Theories and Mechanism of Evolution

Organic evolution is the process by which simple organisms gave rise to complex organisms over time. Evolution by natural selection begins when living forms show differences in metabolic capability, and the better-adapted forms survive under changed conditions.

Lamarckism:

• Lamarck stated that evolution occurred through the use and disuse of organs.
• According to this view, acquired characters were passed to the next generation.
• The standard example is the giraffe, whose neck was said to become longer by repeated stretching to reach leaves on tall trees.
• This view is not accepted now.

Darwinism:

• Darwinian theory is mainly based on branching descent and natural selection.
• Branching descent means present-day forms arise as modified descendants from earlier forms.
• Natural selection means nature selects those organisms that are better fitted to survive and reproduce.
• The basis of natural selection includes limited natural resources, a stable population size except for seasonal fluctuation, variation among members of a population, and inheritance of most variations.
• Since populations can grow rapidly but resources remain limited, competition for resources occurs.
• Organisms with heritable variations that improve resource use and adaptation survive and leave more progeny.
• Over many generations, these changes in population characteristics and new forms arise.
• Fitness means better adaptation under given conditions, and this adaptive ability is inherited and genetically based.

Evolution in Microbes and Higher Organisms:

• In fast-dividing microbes, new forms may appear within days because they multiply into millions within hours.
• In a bacterial colony growing on a medium, variation already exists in the ability to use a feed component.
• When the medium changes, only the part of the population able to survive under the new condition continues and outgrows the rest.
• In organisms such as fish or fowl, the same kind of change takes millions of years because their lifespans are much longer.

Mechanism of Evolution:

• The central question is the origin of variation and the way speciation occurs.
• Mendel had already spoken of inheritable factors affecting phenotype, but Darwin did not develop this explanation.
• Hugo de Vries proposed that mutations are large differences arising suddenly in a population.
• He believed evolution occurs by mutation rather than by the minor heritable variations described by Darwin.
• Mutations are random and directionless, while Darwinian variations are small and directional.
• Darwin considered evolution gradual, whereas de Vries considered speciation the result of sudden mutation, called saltation.
• Later studies in population genetics brought greater clarity to variation and speciation.

• Organic evolution is the process by which simple organisms gradually gave rise to complex organisms over time.
• Lamarckism: evolution by use and disuse of organs and inheritance of acquired characters (e.g., a giraffe's long neck); not accepted today.
• Darwinism: based on branching descent and natural selection - organisms with useful heritable variations survive, reproduce more, and form new species over generations.
• Microbes vs higher organisms: new forms appear in microbes within days, but in higher organisms, it takes millions of years.
• Hugo de Vries's Mutation Theory: evolution occurs by sudden, random mutations (saltation), not by gradual variations as Darwin proposed.