Environmental Science · Science

An Introduction to Zoology


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 (Evolution.berkeley.edu, 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) (Anthro.palomar.edu, 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.
Prokaryote Eukaryote
Location Inside the cytoplasm Inside the nucleus
Origin Two origins per DNA Molecule.  Ranging from 100-200 or more nucleotides. Several ancestors formed about 150 nucleotides.
Initiation At one point carried out by protein DNaA & DNaB Replication occurs at several points, conducted by many proteins.
Stages of Division Only two replication forks Many replications forks
No. of Replicons One Numerous in one replicating DNA (50,000 and above)
Okazaki Fragments Large (1000-2000) nucleotides long Short (100-200) nucleotides long
Speed of replication Very fast and rapid. Slow
Termination Single termination site Several termination sites.
Direction Bi-directional Process Bi-directional Process
Primers Required Required
Strands Leading and lagging Leading and lagging

 Homeostatic Mechanism

Homeostatic is a process of stabilizing water, minerals and other components’ levels of animal body fluids and other thermal regulations. This mechanism is unique in the animal species; as the organism can maintain it is physiological and behavioral stability in different environmental conditions. There two mechanisms that animal uses; Conforming and Regulation (Pond and Bell, 2005). Conforming is a process that an animal adapts to their environment so their internal body composition will match with external conditions surrounding. For example, the concentration of the marine crab body fluid is the same in the surrounding seawater. Regulation is a mechanism that an animal regulates the internal fluid composition that is different from external environmental conditions. For example, vertebrates are said to be regulators. I believe that the regulation mechanism is better than the conforming. I would maintain that the conforming is a cheap mechanism for survival. It needs an enormous amount of energy for a small organism to adjust its body temperature.

Level of Organization: Architectural Patterns in Animals

There are five major levels of Organization; Protoplasmic Level, Cellular Level, Cell-Tissue Level, Tissue- Organ Level, and Organ-System Level (Biggs, 2004). Protoplasmic Level is found in the unicellular and protozoa organisms. All life activities are limited within the cell boundaries. The organelles can perform special functions. Cellular Level is that the organism is an accumulation of cells. Those cells perform different functions. Each cell is specialized for a particular function, for example, reproduction. Cell-Tissue Level is the accumulation of same cells into a specific pattern to become a tissue, for instance, Jellyfish and Cnidaria. Tissue-Organ Level is the gathering of the tissues in an organ. Different kind of tissues is forming the organ. Platyhelminthes is the first organisms that well-defined organs are first appeared, such as eyespots. Organ-System Level, is all organs work together to perform many functions, such as circulatory system and nervous system. I would like to maintain that it is a vital strategy to understand the level of organization of organisms. I believe it will help to understand how animals’ body work.

The study of the architectural pattern is vital to comprehend the sentience and survival strategies of an animal. I would like to discuss the Nervous system and how, I believe, that this evidence is imperative to proof the theory of architectural pattern. The nervous tissue is responsible for initiating and conducting electrical signals in animals’ body. The tissue consists of several nerve cell – neurons, all are connected. The spinal cord is the nervous tissue that transfers the electrical signals from the brain to the body cells and vice versa. Nervous tissue plays a critical role in controlling the activities of the body cells. The sentience characteristics of the animal help a most important role in animal’s survival. When an electrical signal generated in one neuron, it will inhibit other neurons to produce new electrical signals. These signals may stimulate muscles tissues to contract, or catalyze the glandular cells to release some chemical substances into the animals’ body fluid.

In above example, we can see that all body’s cells, tissues and organs response to the one external activity or thread. The nervous system consists of neurons which are the nervous cells. All neurons accumulate to form nervous tissue. There are two types of nervous cells in the nervous cells; neurons – nerve, and neuroglia. The neuron conducts the basic function of the nervous system while neuroglia is different non-nervous cells that cover the neuron membranes and serve various functions (Brooker, 2011). This classification is critical to understand the theory of level of organization. When I look at the types of nerve, I will see that there are different kinds according to the functions and number of cell processes. According to the function, there are sensory cells- conduct impulses toward central nervous system (CNS), motor – send signals away from CNS, and associate – that initiate signals within the CNS (Jurd and Jurd, 2004). According to the number of cell processes, we can find that there is Bipolar, Unipolar, Multipolar and pyramidal cells.


Jurd, R. and Jurd, R. (2004). Instant notes animal biology. New York: Bios Scientific Publishers.

