Evolution

Evolution

Definition

Ø This is the process by which different kinds of living organisms are believed to have developed from earlier forms during the history of the earth.

Ø The concept behind organic evolution is that pre-existing living things gave rise to all living things present today.

Ø This is based on the premise that a cell cannot be created.

Ø Evolution is the change in the inheritable characteristics of biological populations over successive generations.

Ø In other word the present day species have not always existed in the present form.

Evidence of Evolution

There are various evidences that proves the evidence of Evolution. Evidence for evolution has been obtained through fossil records, , geography,comparative anatomy, embryology and molecular biology.

a. Fossil Records: Fossil remains have been found in rocks of all ages. Fossils of the simplest organisms are found in the oldest rocks, and fossils of more complex organisms in the newest rocks. This supports Evidence for early forms of life comes from fossils. By studying fossils, scientists can learn how much (or how little) organisms have changed as life developed on Earth.

b.

Geographical Distribution: The geographic distribution of organisms on Earth follows patterns that are best explained by evolution, in combination with the movement of tectonic plates over geological time. Organisms tend to show some forms of variations in form, structures, and functions in relationship to differences in climate. Also, organisms of one climate tend to differ slightly from organisms of the same climate after several years of isolation.

c. Comparative Anatomy: In examinning the bodies of today’s animals in detail, it can be clearly seen that there are some remarkable similarities. For example, the skeletons of four-limbed creatures (tetrapods) are only slight variations on the same body plan. The bones are longer in some animals, and in others they are fused together, but they are arranged in the same pattern. Skeletons don’t have to be this way for animals to function, and in fact they result in some inefficiencies (such as people with lower back or knee problems). But this is the sort of pattern that would be expected if the body plans of tetrapods changed slowly and diversified over many generations.

d. Vestigial Organs: Also, looking at the bodies of animals today, features that are similar to what other animals have, but which no longer seem to function (or have different functions) can be found. These are called vestigial traits. Some classic examples are non-functioning eyes in blind cave fish, the hip bones on whales, and leg bones buried in the muscles of some snakes. In our own bodies, we can point to the appendix, wisdom teeth, goosebumps, and many other features. These are more clues that today’s animals have a history that extends back to ancestors that were quite different.

e. Embryology: The similarities in comparative embryology are also evident in the early stages of development. For example, fish, bird, rabbit, and human embryos are similar in appearance in the early stages. They all have gill slits, a two-chambered heart, and a tail with muscles to move it. Later on, as the embryos grow and develop, they become less and less similar. (The branch of biology that focuses on embryos and their development is called EMBRYOLOGY)

f. Domestic breeding: This brings about new forms that differ from ancestral stock. For example, pigeon fanciers have developed many varieties of pigeons through domestic breeding experiments. In effect, evolution has taken place under the guidance of human hands. The development of new agricultural crops by farmers and botanists provides more evidence for directed evolution.

g. Comparative biochemistry: This is comparison of biochemical processes with ancient species. For example, the same mechanisms for trapping and transforming energy and for building proteins from amino acids are nearly identical in almost all living systems. DNA and RNA are the mechanisms for inheritance and gene activity in all living organisms. The structure of the genetic code is almost identical in all living things. This uniformity in biochemical organization underlies the diversity of living things and points to evolutionary relationships.

Theories of Evolution

The theories of Evolution attempt to explain how evolution have taken place. There are several theories, and these includes the following:

French zoologist Jean-Baptiste Lamarck propounded a theory for evolution in 1801, based on the development of new traits in response to a changing environment. This theory is referred to as the theory of “Use and Disuse”. For example, the neck of the giraffe stretched as it reached for food. Lamarck’s theory gained favor, and his concept of “acquired characteristics” was accepted 1859. His theory is however not accepted by modern scientist, because findings have shown that physical features of a parent acquired through use and disuse are not inheritable by offspring. His postulates are as follows:

Ø Great changes in environment results in corresponding changes in species.

Ø Environmental changes cause organisms to form new habits or structures to help adjust to the new environment.

Ø Organisms develop specialized characters based on used and disused organs.

Ø Frequently used organs become well developed while the ones not used degenerates and become atrophied or useless.

Ø The well developed or dominant acquired characters are inheritable.

Charles Darwin was a British naturalist and after travelling to the remote regions of South America propounded his own theory known as the “Theory of Natural Selection” in 1859. The mechanism that Darwin proposed for evolution is natural selection. Because resources are limited in nature, organisms with heritable traits that favor survival and reproduction will tend to leave more offspring than their peers, causing the traits to increase in frequency over generations. Natural selection causes populations to become adapted, or increasingly well-suited, to their environments over time. Natural selection depends on the environment and requires existing heritable variation in a group. It should be noted that Darwin’s theory is popularly upheld by modern scientist. His postulates are as follows:

Ø Species of living organisms can reproduce large numbers of offspring into an environment with limited resources.

Ø These encourages competition within the offspring.

Ø Offspring that survives must have inherited some useful traits that aided them to survive.

Ø These then reproduce, passing on the good traits to their offspring.

Ø The organisms that were unable to survive the struggle die off and are thus eliminated.

Ø These leads to great variability within the population as this process continues through many generations.

Ø The population gradually become better adapted to the environment, leading to the origin of new species.

Although, both Lamarck and Darwin, recognize the importance of environment on evolution their theories differ in many ways.