Evolutionary Trends in Vertebrates

1. Directional selection and disruptive selection

Directional selection is a type of natural selection that occurs when an extreme phenotype is favoured over the other phenotype, causing the allele to shift over time in the direction of that phenotype. For instance, fossil records show that the size of the black bears decreased in Europe during the interglacial periods of the ice ages, but increased during each glacial period. Another example is the beak size in the population of finches. On the other hand, disruptive selection occurs when two or more phenotypes are extreme, over the intermediate types. For example, a black rabbit with genotype BB will be well adapted to mountains with black rocks, and a white rabbit with genotype bb is well adapted to mountains with white rocks to evade predation. A grey rabbit with genotype Bb will be well adapted to both environments.

2. Pathogenic bacteria and antibiotic resistance

Pathogenic bacteria have become resistant to antibiotics due to natural selection. The resistance is due to the following conditions; pathogenic bacteria have variations, and the traits are inheritable. The genetic composition is varied between the individuals, and the unique traits are inherited to the next generation. The number of pathogenic bacteria becomes more or less stable with time than the environment can support. During bacteria infection, there is a rapid multiplication of new bacteria, therefore, becoming resistant and tolerant to the antibiotics. Bacteria adapt to the competition for resources such as water, nutrients, oxygen, and space for survival. The adaptation to competition makes the bacteria tolerant to antibiotics thus survive. Individuals with the traits that are suited for the environment will multiply and produce offspring that are suitable and very resistant to antibiotics.

3. Major evolutionary trends for the transition to terrestrial life

Major evolutionary trends that favored the move from aquatic life to terrestrial life among major vertebrate groups include the development of strong legs for movement on dry land. For instance, some vertebrates such as frogs developed strong hind legs for leaping. The vertebrate eggs are large and yolky, hence suitable for reproduction. The egg is self-contained and supplies all the necessary nutrients and other requirements for the survival of the embryo. The production of large numbers of eggs also increases the chances of survival on land. Gills alone could not be enough for respiration on land; therefore, the move from aquatic environments to terrestrial life needed more structures for gaseous exchange. Lungs developed for gaseous exchange in most vertebrates. Some have numerous structures; for instance, a frog uses lungs, buccal cavity, and skin for respiration. It favored survival on land. The complex digestive tract diversified their feeding habits, thus enabling terrestrial vertebrates to multiply.

4. Evolution of seeds and reproductive advantage

The evolution of seeds allowed plants to reproduce sexually in the absence of water. The presence of pollen grains enables seed dispersal to greater distances. For example, the lush palm does not rely on water for fertilization, pollen dispersal, and even survival of the zygote. Plants like fern, moss, and liverwort depend solely on water for reproduction. Seeds contain a diploid zygote that will grow into a sporophyte. Seeds have storage tissue that enables the growth and development of the new plant at an early stage, thus having an evolutionary advantage. Seeds also undergo a dormancy period induced by abscisic acid, and the seed remains inactive at a desiccated state until favorable conditions for germination prevail. The whole structure is transported to the female either by cross or self-pollination and is protected from damage because of desiccation. The development of fruits and flowers has promoted the dispersal of pollen and fertilization in flowering plants, thereby reducing the need for water in sexual reproduction.