All above evidence is imperative to understand how the architectural pattern works, and significant is this. We can do the same analysis for all different body systems, for example, reproductive system, circulation system, and respiratory system. Some people may argue that each system is different and may have more complex levels. I can partially agree with this idea, but on the other hand, it is very clear that all body levels of organization are the same in the complex animals.

Modern Evolutionary Theory

Charlies Darwin, 1859 has discovered that all known animals are related by descent from common ancestors (Darwin, Einstein, and Smith, 1859). This theory is what Darwin stated and discovered when he invented the Theory of Evolution. Darwin argued that the evolution may happen from generation to another due to genetic variation and natural selection. The Genetic Variation occurs among individuals of a given species. The genetic characteristic passes from parents to offspring. The Natural Selection is a process that in each generation, offspring may survive and reproduce or die. It is according to how they will adapt and cope with environmental changes (Brooker, 2011). I can say that I fully agreed with the Natural Selection theory, as it can generate new species with a combination of several traits that may be different from those of the origin species. Finche species of larger beaks and different color are an example.

Despite Darwin’s theory is a milestone in the science of evolution and biology, many people argue that it not be true. I can say that his theory has made considerable implications. His theory has a significant impact on the genetic science. By Darwin theory, scientists recognized that the genetic structure of populations is very complex. I also can argue that Darwin influenced the medical field. His theory helped to understand the history of the human disease and health. It also contributes to predicting the human resistance to pathogens. Darwinian theory has religious implications. Let me tell you a personal story. Couple weeks ago, I was discussing the theory of evolution with one of my colleagues from the science point of view. He was arguing that this theory is against his beliefs. He believes that God, who create life and there is no evolution. This discussion is not a one-to-one discussion. I think this debate has been alive until today. Darwin states that complex organisms like human beings evolve from less complex living beings (Dinis, 2010). This process took place through the mechanism of natural selection. Church and some Mosques, violently attacked Darwin’s ideas as they think that his ideas are against the Role of God. The most difficult to accept was the claim of the human, and the apes have common ancestors. This claim led some Christians in the U.S. during the 1920s to ban the teaching of Darwin’s theory in the Government Schools (G, 2014). The U.S. Supreme Court repeal the ban because of the constitution’s grant of religious freedom. I believe that Darwinian Theory is a magnificent and unique theory. We have to separate the scientific point of view from religious. Religious books are not scientific books; they are only a guide of life. However, we cannot connect the scientific knowledge with religious as some people are not religious at all.

The environmental changes influence phenotype and inheritance patterns. There are several evidence that can be considered as a proof of the impact of environmental changes on living organisms. We can find that environment can impact the phenotype of organisms. Poor diet as an example of an environmental factor can affect human height. For example, if someone has inherited genes to be tall has an improper diet, it might not reach their expected height (Monaghan, 2008). Phenotypic plasticity or genetic change may result in a phenotypic change. For instance, Teplisky et al. (2008) provided evidence that climate-driven plastic decreases in the body size of red-billed gulls ‘Laris novaehollardiae’ happened due to the environmental stress. (Merilä and Hendry, 2014). The environmental changes can affect the control of the development of organism’s genetic structure. When poor conditions of maternity exist, it will result in poor quality of offspring. The change of environment is very clear in the European Map butterfly ‘Araschnia levana’. This species can have different color forms in different environmental temperature and photoperiod. Personally, I can say that the changes in the environment can affect the phenotype and inheritance pattern of an organism. I would like to argue that, I do not think that the poor diet may influence the human inheritance pattern. If an individual has inherited gene of tall, how can the poor diet affect that gene? I cannot find any scientific evidence on this debate.


Theory of evolution can be divided into the pre-Darwinian period, Darwin Theory of evolution and post-Darwinian period. The theories of the pre-Darwinian period are inaccurate and had lots of implications. Study of the butterfly lifecycle is vital to understanding how different organisms grow and reproduce. Studying of lifecycle gave a very deep dive on how animals develop and reproduce. Cell division and mitosis have several steps and phases. One of those steps is DNA replication. The replication is different in Prokaryotes than that in Eukaryotes. As we have seen, the homeostatic mechanism exclusively exists in animals. It helps animals to adapt easily to various environments. It is very clear that the study of Levels of organization gave a deep insight on how life evolved from unicellular to a complex organism. It also gave an opportunity to understand how it helps in animal survival, for example, the importance of the nervous system in animal survival. Darwin’s theory made and still make a great debate between scientists and religious people. This debate should be fixed by separating the religion beliefs and science. Also, the phenotype and inheritance patterns may change by changing the environmental conditions. Many examples and strategies can prove this principle.


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