important aspect of human life, as well as of the lives of many other organisms, the higher or greater your learning capacity the more important learned habits and responses are for you as a living organism. Because the relationship between what you learn and the genes that generate your learning ability are not obvious or direct many people tend to consider learned behavour as non-genetic in origin and then use the concept of 'nuture' (often specifically referring to taught learned behavour) as proof that all our lives are not the result of our genes. This arguement is foolish, no biologist should ever forget the difference between genetic potential or 'genotype' and the observed result of the expression of this genotype in the macro (real) world or 'phenotype', which expression includes the effects of environmental constraints as well as the results of learning, both taught and otherwise.

It is interesting to think about the generation and 'evolution' of ideas in human society, though these ideas are not genetically propagated but passed from generation to generation as a result of successive acts of individual learning, and though they are in their own way subject to change and selection and as such can be seen to evolve in much the same ways as more material organisms, they are ultimately dependant on the existance of those genes which give human beings their capacity for learning and intelligence exhibited through thought.

Back to Evolution

Because evolution is the result of inheritable changes in individuals and the differences these changes make in that individuals reproductive success it follows that one of the key causes of evolution is genetic change.
Each cell in our body contains a complete set of our genes, this means that all the cells in your ears have a copy of the genes telling your eyes to be blue or brown and your hair to be black or whatever, to avoid confusion a cell only activates those genes it needs to to do its particular job, i.e. the genes relevent to ears are turned on in the ear and the rest are turned off.
Genetic change can be large or small and can occur in many ways, we call any genetic change a 'mutation'. Mutations occur all the time in our genes, , most of these are harmless and go un-noticed, some can cause changes to an individuals form or phenotype such as the skin melanomas resulting from mutations caused by ultra violet light. Only those mutations which occur in the genetic material of reproductive cells, i.e. eggs and sperm, are inherited. Thus if a mutation occurrs in a sperm cells genetic material which tells your hair to have 'x' amount of brown colour in it, such that it now says have twice as much e.g. 2x brown colour then any children resulting from that sperm cell will have the possibility to have hair that is twice as brown as it would have been otherwise.

Thus we can see life as a testing ground for the genetic changes resulting from mutation, with only those changes which result in a greater reproductive success for their possessors becoming common, though those which have no effect at can be passed on in otherwise successful individuals.

Other Evolution Sites






ENTER EVOLUTION: Theory and History An Excellent Site, essential reading for anyone interested.
Read Darwin On Line :- The Origin of Species     The Voyage of the Beagle     The Descent of Man

Mendelian Genetics
The F0 Generation. True breeding Stock
Homozygous Tall
I
Homozygous Dwarf

I

The F1 Generation. The result of crossing the above.
100% Heterozygous Tall
I I I I
The F2 Generation. The result of selfing the above, it is the same for each of the offspring.
25% Homozygous
I
50% Heterozygous
I I
25% Homozygous

I

The F3 Generation. Each box = the result of selfing the one above it. Each I represents 25% of the offspring.
I I I I I I I I I I I I I I I I



 

 

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