In 1859 Charles Darwin revolutionized our concept of the world and humanity's place in it with his book On the Origin of Species. Darwin explained the evolution, or change through time, of animals as resulting from a process he called Natural Selection. He modeled his hypothesis, in part, on the un-natural selection which we can all see around us. People breed championship dogs, cats, race horses, etc. by selecting which animals will mate and which will not (a successful race horse commands enormous stud fees, while a loser ends up in the glue factory).
Darwin hypothesized that in the natural world the local environment applies adaptive pressures (fitness tests) to individuals within a species population. These "tests" insure that the individuals best adapted to the local environment will be the ones which reproduce. If the individual passes, it survives because it has characteristics which make it better adapted to the local environment than other members of the same species. The important aspect of Natural Selection is not just survival, but the retention of beneficial characteristics within the population through reproduction. An animal which does not pass the test need not die, but it may just be too weak to successfully breed, so its characteristics are lost.What, exactly, is an adaptive pressure?
We can define an adaptive pressure as any aspect of the environment which can affect the survival of an individual. Food availability, climatic variations, predators, etc., would all be examples of adaptive pressures. As a non-biological example, consider the likelihood of success of a computer company which was still trying to market PC's with 286 chips, 1Mb of RAM and 28Mb hard-drives, but no other peripherals. Probably somewhat less that zero! These companies are under an adaptive pressure to make sure that their product keeps up with advances in computer technology. If not, they go extinct and their characteristics are lost. When was the last time you saw a computer like the one described above that wasn't gathering dust in a closet? Fossil computer??
Natural selection, however, works in an extremely variable world, so the population generally will posses a wide range of variation. Consider the hatching of insects in the spring. It is advantageous to a species to be the first to have representatives exploring the local environment in order to find new food sources, nesting sites, etc., before some other species population finds them. We would therefore expect that insect individuals that hatch on the first warm spring day would be selected as most adaptive. In many cases this is true, but what happens when the first warm day turns out to be one of those "January thaw" days, followed quickly by a return to sub-freezing conditions? Well, if all members of the local population of a species had hatched, the species would be quickly frozen to extinction in that area. So having individuals that don't hatch until there have been five or more successive warm days becomes an insurance policy for the species. Variation within a population helps to insure its survival.
Well, all this business about Natural Selection seemed to make sense to some people, but Darwin kept being asked, " But how do these characteristics get passed down,?" Unfortunately, Darwin had no answer. This was especially unfortunate because Gregor Mendell had presented the groundwork for the answer only a few years after Darwin first published "On the Origin of Species"! Mendell, however, published his work in a very obscure journal which no one read.
Be that as it may, Mendell did give us the answer. As you may well know, we all have two alleles for any given character trait - one from our mother, and one from our father. All together, the overall genetic package which we receive from our parents is called our genotype - in effect, our individual genetic blueprint. Based upon that blueprint, our phenotype, or the actual physical appearance of our bodies, is built. In numerous cases though, our parents have very different characteristics. Mom is a brunette, dad has blond hair; or mom has a slender build while dad is rather short and spherical. What determines which of those characteristics we end up with? In a very simple sense, the determining factor is that certain character traits are Dominant - if they are present in the genotype, they will always be the ones which are manifested in the phenotype - whereas, others a Recessive - they will only show up in the phenotype if you get them from both parents.
So what does all this have to do with evolution? Consider Figure 3 and Figure 4. Figure 3 is a population probability curve based on the Mendelian Cross similar to the one in figure 2. In any population which has two alleles for a trait, and if there is no difference in the possibility of survival for individuals with either trait, we would expect the population to consist of 25% homozygous a individuals, 25% homozygous A individuals and 50% heterozygous individuals. However, if either allele confers an increased (or decreased) likelihood of survival, then the probabilities will change.Eye Color - an Insightful Example (sorry, but I couldn't resist!) Figure 2. A Mendelian cross between two heterozygous individuals. Note that while the possible results are 25% homozygous brown eyed, 25% homozygous blue eyed, and 50% heterozygous in terms of the genotype; the phenotypic results will be 25% blue eyed and 75% brown eyed. Since the brown eyed trait is dominant, any individual carrying the Br allele will be Brown eyed.
Most people have either Blue or Brown eyes. The Brown eyed trait (Br) is the dominant trait in humans, while Blue (Bl) is recessive. This allows me to say something about your genotype. If you have blue eyes, then you received the Bl trait from both of your parents (if both of your parents have blue eyes there was never any doubt that you would have blue eyes, since both of your parents must be homozygous for the blue eyed trait. A homozygous individual is one in which both alleles for a given trait are the same (e.g., Bl,Bl). On the other hand, if you have brown eyes, then your genotype is not as definite. You may be homozygous for the brown eyed trait (Br,Br), or you may have received different traits from each of your parents (e.g., Bl, Br). If you carry two different alleles for the same trait, then you are heterozygous. (Note, homo=same, hetero=different). Why will you have brown eyes if you are heterozygous? Because the brown eyed trait is the dominant, while the blue eyed trait is recessive. See figure 2 above.
Figure 3. A probability distribution for a population in which a character trait has two alleles ( A=dominant, a= recessive). Note the bell curve shape and that the probabilities of homozygous and heterozygous individuals are those predicted in Figure 2.
Figure 4. Results of a directional adaptive pressure causing aa to be lethal.
In Figure 4, an adaptive pressure from the local environment has made the homozygous a condition lethal - all aa individuals die. Over time, the population distribution will shift in the direction of AA. In other word A is being selected for. The a allele is maintained in the population in heterozygous individuals (carriers) because, since it is recessive, it does not show up in the phenotype of the heterozygous individual. However, since all aa individuals die, the percentage of a in the population does decrease, but very slowly (How rapidly would a lethal dominant allele decrease?). Nevertheless, prolonged selection for A and against a would cause changes in the population which would be considered evolution.