Types of Natural Selection

Types of Natural Selection

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Natural selection is all about traits that help organisms survive become more common in a population over time. This happens when individuals with those traits are better able to survive and pass on their genes to their offspring. There are three types of natural selection that can affect a population's gene pool:

  1. Directional selection: This happens when one extreme of a trait is favoured over the other. For example, if big seeds are more abundant, birds with bigger beaks will be better able to eat and survive, so their genes will become more common in the population.
  2. Stabilizing selection: This happens when the middle range of a trait is favoured over the extremes. For example, if being too big or too small is a disadvantage, organisms with an average size will be more likely to survive and pass on their genes.
  3. Disruptive selection: This happens when both extremes of a trait are favoured over the middle. For example, if there are two different types of prey in an environment, birds with either a very long or very short beak may be better able to eat and survive than those with an average-sized beak.

Understanding these different types of natural selection can give us insight species evolve and adapt to

Process of natural selection
Process of natural selection

What is the importance of natural selection?

Natural selection plays a crucial role in the evolution of species. It allows for advantageous traits to be passed down from one generation to the next, leading to the diversity of life on earth. This diversity is important because it ensures that populations can adapt to changing environments, reducing the risk of extinction. Without natural selection, populations would be less likely to evolve and could be at risk of dying out. That's why it's so important to understand how natural selection works and how it shapes the world around us.

What are the three types of natural selection?

There are three primary types of natural selection that influence the characteristics of a population in unique ways. These are:

  1. Stabilizing selection: This type of selection favours the average phenotype and eliminates extreme variations. An example of this is the birth weight of human babies, where babies with an average weight have a higher chance of survival than those that are either too small or too large.
  2. Directional selection: This type of selection favours one extreme phenotype over the others, leading to a shift in the population towards that extreme. An example of directional selection is the evolution of giraffe necks, longer necks were favourable for reaching leaves on taller trees, leading to a shift in the population towards longer necks.
  3. Disruptive selection: This type of selection favours more than one extreme phenotype, leading to a bimodal distribution in the population. An example of this is the beak size of Darwin's finches, where larger beaks were favourable for cracking hard seeds and smaller beaks were better for eating insects, leading to a split in the population with two distinct beak sizes.

Understanding these different types of natural selection can help us understand how traits evolve and how populations adapt to their environment.

What is stabilising selection?

Exactly! Stabilising selection is a key mechanism for maintaining the status quo in a population. It acts to preserve the existing traits that are already well-suited to the environment, while eliminating individuals with extreme or unusual traits that may be less fit. This type of selection is common in stable environments where there is little change in the selection pressures acting on the population. Over time, stabilising selection tends to keep the allele frequencies of a population relatively constant, resulting in a stable, intermediate phenotype that is well-suited to the environment. However, if the environment changes, stabilising selection may no longer be the most adaptive strategy, and other types of selection may come into play.

Stabilising selection
Stabilising selection

Stabilising selection is a common mechanism for maintaining the optimal number of eggs laid by birds. Too many eggs can be energetically costly for the mother and result in malnourished offspring, while too few eggs may not produce enough viable offspring to maintain the population. By selecting for an intermediate number of eggs, stabilising selection helps to balance these conflicting pressures and ensure the survival of the species.

Similarly in humans, stabilising selection acts on traits such as height and weight. Individuals who are too tall or too heavy may experience health problems, while those who are too short or too light may be less competitive in certain environments. By selecting for an intermediate or average phenotype, stabilising selection helps to maintain a stable and healthy population.

What is directional selection?

Absolutely! Directional selection occurs when selection pressures favor one extreme phenotype over the other, causing a shift in the mean phenotype the favored extreme. This can happen when the environment changes, or when new selection pressures are introduced. For example, if a population of birds with short beaks experiences a drought, and only birds with longer beaks can reach the available food, then directional selection would favor birds with longer beaks, causing the mean beak size to increase over time.

Regarding continuous and discontinuous variation, you are correct! Continuous variation refers to traits that can take on a range of height or and by genes and environmental factors. Discontinuous variation, on the other hand, to traits that can only take on a limited number of values, such as blood type or eye color, and are controlled by one or a few genes.


Directional selection
Directional selection

Climate change is a major driver of directional selection, as it can cause significant changes in the environment and shift the phenotype in one direction. In the case of polar bears, a shift towards smaller body sizes may help them adapt to warmer temperatures and changes in their habitat.

The peppered moth is another classic example of directional selection, as the changing environment caused by industrialization created a new selection pressure that favored darker moths over lighter ones. This is a great example of how human activities can have a significant impact on the evolution of other species.

Overall, directional selection is an important mechanism in evolution that can drive significant changes in a population over time. It highlights the dynamic nature of the natural world and the constant interplay between organisms and their environment.

What is disruptive selection?

Disruptive selection, also known as diversifying selection, favours several extreme phenotypes rather than intermediate phenotypes and is the least common form of selection. This type of selection occurs when the extreme phenotypes are more successful than intermediate forms within the same habitat, as depicted below.

Disruptive selection
Disruptive selection

Disruptive selection can lead to the evolution of polymorphism, where many distinct forms or morphs of a species exist in the same population. This can be seen in both physical traits, like body coloration in oysters and rabbits, as well as in behavioral traits, like mating strategies in fish.

In the case of rabbits, disruptive selection favors both black and white individuals over intermediate grey individuals, leading to the evolution of polymorphism in the population. Similarly, in oysters, light and dark morphs can exist together in the same population, allowing them to blend in with different backgrounds.

Overall, the different types of natural selection play a crucial role in driving the evolution of species over time. By selecting for certain traits and against others, they shape the genetic makeup of populations and allow organisms to adapt to changing environments.

Types of Natural Selection

How many types of natural selection are there?

There are 3 main types of natural selection: Stabilising selection Directional selection Disruptive selection

What are the 3 types of natural selection?

The 3 types of natural selection include: Stabilising selection Directional selection Disruptive selection

Is natural selection random?

Natural selection is not random however the mutations that contribute to it are. This is because natural selection favours individuals that are more likely to survive in the wild amidst selection pressure due to possessing advantageous alleles. 

Why does natural selection happen?

Organisms harbour advantageous alleles and therefore an advantageous phenotype that increase their chances of survival. As a result, they will live on to reproduce and create offspring which inherit the advantageous allele.

Does natural selection affect humans?

These days, many human shortcomings are dealt with by innovation. However, humans are still evolving. For example, Tibetans who live at high altitudes of around 4000 meters (that would cause many of us altitude sickness) have evolved to be more efficient in metabolising oxygen. This is due to mutations in the EPAS1 gene which is over expressed in 87% of Tibetans.

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