Evolution is the process by which species change over time due to mechanisms like natural selection, genetic drift, gene flow, and mutation. These processes lead to adaptation, where organisms become better suited to their environments, and can result in speciation, the formation of new species. Ev...
1. Evolutionary Theory: Foundations and Definitions
Evolution is the scientific explanation for the diversity of life on Earth. It refers to the process by which
species change over time through mechanisms such as natural selection, genetic drift, gene flow, and
mutation. The theory of evolution is central to biology and is supported by evidence from multiple
scientific fields, including genetics, paleontology, comparative anatomy, and molecular biology.
2. Mechanisms of Evolution
A. Natural Selection
Natural selection is a key mechanism by which evolution occurs. It was first articulated by Charles Darwin
and Alfred Russel Wallace. The process works based on differential survival and reproduction.
Conditions for Natural Selection:
1. Variation: There must be genetic variation within the population. These variations can be in
physical traits (morphology), behaviors, or biochemical processes.
2. Heritability: Traits must be heritable (passed from one generation to the next through genetic
inheritance).
3. Differential Reproduction: Some variants of the trait are more advantageous than others,
meaning organisms with those traits are more likely to survive and reproduce.
4. Adaptation: Over time, advantageous traits become more common within the population,
leading to adaptation to the environment.
Example: Darwin’s Finches
• In the Galápagos Islands, Darwin observed finches with varying beak sizes and shapes. When
drought conditions reduced the availability of smaller seeds, finches with larger beaks (better
suited for cracking larger seeds) had higher survival and reproduction rates, leading to a change
in the population’s beak size over time.
B. Genetic Drift
Genetic drift refers to the random changes in allele frequencies that can occur in a population,
particularly in small populations. Unlike natural selection, genetic drift is not driven by any
environmental pressures or adaptive advantages. It occurs due to chance events.
Types of Genetic Drift:
1. Bottleneck Effect:
o Definition: A significant reduction in the size of a population due to environmental
events (e.g., natural disasters), resulting in a loss of genetic diversity.
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, o Example: The cheetah population went through a genetic bottleneck, and today,
cheetahs have extremely low genetic diversity, which makes them vulnerable to disease
and environmental changes.
2. Founder Effect:
o Definition: When a small group of individuals from a larger population colonizes a new
area, carrying only a subset of the genetic diversity of the original population.
o Example: The Amish population in the United States descended from a small group of
European immigrants. Due to limited genetic variation in the founder group, certain
genetic disorders are more prevalent in the Amish population (e.g., Ellis-van Creveld
syndrome).
Key Point: Genetic drift has a more significant effect in smaller populations, and over time it can
reduce genetic variation, even eliminating certain alleles from the population.
C. Gene Flow
Gene flow (also called migration or allele flow) occurs when individuals from one population migrate to
another population and interbreed. This introduces new genetic material into the gene pool of the
recipient population.
Effects of Gene Flow:
• Increased Genetic Variation: Gene flow introduces new alleles into a population, potentially
increasing genetic diversity.
• Reduced Differences Between Populations: If populations are regularly exchanging individuals,
they may become genetically similar, reducing the likelihood of speciation.
Example: If a population of foxes in one region migrates into another population of foxes in a different
region, the gene pool of the new combined population will include alleles from both populations,
potentially increasing genetic diversity.
D. Mutation
Mutations are random changes in an organism's DNA that create new genetic variations. Mutations are
the ultimate source of new genetic material and can be beneficial, neutral, or harmful.
Types of Mutations:
1. Point Mutation: A change in a single nucleotide base in DNA (e.g., A → T).
o Example: Sickle-cell anemia results from a point mutation in the hemoglobin gene,
where glutamic acid is replaced by valine.
2. Insertion/Deletion: Insertion or removal of nucleotides in the DNA sequence.
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