Theories of Life's Origin and Species Evolution

Hypotheses on the Origin of Life

Panspermia Hypothesis

The Panspermia Hypothesis suggests that life originated in space and traveled in spore form from one planetary system to another.

Prebiotic Synthesis Hypothesis

The Prebiotic Synthesis Hypothesis posits that life arose from organic molecules on Earth, which in turn originated from inorganic matter. This hypothesis was defended by Haldane and Oparin. In 1953, Stanley Miller experimentally tested the hypothesis of Oparin and Haldane. Miller designed an experiment that reproduced the alleged conditions of early Earth in specific areas: one section simulated the primitive atmosphere's composition, while another simulated solar radiation and lightning.

Fixity of Species

Fixity, or the theory of fixed species, proposes that species have remained unchanged since their inception. Currently, this concept is not scientifically accepted. Carl Linnaeus and Georges Cuvier were notable defenders of fixity.

Evolutionism

Evolutionism states that species can change and generate other species over time. There are different evolutionary theories, including Lamarckism, Darwinism, the Synthetic Theory of Evolution, and the Theory of Punctuated Equilibrium.

Lamarckism: The Theory of Acquired Characteristics

Jean-Baptiste Lamarck proposed that species transform continuously over time. His theory, also known as the Theory of Acquired Characteristics, was based on the following points:

• Organisms show a trend towards complexity.
• The repeated use of an organ or body part produces its development; conversely, disuse leads to its degeneration or obsolescence. This concept is often summarized as "the function creates the organ."
• Original characteristics are slowly replaced by acquired characteristics.
• Acquired characteristics are inherited by offspring.

Darwinism: Natural Selection and Survival

Charles Darwin's theory of evolution by natural selection is based on these key principles:

1. Struggle for Survival: There is a constant struggle among organisms for survival due to limited environmental resources. If more individuals are born than can survive, competition for resources intensifies.
2. Variability within Populations: Variability exists among individuals within a population. Not all individuals are exactly alike; there may be differences among them, such as size, color, or other traits.
3. Natural Selection of the Fittest: The environment selects the best-adapted organisms. Within a population, individuals with an advantageous variation for a given environment will have a greater chance of survival and reproduction than those without it. Thus, gradually and steadily, species change over time.

Evidence for Evolution

Various lines of evidence support the theory of evolution:

1. Homologous Organs

   These are organs that have the same internal structure but different external forms and functions. An example is the forelimbs of mammals (e.g., human arm, bat wing, whale flipper).

2. Analogous Organs

   These are organs that perform the same function in different organisms but have a different origin and internal structure. An example is the wings of insects and birds.

3. Vestigial Organs

   These are organs whose original function has been lost or significantly reduced over the course of evolution. Examples in humans include the appendix or wisdom teeth.

Neodarwinism: The Modern Synthesis

Neodarwinism, also known as the Modern Synthesis, integrates Darwin's theory of natural selection with the principles of Mendelian genetics. Its main tenets include:

1. It explicitly rejects Lamarckism.
2. Genetic variability is primarily due to two processes: mutation and genetic recombination.
3. Natural selection acts on this genetic variability.
4. Natural selection leads to changes in the allele frequencies within a population over generations.
5. Populations evolve, not individual organisms.
6. Evolution generally takes place gradually.