Environmental Science · Science

Insects, Fish, Reptiles, and Birds


The process of evolution is crucial to study and understand the evolution mechanism. To understand this evolutionary process, biologists and paleontologists started to explore the anatomical structure and morphological processes of each species. The most prominent species we need to study are insects, fish, reptile, and birds. Insects are different fish, reptiles, and birds as it is a class of phylum Arthropod. However, before going into details in those species, we need to understand the morphology and anatomy of the chordates. We also need to relate these characteristics to fish, reptiles and birds. It will help us to understand the evolutionary processes that are related to the morphological processes and anatomical changes of each species. Finally, determining the factors that affect the evolutionary processes are vital to understanding.

Anatomical Structure and Morphological Processes

Anatomical structure and morphological processes of insects, fish, reptiles, and birds are different. However, fish, reptiles, and birds are too close as they are classes of Subphylum Chordata. It is vital to discuss each class separately to explain their features, anatomical structure, morphological processes, and their origins adequately.


The insect is a class under phylum Arthropod. Its body is hard, segmented, and covered by a hard cuticle. It is also an exoskeleton contains chitin and protein layers. Many of the segments have appendages to allow the insect to move, handling food and reproduce. The body includes the head, thorax, and abdomen. The sensory organs are developed in insects. They have many sensory organs such as organs of sight, balance, touch, smell, and hearing. Insects have two types of eyes; compound and simple (Cornell University, 2016). Compound eyes have several ommatidia – visual units, and simple eyes that make the insect only able to distinguish light from dark. The brain is developed in insects. It has two or three ganglia that are connected to many ventral nerve ganglia. The most important features of insects that they have two pairs of wings, six legs, and two antennae. They have a pair of antenna that used to determine vibration, food, and other environmental pheromones. They are vital features, I believe, that help insect to survive. However, the mere structure of the insect’s body minimizes their chances in a food competition.

Biology Robert Brooker.JPG

Brooker, R. (2011). Biology. New York: McGraw-Hill.

Insects have unique morphological changes during their life cycle. These changes called a metamorphosis process. The earliest insects did not metamorphose as they hatched from the eggs the juveniles resemble adults. However, between 280 and 300 million years ago, some insects began to produce embryos that are different than the adults (Jabr, 2012). Some scientists argued that the adult raised from imaginal discs– the first form that embryo develops in the eggs. I would like to maintain that not all insects do the complete metamorphosis. Some species conduct incomplete metamorphosis, as in the silverfish and bristletails, they undergo no metamorphosis. The complete metamorphosis in insects includes egg, larva, pupa, and adult (Pacific Science Center, 2016). The egg is the embryo stage. Larva hatches from the egg. Its shape is different than adults. The exoskeleton of larval cannot stretch to grow up. The pupa is a transforming stage. In this stage, new organs and muscles are developing. The adult is the reproductive stage, and it has all insect’s features.

Fish, Reptiles, and Birds

They are classes of the phylum Chordata. It includes three subphyla; Urochordata (Sea Squirts), Cephalochordata (the lancelets), and Vertebrata (the vertebrates). Fish, reptiles, and birds are species under subphyla Vertebrata. All chordates have a notochord – a long, semirigid, rod-like structure that exists between the digestive system and the dorsal hollow nerve chord (Prior, 2014). In vertebrates, the backbone replaced the notochord, such as in reptiles, and birds. The primary function of it is to fasten body muscles. It adapted in the bony fish, as vertebral column replaced it. Chordates have dorsal hollow nerve cord. It is a tubular nerve cord, filled with fluid. It exists above the notochord and outside the coelom and extend along the body length. The nerve cord anterior region forms brain which is located in a bony cranium. The outer part of the nerve cord change to the spinal cord. Pharyngeal slits are a series of longitudinal openings on the wall of the pharynx. In fish, gills are located near to the pharyngeal slits. It exists in vertebrates in only the embryonic stages.  The function of pharyngeal slits is feeding the primitive chordates. Post-anal Tail is a posterior elongation of the body extending beyond the anus (Kardong, 2012). This is evident in the fish species as it is used for swimming.

