# Natural selection

Natural selection is a fundamental biological process where certain alleles become more common in a population because they code for characteristics that increase an organism's likelihood of survival and reproduction. It is the driving force behind evolution, leading to gradual changes in species over time.

**Mechanism of Natural Selection** The process of natural selection works through a series of steps:

* **Genetic Variation**: Individuals within a species show a wide range of variation in their phenotypes, which are observable characteristics. This variation is primarily due to differences in alleles. The main source of new alleles and genetic variation is **mutation**, which are random changes in the DNA base sequence. Further genetic variation is introduced during sexual reproduction through **meiosis** (via crossing over of chromatids and independent segregation of chromosomes) and **random fertilisation of gametes**. Gene flow, where different alleles are introduced by migrating individuals, also increases genetic diversity.
* **Struggle for Survival**: Organisms produce far more offspring than can survive to maturity, leading to competition for limited resources. This creates a "struggle for existence" where individuals face various **selection pressures**. These pressures can be **biotic** (e.g., predation, disease, competition with other species) or **abiotic** (e.g., climate, food availability, temperature, light intensity).
* **Differential Reproductive Success**: Individuals with phenotypes that give them a **selective advantage** (i.e., make them better adapted to the prevailing environmental conditions) are more likely to survive, reproduce, and pass on their advantageous alleles to the next generation. Less well-adapted individuals are less likely to survive or reproduce effectively.
* **Increase in Allele Frequency**: This differential reproductive success means that a greater proportion of the next generation inherits the beneficial alleles. Over many generations, the frequency of these advantageous alleles in the population's gene pool increases, leading to **evolution** and improved adaptation to the environment.

**Types of Natural Selection** Natural selection can manifest in different ways, altering allele frequencies depending on the selection pressures:

* **Directional Selection**: This occurs when individuals with alleles for characteristics of an **extreme type** are more likely to survive and reproduce. It often happens in response to an environmental change, causing the average phenotype to shift in one direction. Examples include antibiotic resistance in bacteria, changes in fur length of caribou in warmer climates, and industrial melanism in peppered moths.
* **Stabilising Selection**: In this type, individuals with alleles for characteristics **towards the middle of the range** are more likely to survive and reproduce. It typically occurs when the environment is stable and unchanging, reducing the range of possible characteristics and maintaining the existing mean phenotype. Human birth weight is a classic example, where very low or very high birth weights are selected against.
* **Disruptive Selection**: This is the opposite of stabilising selection, where individuals with alleles for **extreme phenotypes** are more likely to survive and reproduce, while intermediate characteristics are lost. It occurs when the environment favors more than one phenotype, leading to a divergence within the population.

**Importance of Genetic Variation for Natural Selection** Genetic variation is paramount because it provides the **raw material** for natural selection to act upon. It enables populations to **adapt to changing environments** and selection pressures. Without sufficient genetic diversity, a population may lack the necessary advantageous alleles to adapt, making it vulnerable to being wiped out by events such as disease. Natural selection is crucial for evolution, driving the gradual change in species over time and contributing to the immense diversity of living organisms on Earth.

**Natural Selection vs. Genetic Drift** While natural selection is a non-random process leading to adaptation, **genetic drift** is a **random process** that also causes changes in allele frequencies over time. Genetic drift has a greater effect in **smaller populations** because chance factors tend to even out in larger ones. The **founder effect** is a specific instance of genetic drift where a new colony is started by a few individuals from a larger population, resulting in a small gene pool that may have very different allele frequencies from the original population.

**Natural Selection vs. Artificial Selection** Natural selection is distinct from **artificial selection (selective breeding)**, which is the process where humans deliberately choose organisms with desired traits to breed over many generations. While both alter allele frequencies, artificial selection has a human goal, can produce bigger changes in fewer generations, and often reduces genetic diversity within the population.