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Analyse Of Vertebrate Forelimbs

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Autor:  edward  16 December 2009
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1. Gather information from secondary sources (including photos/diagrams/models) to observe, analyse and compare the structure of vertebrate forelimbs.

Comparative anatomy is the study of the differences and similarities in structure between different organisms. An example of comparative anatomy is the pentadactyl limb which is the basic five-digit bone structure of many vertebrate’s forelimbs. The vertebrate forelimb (or the pentadactyl limb) comprises of the humerus (upper arm), radius and ulna (forearm), carpals (wrist), metacarpals (palm of hand), and phalanges (fingers).

The pentadactyl limb is evidence that present-day vertebrates (which include fish, birds, amphibians, reptiles and mammals) are closely related. It is believed they inherited this structure from an ancient aquatic ancestor, the lobe-finned fish. Due to natural selection, all vertebrate forelimbs, although have the same structure, are modified for very different uses. Therefore vertebrate forelimbs are homologous structures. Homologous structures are structures in different species that are similar because of common ancestry. They have the same basic structure but may appear somewhat different depending on the degree of modification produced by evolution.

One use for the vertebrate forelimb is swimming. Whales, otters and penguins have forelimbs which are modified to allow them to swim efficiently. The bones in whales are relatively compact and do not move within the flipper. This provides them with a strong ‘paddle’ which allows the whale to control the direction of their swimming. However, Otters have more flexible arms which allow for more movement. The metacarpals and phalanges are connected by skin which provides a webbed appearance. Penguins bone arrangement is only slightly different from a whale, although penguins are birds and their forelimbs were first adapted for flight and then swimming. Running is another use for the vertebrate forelimb, which can be seen in organisms such as horses, wolves and deer. Flight is a movement which requires special adaptations. Even though bats and birds forelimbs are suited for flying, they are quite different.

Forelimb structures of different vertebrates

2. Analyse information from secondary sources to prepare a case study to show how an environmental change can lead to change in species.

Evolution is a process of development, as from a simple to complex form or of gradual, progressive change as in social and economic structure. Whales have undergone many evolutionary changes over millions of years. They are large, intelligent aquatic mammals which belong to the Kingdom - Animalia, Phylum - Chordata, Class - Mammalia, Order - Cetacea, Suborder - Odontoceti, Family - Monodontidae, Genus - Delphinapterus and Species - Leucas. They breathe air through a blowhole into their lungs and have sleek, streamlined bodies that move easily through the water and are the only mammal besides manatees that live their entire life in water. They belong to the order Cetacea which is divided into the following groups: Toothed whales and Baleen whales. Toothed whales are called Odontoceti while Baleen whales are called Mysticeti. Primitive whales evolved from hoofed land mammals and may have evolved from shore-dwelling hyena-life Mesonychid that evolved into water living mammals. Whales may also have evolved from otter-like creatures which were approximately 10ft. long and 650lbs. This mammal had limbs that allowed it to swim but could also walk on land.

Our perceptions of whales are that they are like fish but whales are only like fish in that they live in water and have fins. A whale’s body contains a bone structure like mammals; its flipper has bones like a mammal’s forelimb and it is interesting to note the pentadactyl five digit bone structure (pictured below):-

Evolution occurs when natural selection causes changes in relative frequencies of alleles (gene determination) in the gene pool. Three examples of evolution which whales have experienced are
1) The change in a whale’s inner ear, 2) changes in a whale’s bone structure, and 3) the transition from legs to flippers for swimming. Sound transmission mechanisms change was an early whale evolution as the whale could hear both in and out of the water. However, eventually, the whale could hear only in the water. This was discovered by finding remains of some fossils. When the land mammals evolved into water mammals, the inner ears of whales evolved to help them be flexible swimmers and prevent them from becoming dizzy when they swam. The semicircular canals located in the inner ear and responsible for balance, helped whales adapt to aquatic life as they made the transition from land to water. The cause of this is change in the environment. Hearing needed to develop better due to the noise level boats created in the ocean as it made it difficult for the whales to hear one another sing subsequently decreasing their chances to find mates and food. Baleen whales sing low-frequency songs, while toothed whales emit whistles and clicks for echolocation. These songs are used to attract mates and keep track of their offspring.
When the ancestors of whales made the transition from land mammals to sea mammals their bone structure changed as they were able to run when on land and then swim when in water which changed their legs into flippers and fins. Marine whales have hand-like bone structure in their flippers resembling human hands.
The oldest known whale is the Himalayecetus subathuensis which is thought to be approximately 53.4 million years old. Fossils of this mammal were found in the Simla Hills of northern India as the area was underwater during the Tertiary period. It is thought that this whale may have spent time on land starting with the suborder, Pakicetus which has evolved into a bigger, aquatic mammal, Odontocetes. The reason that could have influenced such a change in bone structure was predation. Whales can swim fast and escape predators easier than when they lived on land and it was more effective for them to get food by swimming. Another influence for this change could have been the
competition for food. These once land walking mammals could have experienced food shortages therefore changing into water mammals increased the food supply. Moving from land to sea could have been the best way for these mammals to survive. There may have been a predator killing the Mesonychids mammal or some other cause which may have best suited the mammal to adapt to living in the ocean. The evolution of whales over time from Mesonychids to Odontocetes is an evident example of natural selection due to changes in environment. Whales contain human-like bone structure and vertebrae as well as breathe air. Physical links to land mammals such as having human-like bone structure, breathing air through lungs as well as having hair, help evolutionists explain the progression of the land mammal Mesonychids to the water mammal Odontocetes.
In conclusion, Evolution is a process of development from a simple form to a complex form or of gradual progressive change as in social and economic structure. It includes all the changes that have transformed life on Earth from its earliest beginnings to the diversity that characterises it today. Whales have undergone extreme evolutionary changes to make the transition from land mammals to sea mammals. The changes in their environment from land to sea and then the evolutionary adaptations to life in the oceans has altered each generation of whales over millions of years in order for their survival.

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