EVOLUTION BY NATURAL SELECTION Evolution Change over time

EVOLUTION BY NATURAL SELECTION

Evolution – Change over time. It the Process by which modern organisms has descended from ancient species. In 1859, On the Origin of Species, Darwin proposed a mechanism for evolution called natural selection. He describe Natural selection is the process by which genetic mutation that lead to selective advantages and increased fitness become, and remain, more common in successive generation of a population. Natural selection operates on the phenotypes of individuals produced by their particular combination of allele.

Darwin’s process of natural selection has four components. � Variation. Organisms (within populations) exhibit individual variation in appearance and behavior. These variations may involve body size, hair color, facial markings, voice properties, or number of offspring. On the other hand, some traits show little to no variation among individuals—for example, number of eyes in vertebrates. � Inheritance. Some traits are consistently passed on from parent to offspring. Such traits are heritable, whereas other traits are strongly influenced by environmental conditions and show weak heritability.

� High rate of population growth. Most populations have more offspring each year than local resources can support leading to a struggle for resources. Each generation experiences substantial mortality. � Differential survival and reproduction. Individuals possessing traits well suited for the struggle for local resources will contribute more offspring to the next generation.

Natural selection describes how populations, over time, adapt to their environments. Adaptation is based on the generational selection of certain beneficial alleles that will, over time, increase in frequency (and conversely, less advantageous alleles will decrease in frequency). For example, let's say in a certain species of bird there is an allele that leads to curly wing feathers. While these curly feathers may be nice to look at, they may not lead to an increase in fitness.

On the contrary, these curly-feathered birds are likely poor flyers and they do not live sufficiently long enough in the wild to reproduce. As a consequence, the curly-wing allele is not very prominent in the population because birds having this allele don't live long enough to breed and contribute the allele to the next generation.

There are main three types of natural selection: Directional selection: Directional selection is associated with gradually changing conditions where the adaptive phenotype is shifted in one direction and one aspect of a trait becomes emphasize. So, in Directional selection Smaller individuals may have higher fitness (i. e. produce more offspring) than larger individuals. It favors smaller individuals and will, if the character is inherited, produce a decrease in average body size. Directional selection could, of course, also produce an evolutionary increase in body size if larger individuals had higher fitness.

An example of directional selection: pink salmon Pink salmon (Onchorhynchus gorbuscha) in the Pacific Northwest have been decreasing in size in recent years. In 1945, fishermen started being paid by the pound, rather than per individual, for the salmon they caught and they increased the use of gill netting, which selectively takes larger fish. After gill netting was introduced, smaller salmon had a higher chance of survival; the selection favoring small size in the salmon population was intense, because fishing is thorough: about 75 - 80% of the adult salmon swimming up the rivers under investigation were caught in these years. The average weight of salmon duly decreased, by about one -third, in the next 25 years.

Figure: the graph shows the decrease in size of pink salmon in two rivers in British Columbia. Two lines are drawn for each river: one for the salmon caught in odd-numbered years, the other for even years. Salmon caught in odd years are presumably heavier because of the two year life cycle of the salmon. From Ricker (1981).

Professor E. W. Bride wrote in "Nature": Natural Selection' affords no explanation…of any…form of evolution. It means nothing more than 'the survivors survive. ' Why do certain individuals survive? Because they are the fittest. How do we know they are the fittest? Because they survive. In his book he describes neither natural selection nor mutations introduce any new genetic data into the organism's DNA. Natural selection only selects out the disfigured, weak, or unfit individuals of a population. It cannot produce new species, new genetic information, or new organs and thus cannot make anything evolve.

One commonly cited evidence for evolution is the development of resistance in bacteria against antibiotics. Fifty years ago, penicillin killed many types of disease-causing bacteria. However, it is not that effective today. The development of resistance against antibiotics is hypothetically direct evidence for evolution. The magazine Scientific American has to say the following in its March 1998 issue: "Many bacteria possessed resistance genes even before commercial antibiotics came into use. "

Before the development of penicillin, some bacteria species were already resistant. Many were not. After penicillin was used against the bacteria, the nonresistant bacteria were killed. The resistant bacteria survived and reproduced to produce more resistant bacteria. The population increase of resistant bacteria is not evolution. A new species of bacteria did not evolve. The bacteria are still the same species as they were before. What happened was only a weeding out of non-resistant bacteria.

Evolutionists frequently point to the development of antibiotic resistance by bacteria as a demonstration of evolutionary change. However, molecular analysis of the genetic events that lead to antibiotic resistance do not support this common assumption. Many bacteria become resistant by acquiring genes from plasmids or transposons via horizontal gene transfer. (Is Bacterial Resistance to Antibiotics an Appropriate Example of Evolutionary Change? Kevin L. Anderson; Creation Research Society Quarterly; volume 41, No. - 4; March 2005)

Disruptive selection Diversifying selection (also referred to as disruptive selection) favors individuals at both extremes of the phenotypic range, usually during periods of environmental change. Disruptive selection could favor both extremes over the intermediate types. Disruptive selection favours individuals with variation at opposite extremes of trait over individual with intermediate variations. Sometimes environmental conditions may favour more than one phenotypes. It is associated with a fluctuating environment and gives rise to balance polymorphism in the population.

Imagine a population in which the main food supply has decreased. Because less preferred food exists, the variants of both extremes may be able to utilize different food supplies, whereas intermediates may not be able to adjust. Selection against intermediates would result, due to the decrease in available preferred food.

AN EXAMPLE OF DISRUPTIVE SELECTION: BRISTLES ON FRUITFLIES. In nature, sexual dimorphism is probably a common example of disruptive selection; but here we use an experiment by Thoday and Gibson on the Drosophila melangaster fruitfly as an example. Thoday & Gibson (1962). Thoday and Gibson bred from fruitflies with high, or low, numbers of bristles on a certain region of the body; individuals with intermediate numbers of bristles were prevented from breeding. As the graph shows, after 12 generations of this disruptive selection, the population had noticeably diverged.

Figure: experimental disruptive selection on sternopleural bristle number in fruitflies. Individuals with many or few bristles were allowed to breed, those with intermediate numbers were not; and the population rapidly diverged.

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SEXUAL SELECTION Definition: Sexual selection involves any features physical or behaviour that affect reproductive advantage over members of the same sex. It has led to sexual dimorphism which is the differences between male and females in the same species.

SEXUAL SELECTION Two types of sexual selection (i) Intrasexual : where male compete with one another for access to females or for resources required by females (ii) Intersexual: when females decide on which mate to choose base on personal attributes such as song morphology or coloration.

SEXUAL SELECTION Intrasexual Where males compete with one another. Main forms of competition are: - Contests - Scrambles - Endurance rivalries

SEXUAL SELECTION Intersexual - Where female choose. - In 95% of species females spent a lot more time raising offspring than males so they have a lot more vested in who they choose as a mate. Tungara Frog

SEXUAL SELECTION SEXUAL DIMORPHISM Why are males and females so different in many species? Darwin believed that sexual selection would account for differences in colouring in both sexes. THE PEACOCK

SEXUAL SELECTION NATURAL SELECTION VS SEXUAL SELECTION - Natural selection deals with forces between species while sexual selection deals with forces within a species - Sexual selection focuses on reproduction rather than on survival while natural selection relies on both.