An Introduction to Zoology

Source: An Introduction to Zoology by Sameh Fouad


Many scientists invented the evolution of living organisms long time ago. Different theories have been discussed. Some of them are good, others are not accurate. The study of evolution had led to the study of the life cycle of organisms and the survival strategies that organisms follow in the changing environments. It is crucial to study mitosis and cell division processes in the animals and draw a comparison between replication processes in different species. It shows that there are several differences and little similarities in those species. Another an important mechanism that is unique in animal species is the homeostatic mechanism. Another area of interest that we need to discuss is the architectural patterns of the levels of organization of animal. The importance of understanding these patterns in the sentence and survival of organisms. The modern theory of evolution has several implications in several areas. The critical evaluation of these implications can help to understand the impact of this theory on our life. Finally, phenotype and inheritance patterns are influenced by the environmental changes.

Evolution Theories

Several theories of evolution related to the animals. I will discuss the most relevant theories that I believe. In 1801, John Baptise Lamarck (, 2016) proposed a theory of evolution. It states that the organisms change their behavior as the environment change. If organisms’ organs are used more frequently, it will develop in their lifetime. He also stated the theory of Use and Disuse. He argued that the giraffe, for example, had a short neck. It made the nerve fluid to flow into its neck and stretch it. This property would inherit in its offspring. He also invented the theory of Transition of Acquired Characteristics. I think this theory is not accurate for two reasons. Firstly, there is no evidence in the fossil records that there were different species of giraffe that has a short neck. Secondly, if we considered this theory, so the offspring of the swimmer will have long legs or stretched body.  Another theory that based on the catastrophic events on the Earth, George Cuvier (1769- 1832) (, 2016)argued that the significant events such as great floods, and the rapid formation of mountain chains have led to killing plant and animals that are lived during these periods. I partly agreed with this idea, as the fossil records showed some of the unconformities in the strata analysis, but again here there is no evidence that these events had killed all animals and plants. There is also no evidence that these events helped in the evolution of animals. Darwin Theory and Natural selection have been discussed as the most accurate theory that explains the evolution of animals. Some animals can survive a specific environment by natural selection. According to the hereditary characteristics of animals, they have different chances to survive. The number of offsprings and the survival of the offspring determines how big the next generation will be (G, 2014). I would maintain that I like the theory of natural selection and it is more accurate and reasonable rather than Lamarck’s and Cuvier’s theories.

The Life Cycle of Organisms

A life cycle can be defined as a description of stages that an individual organism passes through between the time of born until the date of producing offspring (Olson, 2016). The life cycle of an organism has many stages. It starts from the juvenile stage, as the organism grows and mature, then the reproductive stage in which the organism can produce offspring. Some organisms have different life cycles in order to adapt to survival. The life cycles of an organism can be simple such as in human as the morphological characteristics still the same in all stages. Another type of life cycle is complex. In this life cycle, the organism can change morphology, its environment, and diet as they transfer from a stage to another, for example, the monarch butterfly ‘Danaus plexippus’. The stages of the monarch butterfly life cycle are divided into four phases. The first stage is the Egg stage which is very small, oval, rounded or cylindrical. The second stage is the larva or caterpillar. It comes after the egg stage. The third stage is Pupa or Chrysalis. The caterpillar rapidly changes inside the chrysalis. The metamorphism process occurs in pupa as the old body parts of caterpillar undergoes a transformation process. The fourth stage is Adult Butterfly. I can say that this is a good example of the complex life cycle. The body of the butterfly changes in every stage. However, this strategy is only applied to butterfly. Some organisms have the same physiology in the embryonic stage until the mature stage.

Butterfly life cycle.JPG

Kalman, B. and Reiach, M. (2002). The life cycle of a butterfly. New York: Crabtree Pub. Co.

Adaptation Strategies

Animal survival in the desert is one of the most important adaptation strategies. In the desert, animals adapt their behavioral and physiological mechanism to overcome the heat and water issues. Birds, such as the Phainopepla, can breed during the cool periods of the year. It leaves the desert and move to higher cooler areas. Many animals restrict their activities in the day and start their activities in the cooler temperature of the night. Some animals retain water by occupying deep moist soil during the daylight hours. I like this type of adaptation, as it proofs that animals are intelligent, and they can keep their body water stable. It supports the theory of Darwin of Natural Selection. In polar regions, some animals hibernate, take shelter, or even migrate to warmer places (BBC Nature, 2016).  An excellent example of the survival strategies of animals can be found in Chameleon. It can change their skin color to avoid predators. However, I believe it does not have any strategy to adapt to climate or environmental changes. If the environment change, I guess, it might be threatening.

Mitosis and Cell Division in Animals

Mitosis is a process to split the cell nucleus into two nuclei.  It helps in the cell division processes. It divided into several phases. Interphase is the phase that occurs before the mitosis processes. It includes the G1 (first gap), S (Synthesis of DNA), G2 (Second Gap), and M phase (mitosis and cytokinesis). Prophase starts at the beginning of mitosis process. The chromosomes’ replication takes place. It produces 12 chromatids in 6 pairs of sister chromatids. The nuclear envelope begins to divide into small vesicles. In Prometaphase, the centrosome moves to the two poles of the cell. In the Metaphase, the sister pairs of chromatids gather in the center of the cell align along metaphase plan. In the Anaphase, the polar microtubules separate the chromatids and push the chromosomes to each pole. Finally, in the Telophase, the two cell daughters are formed, and the cell separate to produce two distinct nuclei. I guess, the mitosis process of animals is little similar to those in the plant, but I can argue that it is more complicated in animals than a plant.

There are some differences and little similarities between replication processes in different species. I will here compare between Prokaryotes and Eukaryotes as they are the two major classes of species (Stillman, 2008).


Biggs, A. (2004). Biology: The Dynamic of Life. 3rd ed. New York: Glencoe/McGraw-Hill.


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