Darwin's theory of evolution by means of natural selection drastically changed our perception of the natural world. His revolutionary theory says that in the struggle for existence, individuals with some heritable variations will be favored over others. The advantageous variations will then become more frequent in the population. The theory includes four postulates: 1. Organisms have traits that vary within their population. 2. Some of the variations are consistently passed on to offspring. 3. More offspring are produced than can be supported by local resources, leading to mortality in every generation. 4. Survival and reproduction are not random; they are influenced by how fit an individual's traits are (Riley, 2011). These four postulates can be demonstrated every time an instance of evolution takes place in a population.
We can learn a lot about the process of natural selection by evaluating its effects on a population. Geospizae are excellent model organisms to demonstrate these effects because of the extent to which they have been studied. Peter and Rosemary Grant, evolutionary biologists from Princeton University, have spent six months of every year since 1973 capturing, tagging, observing, and recording data on thousands of birds on Daphne Major Island (Riley, 2011). Their data has shown the forces of natural selection at work in the Geospiza populations.
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Through studies of Geospiza, we can observe that the pressures of natural selection can affect the range of phenotypic traits in a population in several different ways. For example, sometimes natural selection works to maintain the status quo. Over a range of phenotypes, individuals on either ends of the spectrum are selected against and individuals near the mean are selected for. This is known as stabilizing selection. Another way populations can be affected is by disruptive selection, in which extreme values for a trait are selected over mean values. Directional selection favors one extreme in the range of phenotypes over the other. This causes allele frequency to continuously shift in one direction (Campbell and Reece, 2009).
The Grants' study on bill size in Geospiza conirostris is an excellent example of natural selection affecting a population. By measuring bill depth, length, and width of all birds born in a year and seeing which ones survived their first 100 days, the Grants were able to show a correlation between these traits and survival (Grant, 1985).
During the yearly dry season, which lasts from May to December, food on the island that G. conirostris inhabits becomes scarce. The limited food results in a period of high mortality, especially for birds less than 1 year old, as many die of starvation. In fact, 88.9% of fledglings do not survive to breed (Grant, 1985). Although all individuals of the species eat similar foods during the rainy season, this is not always possible during the dry season; birds with different bill sizes exploit different foods more efficiently. For example, birds that survive the dry season by feeding on Opuntia cacti have to crack open the fruit and reach the enclosed seeds. This requires a significantly longer bill than birds that do not feed on Opuntia (Grant, 1985). There are also other food sources that are only a viable option for birds whose bills are significantly smaller than the average.
Fledgling mortality in the studied years was the highest for birds with average sized bills. Competition for shared resources reduces the success of individuals who eat medium-sized seeds (Hendry, 2009). Individuals closest to the large or small bill size modes had the highest survival rate (Grant, 1985). This results in a disruptive pattern of selection, with the two peaks corresponding to the sizes of birds which were best adapted to their particular food source. According to Darwin's postulates, not all offspring produced can survive. Since this particular trait varies within the population, the individuals whose variations are most fit are the few who survive and pass those genes to their offspring.
The pressures that natural selection can place on a population are not static; a trait that is selected for at one point in time may be selected against at another. The drought that took place in Geospiza's habitat in 1977 caused a change in bill size frequency that differed from previous and subsequent years.
Rainfall that was less than 1/5th as much as normal resulted in a change in the availability of seeds that form the birds' diets (Grant and Grant, 1995). Of all the birds recorded and measured in 1975-1976, only 15% survived to 1978 (Schluter et al., 1985). Small seeds were much less abundant than larger, harder ones, so the individuals that survived had to do so by exploiting the large seeds Opuntia echios and Tribulus cistoides (Schluter et al., 1985). Birds with larger beaks could crack open larger seeds than smaller birds could, suggesting that many smaller birds died of starvation.
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Since beak size is highly heritable (h2 = 0.5 - 0.9), the effects of this selection were passed to the next generation (Grant and Grant, 2002). The population started out with a large amount of variation in sizes, but one end of the spectrum contained individuals whose variation of the trait was more fit for the environment than the others. Large beak size, the more favorable trait, was passed to offspring and accumulated in the population, causing a shift in bill size allele frequencies consistent with directional selection. The pressures of natural selection affected this population so that the generation of offspring born in the 1978 breeding season had larger bills than their parents' generation did (Grant and Grant, 1995).
When male members of the Geospiza fortis population are first born, their feathers are brown and streaked, resembling those of a female. After a few months, they begin a series of molts. At each molt, they acquire plumage that is either as black or blacker than their previous plumage; they never become browner. After a series of molts, they eventually reach their fully mature, completely black state. The rate at which birds acquire their adult (black) plumage varies significantly among individuals in the population. The time it takes for them to become completely black varies from 15 months to 3 years (Price, 1984). The length of time that they are in their subadult plumage stage can cause them to be selected for or against in nature. By comparing the maturation of birds born in 1983 to the maturation of their sons born in 1987, the Grants found that the time it takes for a birds' feathers to fully mature is highly heritable (h2 = 0.62 ± 0.23) (Grant, 1990).
The more black a bird's feathers are, the more dominant he is compared to other birds. According to the status-signaling hypothesis, if a blacker male enters the territory of a bird with subadult plumage, the less mature bird will give up the territory without aggression (Grant, 1990). Although it would seem that giving up territory or food would be a disadvantage for birds with subadult plumage, the energy lost by giving up their possession is far less than the energy they would have spent fighting. It is fairly easy for the birds to scavenge more food, but it takes a while for them to recover from a fight (Grant, 1990). However, adult plumage has the opposite effect when it comes to mating. Females chose males with completely black plumage over those with partially black plumage (Price, 1984). Fully black males are also much more likely to protect their nest from intruders (Grant, 1990).
The overall affect becomes that of stabilizing selection. The males who mature the quickest are likely to become injured or die in a fight; the males who mature the slowest have difficulties finding a mate and protecting their nest. The most successful G. fortis males are those whose maturity is of average length. This scenario also shows that selection can be influenced by sexual or social factors, not just by an organism's environment.
The four postulates of Darwin's theory of evolution can be shown in each of the examples I have presented. In each situation, there existed some trait that varied between members of the population and was passed from parents to offspring. In each generation, an overabundance of offspring was produced, and only the individuals most fit for their environment survived. The surviving individuals caused a change in their populations' allele frequencies, and changed the distribution of phenotypes of traits.