Understanding Evolutionary Biology and Natural Selection
1. Fixism
Fixism states that species were created exactly as they are and do not change over time.
- Georges Cuvier: Defender of catastrophism, which posits that Earth experienced major catastrophes and fossils are remains of organisms that disappeared in those events.
- Carl Linnaeus: Father of taxonomy, creator of binomial nomenclature, who believed species were immutable.
2. Evolutionism
Evolutionism states that species change and transform over time. Key scientists include Lamarck, Darwin, and Wallace.
3. Lamarckism
The first evolutionary theory, based on three laws:
- Law of Progression: Living beings tend to become more complex.
- Law of Use and Disuse: Organs that are used become more developed, while those not used become reduced or disappear.
- Inheritance of Acquired Characteristics: Characteristics acquired during life are passed on to offspring.
Functionalism: Function determines structure.
4. Darwinism
Proposed by Darwin and Wallace, this theory is based on:
- Common Ancestry: All species come from a common ancestor.
- Diversification: New species arise from a common ancestor.
- Gradualism: Evolution is slow and gradual.
- Natural Selection: Requires variation, heritability, and biological fitness.
Individuals with advantageous traits leave more offspring, making those traits more common over generations.
Key Difference
Lamarck: Individuals change during their lifetime and pass those changes to their offspring.
Darwin: Individuals are born with different traits and natural selection favours the most advantageous ones.
Evidence for Evolution
Main evidence includes palaeontology, biogeography, embryology, comparative anatomy, and biochemistry.
1. Palaeontology
Studies fossils, which show that species have changed over time. Intermediate forms (e.g., Archaeopteryx) indicate common ancestry. Phylogenetic sequences, such as horse evolution, show progressive evolutionary changes.
2. Biogeography
Studies the distribution of living organisms, showing that geographic isolation can produce new species (e.g., camelids).
3. Embryology
Studies embryonic development. Related species share similar embryonic characteristics, such as pharyngeal arches in vertebrates, suggesting common ancestry.
4. Comparative Anatomy
- Analogous organs: Same function, different origin (e.g., bird wing vs. insect wing). Do not indicate common ancestry.
- Homologous organs: Same evolutionary origin, different functions (e.g., human arm vs. whale fin). Indicate common ancestry.
- Vestigial organs: Evolutionary remnants that have lost their original function (e.g., whale pelvic bones).
5. Biochemistry
Compares DNA, RNA, and proteins. Greater molecular similarity indicates a closer evolutionary relationship.
Natural Selection and Adaptation
Natural Selection
The process by which the environment favours individuals with advantageous traits. Evolution affects populations, not individuals. Biological fitness is the ability to survive and reproduce.
Artificial Selection
Humans choose which individuals reproduce to obtain desirable traits (e.g., dog breeds).
The Peppered Moth (Biston betularia)
An example of natural selection where the environment determines which traits are advantageous based on camouflage and predator visibility.
Adaptation
A hereditary characteristic that increases an organism’s fitness in a particular environment.
Speciation and Modern Theories
Speciation
The evolutionary process by which new species arise.
- Anagenesis (Phyletic Gradualism): One species gradually changes into another; no new branches are formed.
- Cladogenesis (Divergent Evolution): An ancestral species splits into two populations, leading to reproductive isolation and new species.
Synthetic Theory (Neo-Darwinism)
Proposed by Theodosius Dobzhansky. Combines Darwinism with modern genetics; mutations generate variation, and natural selection acts upon it.
Punctuated Equilibrium
Proposed by Stephen Gould and Niles Eldredge. Long periods of stability alternate with short periods of rapid evolutionary change.
Neutral Theory
Proposed by Motoo Kimura. Most mutations are neutral; molecular evolution occurs mainly by chance, emphasizing genetic drift.
Theory of Symbiogenesis
Proposed by Lynn Margulis. Explains the origin of eukaryotic cells through the symbiotic fusion of prokaryotic cells (mitochondria and chloroplasts).