The adaptive strategies of the chordates can be seen in the vertebrates that are evolved from invertebrates. They have a well-developed head, the endoskeleton that helps species to grow continuously and provide a stable anchorage to muscles. The pharynx makes holes by slits that are used for respiration. They also have a complex nervous system includes brain and spinal cord. In my opinion, the most important adaptive strategy is the pharyngeal slits. They are very different in fish than in reptiles and birds. This is a vital sign or evolutionary processes among chordates. It proves Darwin’s theory of natural selection.


Fish species have a musculoskeletal system. The notochord here is developed to form the backbone that capable of fusing muscles together. The body shape of the fish is a fusiform shape. It helps fish to swim quickly in the water. It also helps the body to bend and create a propulsive force to forward movement. The cranium articulates the bones to support the walls of the body.  Fins are imperative in fish. It helps fish to maneuver. Fish is covered by mucus that is important to prevent skin infection and the formation of ulcers.  It also used to improve the movement of the fish by reducing the frictional drag (Aspinall, and Cappello, 2004). Some fish species has special sense, such as Teleost fish. They have organs of vision, smell, hearing and balance. This is an evidence of the evolution of fishes. As Darwin’s theory of Natural selection, this species modified their organs to adapt to the surrounding environment. They also have a lateral line or notochord. This helps fish to detect the prey quickly in the water and avoid a predator.


Kardong, K. (2012). Vertebrates. New York: McGraw-Hill.

Fish morphological changes are divided into five stages of the life cycle; embryonic phase, larval phase, fry phase, ripe phase, and senescent phase (Aral, Şahınöz and Doğu, 2011). Embryonic phase, the embryo is produced with head and front section of the body. At the end of this stage, tail come out to push embryo out the egg shell. In larval phase, there are two stages; pre-larval and post-larval stages. In the pre-larval stage, the head is small; the eyes are big and without pigment and the nostrils are not developed. At the end of this stage, anus and mouth start to open. The pectoral fins start to appear during this phase. In post-larval phase, some organs form to help larva to cope with the pelagic environment. In fry phase, the developing of scales and a lateral line starts, the circulatory system starts to form, and hemoglobin starts to develop during metamorphosis. The fish gonads are fully formed in ripe phase.

Along with the earth history, fish shows the significant evolutionary processes of their morphology. During late Cambrian, 500 million years ago, ostracoderm, the jawless fishes, were the dominant vertebrates (Biggs et al., 2004). The jawed fish (Gnathostomes) evolved from jawless species (Agnatha) after the extinction of Jawless species. The evolution of jawed mouth is accompanied by the development of head and body that became sophisticated. The hinged jaw evolved from pharyngeal arches, and hard rods. The first and second-gill arches are modified and formed the jaws. I think, this is a vital change in the evolutionary lineage of the fish. However, I do not understand how suddenly the first and second-gill arches are modified. I believe it might be a result of changing to a fish diet.


Reptiles body is bilaterally symmetrical, segmented and coelomates. During their development, they have a notochord, dorsal hollow nerve cord, pharyngeal gills, and a post-anal tail. The skeleton consists of five vertebral structures. They are atlas, un-ribbed neck followed by 100 to 600 mid body vertebrae and then the two to ten pre-pelvic and pelvic vertebrae (Laurin, and A. Gauthier, 2012). Reptiles have keratinized, thick skin that is covered by scale or horny plates, such as in snakes. The skin of reptiles enables them to handle moisture and reduce the loss of water.

