Question

Outline the major events in animal evolution/themes of animal evolution. Including types of body cavities and their descriptions, and give examples of animals that display these themes

Answer 1

Evidences supporting the evolution model come from multiple, distinct areas of biology and geology. These pieces of evidences can be grouped into four categories:

• Direct observation of evolutionary change
• Homology and Development
• Vestigial traits
• Fossil record and biogeography

The oldest animal fossils are about 630 million years old. By 500 million years ago, most modern phyla of animals had evolved. Figure attached shows when some of the major events in animal evolution took place.

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Direct observation of evolutionary change:

Small scale evolution can be observed in nature or generated experimentally in the laboratory. the classic story of the peppered moth in Britain during the industrial revolution is an exapmle of microevolution. Grant and Grant have recorded evolutionary change in beak shape within and among populations of Darwin's finches, over period as short as two years.

Homology and Development:

one of the compelling lines of evidence supporting the common ancestry of species is that of similar structural elements across functionally diverse forms. It becomes evident from comparative anatomy. We find the same bones in many different types of animals, but these bones are often modified to do different things. The hopping legs of the frog contain the same bones as our own legs, but the frog's legs are highly modified to fulfill a different function (hopping). The wing of a bird and the forelimb of a bat contain exactly the same bones as the arm of a human, but the size, shape, and even internal structure of these bones are all adapted to paly a different role in each animal. Bat and bird wings have the same function and the same origin (they are modified limbs) so they are analogous and homologous organs. Whale fans are a modification of the posterior limbs while fish fins although having the same function, do not come from modified limbs; so they are analogous but not homologous structure. The observations of structural homology support the casual hypothesis of evolutionary homology: that the common structural organization results from common ancestry. The evidence of analogous structure further supports the evolutionary homology concept, as they indicate that design is not necessarily canstrained (i.e. there is more than one way to build a functional part). In many cases, during embryonic development of organisms, the embryo exhibits characteristics of the embryos of its ancestors. For example, early in their development, human embryos possess gill slits like a fish, and later exhibit a tail, the vestige of which presents in adulthood as a coccyx at the end of the spine. These relict developmental forms suggest strongly that our development has evolved.

Vestigial structure:

Many species show structural or genetic features that serve no apparent function. Obvious example of these include the presence of pelvic bones in snakes or whales, rudimentary eyes in some cavefish species, presence of the nictitating membrane in human and the presence of pseudogenes throughout the genomes of eukaryotes. These structure suggest strongly that the species has changed over time, and descended from an ancestor with different ecological or genetic characteristics. A related phenomena suggesting descent with modification is evidence of poor engineering or inefficient design in sme characteristics of organisms. For example, gene regulation is often a very inefficeint process with multiple (often conflicting) stages. These observations suggest that existing structures are often co- opted during evolutionary change to serve new functions.

Fossil record and biogeography:

The fossil record remains first and foremost among the databases that document changes in the past life on the earth. Sedimentary rocks are the richest sources of fossils. The fossil record shows that there have been great changes in the kinds of organisms that dominated life on the Earth at different points in time.

Biogeography is the science that explains the distribution of species, and higher taxa, on the surface of the Earth. Geographic distribution of species may be endemic, cosmopolitan or disjunct. The biogeographic distribution of species supports evolution. For example, approximately 2,000 species of flies belonging to the genus Drosophila are now found throughout the world. About one- quarter of them live only in Hawaii. Similarly, more than a thousand species of snails are other land mollusks are found only in Hawaii.