Model Answer
0 min readIntroduction
Charles Darwin’s theory of evolution by natural selection, published in "On the Origin of Species" (1859), revolutionized our understanding of the biological world. At its core, natural selection posits that organisms best adapted to their environment are more likely to survive and reproduce, passing on their advantageous traits. However, this process isn't about creating new traits; it acts upon pre-existing *variation* within a population. This inherent variability, arising from genetic mutations and recombination, is the raw material upon which natural selection operates. Understanding this interplay is fundamental to grasping evolutionary biology.
Defining Natural Selection and Variation
Natural Selection is the differential survival and reproduction of individuals within a population due to differences in their traits. It's not a directed force; it’s a consequence of environmental pressures acting on existing variation. Variation, in this context, refers to the differences in observable characteristics (phenotypes) among individuals within a population. These differences can be physical (size, color), physiological (disease resistance), or behavioral.
How Natural Selection Acts on Variation
The process unfolds in several key steps:
- Variation Exists: Within any population, individuals exhibit variation. This variation arises from random genetic mutations during DNA replication, recombination during sexual reproduction (crossing over), and gene flow (migration).
- Environmental Pressure: The environment presents challenges – limited resources, predators, diseases, climate changes. These act as selective pressures.
- Differential Survival and Reproduction: Individuals with traits that make them better suited to the environment are more likely to survive and reproduce. For example, a moth with coloration that camouflages it against tree bark is more likely to avoid predators than a brightly colored moth.
- Inheritance: The advantageous traits are heritable, meaning they are passed on to offspring. Darwin initially didn't understand the mechanism of inheritance (Mendel’s work was largely unknown at the time), but we now know it’s due to genes.
- Change in Allele Frequencies: Over generations, the frequency of alleles (gene variants) that confer advantageous traits increases in the population. This leads to a shift in the population's characteristics – adaptation.
Examples of Natural Selection in Action
Several compelling examples illustrate how natural selection operates:
- Peppered Moth (Biston betularia): During the Industrial Revolution in England, pollution darkened tree bark. Dark-colored moths, previously rare, gained a survival advantage as they were better camouflaged from predators. The population shifted from predominantly light-colored moths to dark-colored moths. This is a classic case of directional selection.
- Antibiotic Resistance in Bacteria: When antibiotics are used, most bacteria are killed. However, a few bacteria may possess mutations that make them resistant. These resistant bacteria survive and reproduce, leading to a population of antibiotic-resistant bacteria.
- Darwin's Finches: On the Galapagos Islands, Darwin observed finches with varying beak shapes adapted to different food sources. Finches with beaks suited for cracking seeds survived better where seeds were abundant.
Consequences of Natural Selection
The long-term consequences of natural selection are profound:
- Adaptation: Populations become better suited to their environments.
- Speciation: If populations become geographically isolated and experience different selective pressures, they may diverge genetically and eventually become separate species.
- Evolutionary Novelty: While natural selection primarily acts on existing variation, it can, over long periods, lead to the emergence of entirely new traits through the accumulation of small changes.
| Selective Pressure | Example | Result |
|---|---|---|
| Predation | Peppered Moth | Shift in moth coloration |
| Antibiotics | Bacteria | Antibiotic resistance |
| Food Availability | Darwin's Finches | Variation in beak shape |
Conclusion
In essence, natural selection is the engine driving evolutionary change. It’s not a creative force, but a filtering mechanism that favors individuals possessing traits that enhance survival and reproduction in a specific environment. The process hinges on the existence of variation, a testament to the inherent dynamism of life. Understanding the interplay of variation and natural selection remains critical for addressing challenges like antibiotic resistance and adapting to a changing climate.
Answer Length
This is a comprehensive model answer for learning purposes and may exceed the word limit. In the exam, always adhere to the prescribed word count.