The embryo of the reptile is developed in an amniotic egg. It contains the embryo and four extra-embryonic membranes: the amnion, allantois, yolk sac, and chorion. The amnion is surrounding the embryo to protect it. The allantois provides for gas diffusion while yolk sac provides food for the embryo. The chorine provides an enclosure for all membrane. The egg structure helps in preventing injury and dehydration of the embryo. The reptile embryo feature and morphology are the same of the adult. The amphibians are gradually evolved to the reptiles. The early reptiles conducted a backbone structure. The structure of the bone in the temporal opening where the reptile’s ears are located near to the skull (Mattison, 2014). The synapsid reptiles are found to have one temporal opening under the postorbital and squamosal bones. The diapsid reptile has two openings existed above each other’s. The euryapsid has one opening above the postorbital and squamosal bones (Florida International University, 2016).

Amnoit Egg.jpg

Biggs, A., Hagins, W., Kapicka, C., Lundgren, L., Rillero, P., Tallman, K. and Zike, D. (2004). Biology. New York, N.Y.: Glencoe/McGraw-Hill.


The skeleton of the bird is light. The cortex of the bone is thin, and the leg bones are hollow. Birds have large pneumatic bones that contain air sacs that are connected to the respiratory system. This help to reduce the weight of its skeleton. There are pectoral muscles that help birds in down stroke, and supracoracoideus muscles that are responsible for the up-stroke. They have only one body cavity with no diaphragm. Birds have little numbers of joints to support the flight muscles. The head of the birds is lightweight as the beak that covers the mandible replaces the teeth. Beak shape is different of the bird species as it depends on its eating behavior. Leg and foot bones are fused to form a tibiotarsus and a tarsometatarsus (Kotpal, 2010). Feet shape depends on the lifestyle of the bird. Duck, for example, have webbed feet. Birds have different shapes and sizes of wings. The long, narrow wings are adapted for soaring on an updraft, and the short wide wings are used for a rapid flight between trees.

The embryo development of the chick undergoes different stages. After incubation, the embryo grows very fast, the primitive streak developed, the head and backbone of the embryo start to appear. The formation of the precursor of the digestive tract takes place, and the eye begins to form. By the fourth day, all embryo organs are shaped and identified (Mississippi State University, 2016). The beak forms in the sixth day. On the seventh day, the heart enclosed entirely in the thoracic cavity. Feathers and feather tracts start to appear on the tenth day. By the fourteenth day, the formation of claws existed. The embryo takes the position for hatching.

Most scientists argued that the today’s birds are evolved from an evolutionary line of dinosaurs. I totally agreed with this argument. I would maintain that the Archaeopteryx is the ancestor of birds. The reasons for this belief are that the Archaeopteryx had feathers and wings. It also had a tail, teeth and front toes that are clawed.  Birds and Archaeopteryx have flexible wrists and three fingers on each hand. From fossil records, in China, the fossil that is found prove that birds have evolved from a therapod dinosaurs. It had feather as well (Evolution.berkeley.edu, 2016). Personally, I have doubt regarding the fossil that is found in China. This theropod dinosaur is flightless, it might not proof that this species is the ancestor of birds. I can say that it might be a transition species.


Evolution.berkeley.edu. (2016). The origin of birds. [online] Available at: http://evolution.berkeley.edu/evolibrary/article/evograms_06 [Accessed 15 Mar. 2016].

Survival Strategies in each species

The table blow explains the different survival strategies in insects, fish, reptiles, and birds.

Insects Fish Reptiles Birds
Camouflage. They can change their skin color to match the surrounding color. Camouflage is for attack and defense. Flounder mix with their background. Camouflage by countershading. Many Salamanders have different body colors on their abdomen and back. Camouflage by hiding in the dead leaves or inside the forest floor.
The embryo takes a short time to grow. Deception is a perfect strategy. Juvenile Shaded Batfish has a black body and orange fin that make it resemble flatworms. Brumation is used by reptiles to avoid cold temperatures. American Bittern can hide with cattail. It has vertical strips of its breast and can be blended with cattail.
Mouthparts have specialized for chewing, such as grasshopper. Living in schools is one of the most important strategies. They gather and act as one unit to avoid predators. Some species has a gland that can eject toxins to defeat themselves. Birds have the ability to fly high to avoid on-land predators.
The reproduction rate is high. This will increase the possibility of survival Venomous Defense: Some scorpionfish have spines in the dorsal fins. Rufous-sided Towhee can avoid aerial predators by building its nests on the floor of the forests. However, it will not help it to avoid ground predators.

