TYPES OF NATURAL SELECTION

In the previous blog, I explained what natural selection is about. Today, I will delve into the different ways in which it is classified.

SELECTION BASED EFFECT ON A TRAIT

1. Stabilizing selection: This acts in such a way that the trait is held at a stable optimum. For example, tall plants in a forest are susceptible to being blow down by the wind; short plants are susceptible to competition for sunlight; so that compromise leads to the development of medium height plants.

2. Directional selection: This acts during transitional periods when the current mode of the trait is sub-optimal, and alters the trait towards a single new optimum. For example, the shortage of food at ground level made it difficult for the giraffe with short necks to survive. The development of long necks

3. Disruptive selection: This acts during transitional periods when the current mode is sub-optimal, but alters the trait in more than one direction. Example: If there are three types of plants in an area, tall, medium, and short, that are pollinated by three individuals. If the pollinator of the medium plant is no more there, the medium plant's population will decrease while that of the short and tall will remain. 

SELECTION BASED ON EFFECT ON ALLELE FREQUENCY

1. Positive selection: This effect leads to an increase frequency of an allele.

2. Negative selection: This effect leads to a decrease in the frequency of an allele.

SELECTION BASED ON EFFECT ON GENETIC DIVERSITY

1. Purifying selection: This acts to remove the genetic variation in a population.

2. Balancing selection: This acts to maintain the genetic variation in a population.

THEORIES OF EVOLUTION

Charles Robert Darwin was one of the founders of evolution. Other people who contributed to this were Jean-Baptiste Lamarck and Alfred Russel Wallace.

• Darwin Theory or Darwinism: This states that all species of organisms arise and develop through the natural selection of small, inherited variations (traits) that increase the individual's ability to compete, survive, and reproduce.

• Thomas Henry Huxley, for the purpose of economics, coined the word Darwinism in April 1860, in which he used it to illustrate the survival of the fittest in economic terms.

• Jean-Baptiste Lamarck was associated with the theory of inheritance of acquired characteristics (soft inheritance). He proposed the "Use and disuse of characteristics."

• Natural selection was coined by Alfred Russel Wallace and popularized jointly by Darwin in their joint publication in 1858.

• Natural selection is the process by which animals and plants with traits considered desirable by them are systematically favored for reproduction. 

• Survival of the fittest is central to natural selection. Individuals that are more fit have a better chance or potential for survival.

Examples of natural selection:

• The sickle cell anemias have  low fitness potential in the general population but the trait confers immunity to malaria. So in areas where malaria infection rates are high, it has a high fitness value in that population. 
• Finches of the Galapagos Islands differ in the shapes of their beaks. The beaks of over 13 species is suited to its preferred food, suggesting that the beak shapes evolved by natural selection.

Natural selection can act on any inheritable phenotypic traits. The selection pressure can come from any aspects of the environment. This include sexual selection where the female organism chooses which male it prefers to mate with, or competition with same or other species for food or space.

There are different types of natural selection which I will discuss in the next post. Stay tuned!

BENEFITS OF EXERCISE

Regular exercise helps improve the overall health. These benefits include:

• Stronger muscles
• Stronger bones
• Stronger joints
• Improved immune system with reduced risk of infectious diseases
• Reduced risk of pre-mature death
• Reduced risk of heart disease
• Improved psychological well-being
• Improved memory
• Improved weight loss management and reduced risk of obesity
• Lowering of cholesterol
• Reduced risk of certain cancers
• Lowering of blood pressure
• Reduced risk of stress-related disorders

THE FERTILIZATION PROCESS IN PLANTS

Pollination occurs when pollen is deposited on the stigma (in angiosperms) or the ovule (in gymnosperms ).

• Gymnosperms are a group of plants that share one common characteristic: they bear seeds, but their seeds do not develop within an ovary.
• Angiosperms are vascular seed plants in which the ovule (egg) is fertilized and develops into a seed in an enclosed hollow ovary. 

The pollen germinates through a weakening in the pollen wall by forming a slender pollen tube that elongates through its "tip extension" to penetrating between cells of the host parent. Within the pollen tube, two non-motile sperm cells are ultimately formed and are conveyed through the tube, keeping pace with tip growth. The pollen tube uses chemotropic signals to determine the final pathway to the egg cell, deep within the ovule. In angiosperms, pollen tubes penetrate the stigma, style, and ovary until they are amid the ovules. In gymnosperms, pollen germinates directly on the ovule. Pollen tubes enter ovules through a tiny pore called the micropyle and then elongate into the female gametophyte (called the embryo sac in angiosperms). In gymnosperms, the pollen tube directly penetrates the egg cell, but in angiosperms, there are sterile cells in the embryo sac, called synergids, that initially receive the sperm.

At this point, one sperm cell is discharged from the pollen tube and fuses, with the egg cell to form the zygote (the immediate fusion product) and subsequent embryo, which will become the offspring plant. In angiosperms, the second sperm fuses with the central cell to form a nutritive endosperm during double fertilization . The endosperm is needed for successful embryo development.