Customer center

We are a boutique essay service, not a mass production custom writing factory. Let us create a perfect paper for you today!

Example research essay topic: Gene Pool Northern Hemisphere - 1,207 words

NOTE: Free essay sample provided on this page should be used for references or sample purposes only. The sample essay is available to anyone, so any direct quoting without mentioning the source will be considered plagiarism by schools, colleges and universities that use plagiarism detection software. To get a completely brand-new, plagiarism-free essay, please use our essay writing service.
One click instant price quote

Introduction It is unlikely that any family of fish has received as much attention as the Salmoninae. Many have a particularly heart-felt interest in the Salmoninae (trout, salmon, and char) because they love to fish. But man often destroys that which he loves, and the Salmoninae are no exception to this phenomenon. Exploitation of these fish has led to the extinction of certain species in some areas, but the attempt to preserve our valued fisheries has resulted in extensive research on the Salmoninae. This knowledge allows us to speculate on their origin and their prospects for the future.

Taxonomy It is believed that the Salmoninae are descendants of the Isospondyli order of fishes. This group, the most primitive of bony fishes are also called the teleosts and looked somewhat like today's tarpons and lemmings. The earliest fossils of isospondylous fishes have been found in Cretaceous rocks which means these forms were in existence well over 65 million years ago. The sub-order Salmonoidei is separated from other isospondylous fishes due to many similarities in structure. We find the existence of a second adipose fin which is not found in other fish, and scaled bodies but scaleless heads.

Also, the vertebrae of the tail are turned upwards. There are many other similarities such as similar jaws and oviducts. Of the six families of the sub-order Salmonoidei, the Salmoninae family distinguishes itself by having only one or two upturned tail vertebrae rather than three. Within the family Salmoninae, I am interested in the Salmoninae (trout, salmon, and char) rather than the Thymalinae (grayling) and Coregoninae (whitefishes). Before continuing, I would like to mention that there is a lot of confused nomenclature in this sub-family.

By common definition, salmon are anadromous. This means that they live and feed in the ocean and migrate up rivers to reproduce. Trout, on the other hand are said to be completely fresh water fish. Still, we find that some common names of fish go against this general rule. So we hear of anadromous trout, such as steelhead, and landlocked salmon. In the sub-family Salmoninae, there are seven genera.

Brachymystax is an Asian freshwater genus. Hugh is found in Eurasia. Salvelinus is circumpolar and is commonly called "char." Char have smaller scales than other Salmoninae and have light markings on top of a dark background rather than dark markings on a light background like the other Salmoninae, but are commonly referred to asw "lake trout." The genus Oncorhynchus is composed of the five species of Pacific salmon found from Formosa to California. The genus Salmo includes the Atlantic salmon and the brown trout which is native to Europe. Finally, the genus Parasalmo has only recently been distinguished from Salmo by some researchers. Parasalmo includes rainbow and cutthroat trout which are native to western North America and Kamchatka.

As we move from strict taxonomy to evolution, it is important to note that these divisions are based simply on morphology. According to Mayr, Linsley and Us inger (1953), the "essential property of genera is morphological distinctiveness (usually correlated with the occupation of distinctly different ecological niches). " But evolution does not move in only one direction. Morphological traits can be gained and lost again and again, which may render our efforts to construct phylogenist based on appearances alone guesswork at best. This is especially true with a group like the Salmoninae which are very plastic and can change their morphology rather quickly to take advantage of open niches. Our knowledge of the origins of the Salmoninae would increase greatly if extensive DNA analysis were undertaken to determine true evolutionary distances between species. Evolution Although there has been much research on the evolution and speciation of the Salmoninae, there is still a lot left unknown.

One reason for this is that their bones are rather soft and not conducive to fossilization for later study. Another is that the Salmoninae are extremely plastic. Their forms vary greatly with environmental pressures, thus a straight-forward evolutionary progression is not found. Today, significant morphological differences can be found not only in different groups of the same species, but even down to the individual. The taxonomy of this sub-family is under constant revision, with new subspecies being added frequently. Speciation among the Salmoninae is complicated even more by their many specific behaviors.

