Darwinism and natural selection

Darwinism
Natural Selection
Conditions of Natural Selection
Types of Natural Selection
1. Directional Selection
2. Stabilizing Selection
3. Disruptive Selection

1. Darwinism and Natural Selection

2. Darwinism Darwinism  is a theory of biological evolution developed by Charles Darwin and others. It states that all species of organisms arise and develop through the natural selection of small, inherited variations that increase the individual's ability to compete, survive, and reproduce. Also called Darwinian theory.

3. Natural Selection Natural selection is Darwin’s most famous theory. It states that evolutionary change comes through the production of variation in each generation and differential survival of individuals with different combinations of these variable characters. Individuals with characteristics which increase their probability of survival will have more opportunities to reproduce and their offspring will also benefit from the heritable, advantageous character which spread the population.

4. Conditions of Natural Selection 1. Reproduction 2. Heredity 3. Variation in individual characters among the members of the population 4. Variation in the fitness of organisms according to the state they have for a heritable character. If these conditions are met for any property of a species, natural selection automatically results.

5. Types of Natural Selection Natural selection can occur with or without environmental change . It is of three types:- Directional Selection Stabilizing Selection Disruptive Selection

6. Directional Selection The first type of natural selection is called directional selection. It derives its name from the shape of the approximate bell curve that is produced when all individuals' traits are plotted. Instead of the bell curve falling directly in the middle of the axes on which they are plotted, it skews either to the left or the right by varying degrees. Hence, it has moved one direction or the other.

7. Directional selection curves are most often seen when one colouring is favoured over another for a species. E.g. If thicker-shelled oysters are more resistant to breakage than thinner-shelled oysters, crabs will be less able to prey upon them, and thicker-shelled oysters will be more likely to survive to reproduce.

8. Stabilizing Selection The most common of the types of natural selection is stabilizing selection. In stabilizing selection, the median phenotype is the one selected for during natural selection ( favors the intermediate states of continuous variation). This does not skew the bell curve in any way. Instead, it makes the peak of the bell curve even higher than what would be considered normal . Over time, the intermediate states become more common and each extreme variation will become less common or lost.

9. Continuing our oyster example, very light- colored or very dark- colored oysters might be more frequently preyed upon by shore birds, simply because they are more obvious on the oyster bar; as a result, the intermediate hues become more common. A classic example of this is human birth weight. Babies of low weight lose heat more quickly and get ill from infectious diseases more easily, whereas babies of large body weight are more difficult to deliver through the pelvis. Infants of a more medium weight survive much more often. For the larger or smaller babies, the baby mortality rate is much higher

10. Disruptive Selection Disruptive selection is also named for the way the bell curve skews when individuals are plotted on a graph. It occurs when natural selection favours both extremes of continuous variation. Over time, the two extreme variations will become more common and the intermediate states will be less common or lost. To disrupt means to break apart and that is what happens to the bell curve having one peak in the middle, disruptive selection's graph has two peaks with a valley in the middle of them . Disruptive selection can lead to two new species.

11. This might happen in shallow water among rocks. Light- colored oysters are more  cryptic (less easy for a predator to see) because they match the rock color . Dark- colored oysters blend into the shadows cast by the rocks. In this case, intermediate- colored oysters would be most heavily preyed upon by the crabs, and very light and very dark oysters would survive to reproduce.

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