Evolutionary Theories: From Fixism to Biodiversity
Evolutionary Theories
Fixism: The theory that existing species have not changed since creation.
Creationism: The theory that species were created by God. Creationism is associated with fixism because it also defends the immutability of species.
Early Evolutionary Thought
Lamarckism (Theory of Acquired Characteristics):
- Environmental conditions change over time.
- Environmental changes create new needs, causing individuals to alter their habits or behaviors.
- New habits lead to the greater or lesser use of certain organs, resulting in growth or deformation.
- Changes acquired during an individual’s lifetime are transmitted to offspring.
Darwin’s Theory of Evolution by Natural Selection
Individuals within a species exhibit slight differences (size, color, etc.).
There is a struggle for survival due to limited resources.
Some variations are more advantageous than others.
Individuals with favorable variations are more likely to survive and reproduce.
Species change continuously and gradually if environmental conditions remain constant.
The Synthetic Theory of Evolution
The evolutionary unit is the population, not the individual.
Individuals within a population carry different alleles that originated through mutation.
Certain phenotypes conferred by these alleles increase the likelihood of leaving offspring.
Natural selection is the primary evolutionary mechanism.
Evolution is defined as a gradual change in the genetic composition of populations.
Speciation and Extinction
Speciation: The process by which two species arise from a single ancestral species due to natural selection acting on variability.
Extinction: The process by which a species ceases to exist because it cannot adapt and reproduce successfully in new environmental conditions.
Natural Selection in Changing Environments
When the environment changes, natural selection favors individuals with advantageous traits for the new conditions. These individuals survive, while others die. Populations adapt, not individuals, and this adaptation is not intentional.
How Species Arise
Isolation of Populations: A population becomes isolated from the rest of its species by a barrier (river, mountain, etc.), preventing reproduction.
Gradual Differentiation: Mutations occur randomly, leading to differences between the isolated populations. If environmental conditions differ from the original, these differences are amplified by natural selection.
Two Different Species: Prolonged isolation can lead to populations so different that they lose the ability to interbreed and produce fertile offspring.
Biodiversity
Biodiversity is the result of new species forming and others becoming extinct due to their inability to adapt to changing environmental conditions.
Types of Diversity
Genetic Diversity: Variability in the genetic information of individuals within the same species.
Species Diversity: The variety of species existing in different parts of the Earth.
Ecological Diversity: The variety of ecosystems that interact with each other and their non-living environments.