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In this article, we will discuss the environmental factors that can act as stabilizing, disruptive and directional forces of natural selection. But before proceeding to discuss these factors, first, let us recall what is natural selection and how it occurs.
What are Natural Selection and Speciation?
- Natural Selection: Natural selection refers to an increase in the number of advantageous alleles in a population because of selection pressure.
- Speciation: Speciation refers to the development of new and varying species that take place gradually with time. In other words, we can say that speciation is the evolution of new species from existing ones. This is because the sub-populations of the same species can become genetically different and adapt to their environments in varying ways which results in new species.
Reproductive isolation and accumulation of genetic changes via natural selection lead to the formation of new species.
Natural selection can result in the formation of new species if there is a split in the population. The two types of speciation are:
- Allopatric speciation: This speciation refers to the formation of two species from an original one because of geographical isolation.
- Sympatric speciation: This type of speciation refers to the formation of two species from a single original species because of reproductive isolation while having the same geographical location. This can be by behavioural variation, temporal variation, gametic variation, mechanical variation, and hybrid sterility.
In the next section of the article, we will discuss the process of natural selection.
Natural Selection Process
The process of natural selection drives evolution. This process is explained below:
- Existence of Variation
Variation occurs within the species due to individuals possessing different versions of the same genes, i.e. different alleles. Alleles of a specific gene can have advantages or disadvantages for a specific individual within a species.
- Selection takes place
Here comes the survival of the fittest which is the idea that a fit individual possesses the characteristics that enable it to survive and compete effectively for resources. Due to these attributes this individual is able to reach the reproductive age and produce offspring. On the other hand, an unfit individual with poor characteristics is unable to mate and produce offspring.
- Breeding of beneficial alleles
Genes and their alleles that are beneficial and confer a selective advantage to a species gradually gather in their frequency in a population.
Key Points of Natural Selection
- For natural selection, selection pressure is required
Natural selection depends on selection pressure. In the presence of abundant food, there is little to no selection pressure.
- A mutation can offer a beneficial feature
An advantageous feature can often arise because of the mutation. This beneficial feature then spreads due to natural selection.
- Natural selection occurs gradually
Natural selection takes time and occurs over many generations.
In the next section of the article, we will discuss types of natural selection.
Types of Natural Selection
We already know that the environmental factors that affect the chance of survival of an organism are known as selection pressures. For instance, lions can face high competition for food when the supply of prey is low. This environmental factor favours lions that are faster and better at hunting.
The selection pressures can affect the allele frequencies of a population via natural selection in different ways. The three types of selection are:
- Stabilizing
- Disruptive
- Directional
Stabilizing Selection
Stabilizing selection refers to a natural selection that keeps frequencies of alleles almost constant over generations. It implies that things do not undergo any change as long as the environment changes. A good example of stabilizing selection is observed in the birth weights of humans. Extremely low and extremely high birth weights are selected against and resulting in the maintaining of intermediate birth weights.
Directional Selection
Directional selection refers to a natural selection that incurs a gradual shift in the frequencies of the alleles over many generations. This usually occurs when there is:
- An environmental change
- Selection pressures
- The surfacing of a new allele in the population that is beneficial
For instance, according to a recent study, climate change is affecting the size of fish in some habitats.
How Rising Temperature is Affecting the Size of the Fish in Some Habitats?
As the temperature is rising, there is a selection of smaller body sizes as compared to the larger ones. Climate change is causing the seas to warm up which causes the metabolism of fish to speed up. Hence, they require more oxygen, however, the oxygen levels are lower in the warmer seas.
The metabolic requirements of larger fish are more as compared to the smaller fish, hence they are greatly affected by the rising temperatures. The change in temperature affects organisms greatly because this temperature affects the activity of the enzyme. Fish that have a smaller body size are hence fitter and better adapted to living in warmer seas. The body size of the fish is determined by genetic as well as environmental factors. Fish having a smaller body size have a greater chance to reproduce and transfer their alleles to the offspring. Gradually, this results in an increase in the allele frequencies that lead to small body size and a decrease in allele frequencies that produce a larger body size.
Disruptive Selection
Disruptive selection refers to a natural selection that maintains high frequencies of two different allele sets. In other words, we can say that the individuals with intermediate alleles or phenotypes are selected.
Disruptive selection leads to polymorphism which refers to the continued existence of two or more distinct phenotypes in species. This can take place in an environment that exhibits variation.
For instance, birds living on the Galapagos islands employ their beaks to forage for various-sized seeds. The beak size of the bird is either small or large and the medium-sized beak is selected. This is because birds with shorter or longer beaks can efficiently forage different types of available seeds.
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