How can the habitat affect the evolution of an animal?
Investigación
For many people animals are perhaps the most familiar,
and most interesting, of living things. This may be because we are animals
ourselves. As such, we have a number of features in common with all the
organisms placed in the animal kingdom, and these common features indicate that
we have a shared evolutionary history.
All animals and plants are classified as multicellular
eukaryotes: their bodies are made up of large numbers of cells, and microscopic
inspection of these cells reveals that they contain a nucleus and a number of
other organelles. Compared to prokaryotic organisms such as bacteria, plants
and animals have a relatively recent evolutionary origin. DNA evidence suggests
that the first eukaryotes evolved from prokaryotes, between 2500 and 1000
million years ago. That is, eukaryotes as a taxon date from the Proterozoic
Era, the final Era of the Precambrian. Fossils of both simple unicellular and
more complex multicellular organisms are found in abundance in rocks from this
period of time. In fact, the name "Proterozoic" means "early
life".
Plants and animals both owe their origins to
endosymbiosis, a process where one cell ingests another, but for some reason
then fails to digest it. The evidence for this lies in the way their cells
function. Both plant and animal rely on structures called mitochondria to
release energy in their cells, using aerobic respiration to produce the
energy-carrying molecule ATP. There is considerable evidence that mitochondria
evolved from free-living aerobic bacteria: they are the size of bacterial
cells; they divide independently of the cell by binary fission; they have their
own genome in the form of a single circular DNA molecule; their ribosomes are
more similar to those of bacteria than to the ribosomes found in the eukaryote
cell's cytoplasm; and like chloroplasts they are enclosed by a double membrane
as would be expected if they derived from bacterial cells engulfed by another
cell.
Like the plants, animals evolved in the sea. And that
is where they remained for at least 600 million years. This is because, in the
absence of a protective ozone layer, the land was bathed in lethal levels of UV
radiation. Once photosynthesis had raised atmospheric oxygen levels high
enough, the ozone layer formed, meaning that it was then possible for living
things to venture onto the land.
The oldest fossil evidence of multicellular animals,
or metazoans, is burrows that appear to have been made by smooth, wormlike
organisms. Such trace fossils have been found in rocks from China, Canada, and
India, but they tell us little about the animals that made them apart from
their basic shape.
Lamarck's theory
Jean-Baptiste Lamarck was the first naturalist who
formulated a theory explaining the evolutionary processes. Presented in his
Zoological Philosophy, published in 1809.
We can
summarize the design of Lamarck in the following points:
• The influence of the medium. Environmental changes
cause new needs in organisms.
• Law of use
and disuse. Modified to adapt to the environment, agencies must modify the
degree of use of their organs. Continued use of a body produces growth (hence
the phrase: the function "create" the organ). A prolonged disuse
causes their decline.
• Law of
acquired characters. The changes "created" by the different degrees
of use of the organs are transmitted hereditarily. This means that eventually
the bodies will be developed very much used, while those who are not used tend
to disappear.
In short, according to Lamarck evolution is explained
by accumulation of acquired traits in the course of several generations.
Darwin's theory
Charles Robert
Darwin is the father of the current theory of evolution. His theory, outlined
in The Origin of Species (1859), relies on the following principles:
• There are
minor variations between organisms that are inherited.
• Agencies must
compete with each other for existence. In nature, more individuals are born
than can survive.
• Natural
selection: the variations that are better adapted to the environment are those
that will survive and thus more reproductive success, which are advantageous
not end up being eliminated.
• According to
Darwin, biological evolution is gradual and is due to selective accumulation of
favorable variations over many generations.
• The motor
considered Darwinian theory of evolution, adaptation to the environment arising
from the combined effect of natural selection and random mutations.
The synthetic theory
The synthetic
theory (also called neo-Darwinism) is fundamentally an enrichment of Darwinism
because of new discoveries in genetics. The main founders of this theory were
Dobzhansky, Mayr and Simpson.
'According to
the synthetic theory, the mechanisms of evolution are:
• Natural selection, as Darwin's theory.
• Mutations or random changes in genetic structure of
organisms.
• Genetic drift
or random process by which over several generations changed the genetic
structure of populations.
• Gene flow or
process by which populations become genetically uniform. "
The synthetic
theory is largely accepted theory by the scientific community. However, there
are alternative theories such as punctuated equilibrium theory of Stephen Jay
Gould (evolution theory conceived by leaps and not as a gradual process) or
Kimura's neutral (according to which the variations are neutral from the standpoint
their adaptive value).
The theory of evolution from an epistemological
perspective
Front who still
maintains that the theory of evolution is simply a set of hypothesis remains
unconfirmed, it is necessary to insist that it is a proven theory and is
supported by a range of materials that would be unintelligible without it. The
tests confirm that the materials come from various disciplines provided by
heterogeneous, such as paleontology, biogeography, comparative anatomy,
taxonomy, biochemistry, etc.. From the standpoint of the theory of categorical
closure, the truth of the theory of evolution is founded on the confluence of
these contents operative materials.
It is likely that animals were formed from a group
called heterotrophic choanoflagellates protoctista. Here are some single-celled
organisms whose morphology is reminiscent of the choanocytes, a type of cell
found in Porifera and handles food.
Some choanocytes form colonies. We think that the
precursor of the animals would be similar to one of these colonies.
From the common ancestor of all animals would form the
following three branches:
The sponges, which bear little relation to the rest of
the animals.
Of cnidarians and other animals associated with very
simple structure, such as ctenophores or comb the sea.
All other animals, from worms to vertebrates, with
bilateral symmetry.
Presumably the first soft-bodied animals would have no
shells or shells. Therefore, it is very difficult to find fossils that tell us
the start of the first steps in the evolution of animals.
Among the first fossils of animals included the
so-called "Ediacara fauna." This is a set of fossils about 600
million years old. Although it seems clear that they are animals, it is unclear
which group they belong. These may identify some like cnidarians, worms,
arthropods ...
In the evolution of animals found several important
milestones: first, the passage of sponges cnidarians. They already have
symmetry, but radial. Its cells are arranged as two different fabrics, an outer
epidermis and inner gastrodermis. Between them, they have a gelatinous coating.
They also have a diffuse nerve network.
Another important milestone was the emergence of the
first flat worms with bilateral symmetry and a third layer of cells, the mesoderm.
The presence of bilateral symmetry defines an anterior
pole, which begin to concentrate the nerve cells and sensory organs to form a
primitive head.
From the annelid coelom arises, a fluid-filled cavity
in which various organs can be placed. Segmentation is also shown, that is,
your body consists of a series of modules with a similar structure is repeated.
Arthropods develop an articulated skeleton outer
shell, covering the entire body and its many appendices, which becomes more
efficient travel. Insects were the first animals to fully conquered the
terrestrial environment. Also influenced the evolution of flowering plants,
many of whom are involved in pollination.
