The examples of directional and disruptive selections

Question 1

Directional and disruptive selections are both types of natural selection. Directional Selection takes place when an extreme phenotype is favored and the distribution curve frequency shifts towards the direction of the phenotype. The resistance to insecticides and antibiotics are some of the best examples of directional selection. As the use of antibiotics and pesticides increase, many populations of bacteria and insects develop resistance to the antibiotics and pesticides and pass these traits to the subsequent populations leading to directional selection. This is because the extreme phenotype leads to the resistance. Another good example is the anopheles mosquito and how it spreads malaria. When it comes to disruptive selection, the intermediate phenotype is bypassed and then two or more extreme phenotypes are favored ( Mader & Windelspecht. 2018) . This means that disruptive selection has a tendency of favoring polymorphism, which is the existence of two or more forms in a population of the same species. A good example is the British land snails, which inhabit both low vegetation areas and forests at the same time. 

Question 2

The antibiotics have an effect that sees them change the environment in which the bacteria lives. The extreme phenotypes in the pathogenic bacteria are the ones that are favored through the process of natural selection meaning that with time there is a resistance that occurs as the extreme phenotypes make part of the new form of the bacteria in a population of the species. Natural selection as a process enables the population to adapt to the biotic and abiotic environment it finds itself in. In this situation, the most individuals of the pathogenic bacteria population are the ones that will survive in the environment infested with antibiotics and when it comes to the population, it is only the pathogenic bacteria with the extreme phenotype which are most fit. It therefore follows that the extreme phenotype being resistant to antibiotics will eventually translate to a situation where the whole population is resistant. The better adaptation to the environment and the higher reproduction rate is what makes the pathogenic bacteria resist antibiotics through the process of natural selection ( Mader & Windelspecht, 2018) . 

Question 3

The major evolutionary trends that developed among major vertebrate groups, specifically those that allowed for the transition from aquatic to terrestrial life was the ability to develop a breathing mechanism that would draw oxygen direct from the natural air rather than draw the same from the water where they used to live. There was also the shedding off of fins as fins could not easily make it for movement on the terrestrial front allowing them to develop hind limbs. The fish walked on land and could swim in water by flexing the backbone and paddling with the hind limbs. The development of the amniotic eggs was yet another important milestone in the evolution of the vertebrates from being aquatic to becoming terrestrial. The ability of the eggs to survive away from a water point that was stagnant was a major evolutionary trend. The adaptation of the amniotic eggs to the rigor that come with the physical situation of laying the eggs out of the water allowed the shift from aquatic to fully terrestrial in many ways as the new organisms would technically be hatched on land and not water. Some snakes such as the python have developed the vestigial hind limbs that allow them to move in terrestrial environment unlike in the past where movement was purely due to muscular contractions and as such was easy in water as many snakes indicate today. 

Question 4

Sexual reproduction in plants has evolved to become less dependent on water because there are many seeds, which are dispersed with the help of animals and wind. These plants are light in terms of weight to allow them to be carried by the wind while some attach to the animals and in the process get carried to their new destinations. It is also possible that the seeds are eaten by some animals and later excreted in the places where they undergo sexual reproduction thus making no use of water. There are also those plants whose male and female sporangia develop within the same stem such as spike moss, Salaginella ( Fowler et.al. 2018). Such developments means that plants no longer rely on water for sexual reproduction. Initially the gametophyte stages of plant reproduction were dependent on water for reproduction. The flagellated sperm would swim in a film of water in order to reach the egg. Upon the fertilization of the egg, the zygote formed would become the embryo and develop into a sporophyte. The photosynthetic gametophytes would then nourish the sporophyte and later produce spores. The spores are then released in air and dispersed by wind and when they land on a moist environment, they then germinate. Upon germination, they would then produce male and female gametophytes and thus adapting to the life in land. 

References 

Bonner, J. T. (1988).  The evolution of complexity by means of natural selection . Princeton, NJ: Princeton University Press. 

Fowler, S., Roush, R., & Wise, J. (2018).  Concepts of Biology . Samurai Media Limited. 

Mader, S. S. & Windelspecht, M. (2018). Essentials of Biology . (5 th ed.). New, NY: McGraw Hill Education.