Factors Affecting the Evolutionary Processes

Several factors that influence the evolution of any species. One of the most important factors, I believe, is the genetic variation within a population.  It can be provided from the random mutation in DNA sequence.  DNA replication can obtain the variation as nucleotide base can be inserted in the DNA sequence, deleted or substituted. Some of the environmental factors might affect the genetic variation. The UV light, radiation, pollutants and chemicals can affect the random mutation rate. Natural selection is another important factor in evolution (The University of Waikato, 2008). The species which can adapt to the environmental changes and can produce offspring that can adapt too, they can be survived and evolved to the changing environment. When the environmental conditions changed, gradually, the favorable gene will dominate in a population while the less desirable gene will decline. Sexual recombination will allow the gene from parents to recombine and produce new offspring with the new combination. It can increase the genetic variation by crossing over, independent assemblage and fertilization. Finally, gene flow is one of the factors that can affect the evolution process.  It is the simplest mechanism of the evolution (Swaddle and Duffy, 2016). It depends on the movement of individuals from one population to another moving its gene among the new population. I agree with the idea that the high levels of gene flow within a population will lead to being a homogenizing force. I consider the natural selection and the genetic variation the most important factors that may affect the evolution of a species.

Climate change, habitat change, and human interactions have great impacts on the evolution process (Templeton et al., 2001). The impact of climate change is to affect the natural selection process. If the species is unable to cope with the environmental change, it will not be able to transfer its gene to its offspring, or its offspring will not be able to survive. Several examples can explain this issue. If a species can survive and reproduce at a higher temperature than other species. This species has a heat tolerance character. Once the temperature increases, it will be able to breed and produce offspring that will carry the gene for heat tolerance. Habitat change is another factor that can impact the evolution. It might happen by the introduction of a species into a new habitat. The species can go into two pathways; it can adapt to the threats that exist in the new habitat, such as predators or food competition, or it will not be able to cope and extinct.   Habitat change can be a result of climate change. The increasing in the weather temperature will increase the meltdown of glaciers on poles. This will alter the habitat of the polar ecosystem (Mergeay and Santamaria, 2012). Human interaction, in my opinion, plays a vital role in the evolution processes. It also has cons and pros. Human can help species to evolve and cope with different factors that can affect the evolution, by genetic engineering. As a scientist, if I have the genetic information of a threatening species, I can work to modify his genetic sequence to make it better cope with the changing environment. This is not easy as the scientist can mistakenly deform the DNA sequence of this species. Also, human interaction help increases the pollution rate on earth. The pollution may affect the fertility of some species that may lead to the extinction of this species.


Different types of species have been studied to understand their anatomical structure and their morphological processes. Each species has its unique structure that has various functions to adapt to the habitat. Species of fish, reptiles, and birds have common chordates characteristics. However, some of these features are changed to adapt to surrounding environment. Each species has different embryonic development, as the morphology and physiology of embryo changed from egg to adult showing relevant evolutionary processes. The evolution process is a very complicated process that is controlled by many factors such as genetic variation, natural selection, and sexual recombination. Climate change, habitat change, and human activities have a great impact on the evolutionary processes. Mostly, it is an adverse consequence.


