Topic 5: Evolution
1. Understanding Evolution
In the study of advanced biology, evolution is perceived as the fundamental unifying theme that explains both the unity and the diversity of life. It can be defined in two primary ways:
- Genetic Perspective: A change in the genetic composition (allele frequency) of a population over successive generations.
- General Perspective: The process of developing by gradual changes from simple to complex forms.
Organic Evolution
Organic evolution specifically refers to the gradual change in the genetic composition of organisms in a population during successive generations. This process ultimately leads to the formation of new species from pre-existing species. It implies that all living things share a common ancestry but have diverged through various environmental pressures.
Join WhatsApp Group for Full Notes2. Forces & Pressures Driving Evolution
Evolutionary change is not random; it is driven by specific biological forces. These are categorized based on their necessity for the formation of new species.
A. Primary Forces (Essential)
These are the forces that must be present for speciation to occur. Without these, no organic evolution takes place.
Mutation is the sudden, spontaneous, or abrupt change in the amount or chemical structure of a DNA molecule. It is the ultimate source of all genetic variation.
If a mutation provides a selective advantage, the organism is more likely to survive and pass that gene to the next generation. Example: The development of antibiotic resistance in bacteria.
Gene recombination occurs during meiosis through crossing over (prophase I), random assortment of chromosomes, and random fertilization. This creates new combinations of existing alleles, leading to high variation in offspring.
Natural selection is the mechanism by which individuals with favorable traits are “selected” by the environment to survive and reproduce. NB: Natural selection must be present along with either mutation or gene recombination for evolution to occur.
B. Secondary Forces (Modifiers)
These forces modify the rate of evolution. If present, evolution is faster; if absent, it is slower.
- Gene Flow: The exchange of genes between different populations via migration.
- Genetic Drift: Random changes in allele frequency, especially in small populations.
- Breeding: Patterns of mating (inbreeding vs outbreeding).
- Adaptive Radiation: Diversification of a group into forms filling different ecological niches (e.g., Darwin’s Finches).
3. Theories of the Origin of Life
Mankind has long sought to explain how the first living organisms appeared on Earth. There are three prominent historical theories:
The belief that life was created by a supernatural power (God) at a specific time. According to this theory, species are immutable (unchanging). Whatever exists today is simply the result of reproduction from those original ancestors.
Strength: Based on faith and belief.
Weakness: It cannot be proved or disproved scientifically as it falls outside the realm of observable experiments.
The idea that life arises from non-living matter through an “active principle.” For example, Van Helmont (15th century) claimed mice could be generated in 3 weeks from a dirty shirt, wheat grains, and a dark cupboard, with human sweat acting as the active principle.
Weakness: Experiments lacked scientific controls. It was eventually disproved by Pasteur and others who showed life only comes from pre-existing life (Biogenesis).
Life originated elsewhere in the universe and was brought to Earth (e.g., via meteorites or “spacemen”).
Weakness: It explains the *perpetuation* of life on Earth but fails to explain the actual *origin* of life itself (it just shifts the question to another planet).
4. Mechanisms of Organic Evolution
Lamarckian Theory (Lamarckism)
Jean-Baptiste Lamarck proposed that evolution is driven by environmental change creating “new needs.”
- Use and Disuse: Organs used frequently become stronger and better developed; those not used degenerate (become vestigial).
- Inheritance of Acquired Characteristics: Traits developed during an organism’s lifetime are passed to its offspring.
Critique:
Lamarck was right that the environment creates needs, but he was wrong about inheritance. Acquired characteristics (like a boxer’s muscles) do not affect the gametes and thus are not inherited.
Darwinian Theory (Darwinism)
Charles Darwin’s theory is based on Natural Selection and the Struggle for Existence.
Key Observations:
- Individuals have a high reproductive rate (Overproduction).
- Population numbers remain relatively constant due to limiting factors.
- Variation exists among individuals in a population.
Deductions:
From overproduction and limited resources, a “Struggle for Existence” ensues. In this struggle, individuals with favorable variations survive and reproduce (Survival of the Fittest).
5. Natural vs Artificial Selection
Selection is the process determining which organisms survive to pass on their genes.
1. Natural Selection
The environment acts as the selective agent. Forces like natural disasters, competition for food, and disease select for suitable variants.
Example: Industrial Melanism (Biston betularia)
In industrial UK, soot blackened tree lichens. The black moth (mutant) became camouflaged, while the original white moth was easily preyed upon. The black moth population increased because of selective advantage.
2. Artificial Selection
Humans act as the selective agent, choosing traits like high milk yield, wool quality, or disease resistance for mating and propagation.
6. Breeding Strategies
Inbreeding
Selective reproduction between closely related individuals to retain desired traits.
- Increases homozygosity.
- Can lead to “Inbreeding Depression” (reduced fertility).
- Solution: Cross with unrelated strains (Outbreeding).
Outbreeding
Crossing genetically distinct organisms or different varieties.
- Produces Hybrids.
- Leads to Hybrid Vigour (Heterosis) – where offspring are superior to parents.
7. Speciation & Isolation
Speciation is the formation of new species from pre-existing ones. This requires Isolation to prevent gene flow.
| Type | Mechanism |
|---|---|
| Geographical Isolation | Physical barriers like mountains or rivers separate populations (Allopatric Speciation). |
| Reproductive Isolation | Structural or behavioral changes prevent interbreeding (Sympatric Speciation). |
Isolation Mechanisms:
- Mechanical: Incompatible genitalia.
- Seasonal: Different breeding times.
- Behavioral: Different courtship patterns (e.g., bird songs).
- Post-Zygotic: Hybrid sterility (e.g., Mules), Hybrid inviability, or Hybrid breakdown.
8. Evidence for Organic Evolution
1. Paleontology (Fossil Records)
Fossils are preserved remains found in sedimentary rocks. Deeper layers contain simpler, older forms; upper layers contain complex, younger forms.
| Type | Process | Examples |
|---|---|---|
| Entire Organism | Frozen in ice or trapped in tar. | Woolly Mammoths in Siberia. |
| Hard Parts | Sand/clay trapping bones/shells. | Dinosaur skeletons. |
| Impressions | Organic remains in fine sediments. | Archaeopteryx feathers. |
| Coprolites | Fossilized faeces. | Cenozoic mammalian remains. |
2. Comparative Anatomy
- Homologous Structures: Same origin/structure, different function (e.g., pentadactyl limb). Suggests common ancestry.
- Analogous Structures: Different origin, same function (e.g., wings of insects vs. birds). Results from convergent evolution.
- Vestigial Structures: Non-functional remnants (e.g., human appendix, snake limb bones).
3. Biochemistry & Biogeography
Universality of DNA and ATP suggests a common origin. Biogeographical distribution (e.g., elephants in Africa/India vs. South America) suggests continental drift isolated populations, leading to unique evolutionary paths.
9. Steady State Theory
This theory asserts that life has no origin. It proposes that the Earth has always existed in a state capable of supporting life and that species have always existed. In this view, species do not evolve; their numbers simply vary or they become extinct.