If one takes the modern definition of a species as a gene pool, these fish are very difficult to divide into species. First of all, salmon always return to the same place to breed. Although there is surely enough accidental straying of fish into other spawning grounds to consider multiple rivers to compare a single gene pool, there is a definite effect of divergence of characteristics of groups within a given gene pool. Furthermore, different groups of what appears to be one species will spawn at different times of the year in the same river, effectively separating gene pools. Lake trout will often inhabit their own depth of a lake, which separates them from each other genetically. We can assume, from the limited fossil evidence that we have, that the current genera of the Salmoninae family have been separate since at least the Pliocene.

In sediments dating back to the Pleistocene formed on the floor of Idaho Lake, a fossil was found with a pre maxillary bone identical to that of the present day genus Salmo. Furthermore, a fossil skull (below) was found dating back to the Pliocene that greatly resembles the present day genus Oncorhynchus. These two fossils tell us that these two lineages have been separate for at least 5 million years. Origin and Dispersal It is generally believed that the Salmoninae are of fresh-water origin.

Where an animal chooses to reproduce is usually a key to its origin. Just as the sea turtle that originated on land returns to beaches in order to reproduce, salmon return to fresh water to spawn. Trout and char live completely in freshwater although trout may occasionally venture into salt water to feed. Glaciation has greatly affected the evolution and dispersal of Salmonids. The present cycle of glaciation began when the continents drifted into such a position as to cut off the circulation of the Arctic ocean with the rest of the world's oceans.

Cold Arctic water could no longer exchange with warmer water from lower latitudes. Furthermore, the continents were now primarily in the northern hemisphere which created greater seasonality. Glaciers began to creep across North America and Eurasia. The presence of these immense ice caps meant the presence of large amounts of cold glacial runoff water in times of melting.

This was a new niche to be exploited by cold-loving fish. The Salmoninae love cold, swift, oxygenated water. Just as our ancestors changed morphologically to enter the expanding niche of savanna in Africa, the Salmonids changed to fill the expanding niche of cold, running, fresh water in the northern hemisphere. They spread over a huge area.

Natively, salmon swim in the oceans of the highest northerly latitudes, and trout are found as far south as 24 degrees latitude in the Rio del Presidio in Mexico. There was not such an abundance of this cold fresh water niche in the southern hemisphere because the Antarctic ice sheet extends right up to the ocean...


Free research essays on topics related to: northern hemisphere, fresh water, north america, gene pool, million years

Research essay sample on Gene Pool Northern Hemisphere

Writing service prices per page

  • $18.85 - in 14 days
  • $19.95 - in 3 days
  • $23.95 - within 48 hours
  • $26.95 - within 24 hours
  • $29.95 - within 12 hours
  • $34.95 - within 6 hours
  • $39.95 - within 3 hours
  • Calculate total price

Our guarantee

  • 100% money back guarantee
  • plagiarism-free authentic works
  • completely confidential service
  • timely revisions until completely satisfied
  • 24/7 customer support
  • payments protected by PayPal

Secure payment

With EssayChief you get

  • Strict plagiarism detection regulations
  • 300+ words per page
  • Times New Roman font 12 pts, double-spaced
  • FREE abstract, outline, bibliography
  • Money back guarantee for missed deadline
  • Round-the-clock customer support
  • Complete anonymity of all our clients
  • Custom essays
  • Writing service

EssayChief can handle your

  • essays, term papers
  • book and movie reports
  • Power Point presentations
  • annotated bibliographies
  • theses, dissertations
  • exam preparations
  • editing and proofreading of your texts
  • academic ghostwriting of any kind

Free essay samples

Browse essays by topic:

Stay with EssayChief! We offer 10% discount to all our return customers. Once you place your order you will receive an email with the password. You can use this password for unlimited period and you can share it with your friends!

Academic ghostwriting

About us

© 2002-2024 EssayChief.com