  1. Aral, F., Şahınöz, E. and Doğu, Z. (2011). Embryonic and Larval Development of Freshwater Fish. INTECH Open Access Publisher.
  2. Aspinall, V. and Cappello, M. (2004). Introduction to veterinary anatomy and physiology. Edinburgh: Butterworth-Heinemann.
  3. Bergman, J. (2004). Insect evolution: a major problem for Darwinism. Journal of Creation, 18(2), pp.91–97.
  4. Biggs, A., Hagins, W., Kapicka, C., Lundgren, L., Rillero, P., Tallman, K. and Zike, D. (2004). Biology. New York, N.Y.: Glencoe/McGraw-Hill.
  5. Brooker, R. (2011). Biology. New York: McGraw-Hill.
  6. Chandran, A., Rani, G. and Sekar, T. (2007). ZOOLOGY HIGHER SECONDARY – FIRST YEAR. 2nd ed. Tamilnadu: Government of Tamilnadu.
  7. Cornell University. (2016). Insect Biology : A Primer. [online] Available at: http://www.biocontrol.entomology.cornell.edu/bio.php#growth [Accessed 20 Mar. 2016].
  8. berkeley.edu. (2016). The origin of birds. [online] Available at: http://evolution.berkeley.edu/evolibrary/article/evograms_06 [Accessed 15 Mar. 2016].
  9. Florida International University. (2016). The History of Life – Reptiles. [online] Available at: http://www2.fiu.edu/~longoria/gly1101/Mz-reptiles.htm [Accessed 22 Mar. 2016].
  10. Jabr, F. (2012). How Did Insect Metamorphosis Evolve?. Scientific American, (Volume 307, Issue 2).
  11. Jordan, E. and Verma, P. (2013). Chordate Zoology. 14th ed. Delhi: S. Chanda & Co.
  12. Jurd, R. (2004). Instant notes animal biology. New York: Bios Scientific Publishers.
  13. Kaiser, G. (2007). The inner bird. Vancouver [B.C.]: UBC Press.
  14. Kardong, K. (2012). Vertebrates. New York: McGraw-Hill.
  15. Kotpal, R. (2010). Modern Text Book of Zoology Vertebrates. New Delhi: Global Media Publications.
  16. Laurin, M. and A. Gauthier, J. (2012). Mammals, reptiles (turtles, lizards, Sphenodon, crocodiles, birds) and their extinct relatives.. [online] The Tree of Life Web Project. Available at: http://eol.org/pages/3014446/details [Accessed 21 Mar. 2016].
  17. Mattison, C. (2014). Snakes and other reptiles and amphibians. , New Yor: Dorling Kindersley Limited.
  18. Mergeay, J. and Santamaria, L. (2012). Evolution and Biodiversity: the evolutionary basis of biodiversity and its potential for adaptation to global change. Evolutionary Applications, 5(2), pp.103-106.
  19. Mississippi State University. (2016). Stages in chick embryo development. [online] Available at: http://msucares.com/poultry/reproductions/poultry_chicks_embryo.html [Accessed 26 Mar. 2016].
  20. Pacific Science Center. (2016). Metamorphosis. [online] Available at: http://exhibits.pacsci.org/insects/metamorphosis.html [Accessed 29 Mar. 2016].
  21. Prior, R. (2014). Chordates (Biology-Study-Guides). Kindle Edition. 4th ed. Prior Educational Resources Ltd.
  22. Swaddle, J. and Duffy, J. (2016). Evolution. [online] Encyclopedia of Earth. Available at: http://www.eoearth.org/view/article/152700/ [Accessed 23 Mar. 2016].
  23. Templeton, A., Robertson, R., Brisson, J. and Strasburg, J. (2001). Disrupting evolutionary processes: The effect of habitat fragmentation on collared lizards in the Missouri Ozarks. Proceedings of the National Academy of Sciences, 98(10), pp.5426-5432.
  24. The University of Waikato. (2008). Factors that influence evolution | Biotech Learning Hub. [online] Available at: http://biotechlearn.org.nz/focus_stories/evolved_enzymes/factors_that_influence_evolution [Accessed 27 Mar. 2016].
  25. University of California Television (UCTV), (2015). Human-Climate Interactions and Evolution: Past and Future. Available at: https://www.youtube.com/watch?v=nLmCbBVq0xM [Accessed 27 Mar. 2016].



Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s