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Natural Selection

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Biology

Genetic variation exists in populations due to the presence of different alleles. Over time, factors that affect a population’s survival rate (selection pressures) create changes in the allele frequencies of specific genes in favour of organisms that are more suited to survive. As a result, the phenotype frequency also changes. This phenomenon is natural selection.

Natural selection: the differential survival and reproduction of individuals due to differences in their phenotype.

Organisms have different alleles of the same gene, which causes varying levels of reproductive success. This difference in alleles might be due to factors such as the environment. New alleles may also occur due to random mutations and cause more reproductive success. Over time, the frequency of alleles in a population changes. It is the mechanism through which evolution takes place; over time, we can expect the frequency of advantageous alleles to increase in a population and the frequency of deleterious alleles to decrease.

Evolution: a change in heritable characteristics of a population over time.

The process of natural selection

  1. Random mutations produce new alleles of a gene.
  2. Under certain environmental conditions, the new alleles may be beneficial, meaning the organism is better adapted for survival.
  3. The organisms will survive and reproduce to create offspring, which will inherit the advantageous allele.
  4. Over several generations, the advantageous allele will increase in frequency within the population.

To learn more about the different types of mutations and how they arise, check out our article Gene Mutations.

Many mutations are harmful or neutral, but some are advantageous.

Natural selection [+] stages [+] StudySmarterFigure 1. Process of natural selection. Source: Finty Royle - StudySmarter Originals

Natural selection example

As a simple example, let’s imagine a small population of mice that live on a dark sand beach.

  • 90% of mice have the optimum phenotype of dark fur, which helps them camouflage into the sand and avoid predators
  • 10% have light fur, which makes them stand out against the sand.

The light mice are more vulnerable to predators, and as a result of this predation, their population number is kept low compared to the dark mice.

However, a hotel is being constructed on the beachfront, and the owners would like the beach to have white sand instead, so artificial sand is dumped onto the beach. Now, the 10% that were initially poorly adapted possess the optimum phenotype, while the darker mice stand out against the light sand and are vulnerable to predation. Over time, as more dark mice are preyed on, and light mice breed successfully, light fur becomes the most frequently observed phenotype.

Natural Selection [+] Example [+] StudySmarter

Figure 2. Change of predominant phenotype over time due to natural selection. Source: Finty Royle - StudySmarter Originals

What is the theory of natural selection?

Charles Darwin was the English naturalist who first developed the idea of natural selection; after a five-year voyage, he studied plants, animals, and fossils in South America and on islands in the Pacific. His best-selling book, published in 1859, On the Origin of species, brought the idea of natural selection to the world’s attention. The theory was also conceived independently by Alfred Russell Wallace.

In Darwin’s time, genes were not yet known; however, Darwin could see that traits were passed from parents to offspring in the animals he studied.

Darwin chose the name natural selection to contrast with “artificial selection”. An example of artificial or selective breeding in his day was pigeon breeding - a hobby among many people. Through choosing which pigeons mated with others, distinct pigeon breeds with fancy feathers or acrobatic abilities could be created. Have a read of our Artificial Selection article to learn more.

Darwin and other scientists of his day argued that a process similar to selective breeding happened in nature without human intervention due to the environment. He argued that natural selection explained evolution.

Stages of natural selection

There are 4 stages that lead to natural selection. These are:

  1. More organisms are born than can survive
  2. Characteristics vary within a species
  3. Variations are inherited
  4. There are differences in reproduction and survival are due to variations.

What is exponential growth in a population?

In theory, every individual in a population can reproduce and contribute to the growth of the population. If every individual survived to adulthood (or the stage of reproductive maturity) and reproduced, and all their offspring did the same, the population would experience exponential growth.

Natural Selection [+] Exponential growth [+] StudySmarter

Figure 3. Exponential growth. Source: Finty Royle - StudySmarter Originals

However, exponential growth can only be sustained when nothing limits survival and reproduction. Several environmental factors prevent populations from increasing infinitely as it affects their survival rate. These are known as selection pressures.

Selection pressure examples

Selection pressures affect a population or organism’s chances of survival, which can be either abiotic (non-living factors) or biotic (living factors).

Examples of abiotic factors include the availability of:

  • Light
  • Water
  • Temperature

Examples of biotic factors include:

  • Predation
  • Competition for resources
  • Disease (e.g., parasites)

All of these can limit the maximum size of a population. This means not all individuals produced will be able to survive and those that are suitably adapted live on to reproduce. The alleles of these organisms will dominate the gene pool.

Gene pool: the total collection of alleles of all genes in a population.

What is fitness in biology?

When individuals possess the favoured phenotypes, they have higher fitness.

Fitness is defined as an organism’s ability to survive and pass its genes on to future offspring. Organisms that are better adapted to their environment – that is, they possess phenotypes that increase their chances of survival within that particular environment – will have higher fitness than those who are not.

What is evolution?

Evolution is the change in the heritable characteristics of populations over several generations. This is made possible by natural selection acting upon variation within a population. Through evolution, populations can adapt to suit their environmental conditions.

Evolution is different from speciation, an evolutionary process by which populations evolve to become different species.

Why is variation crucial for natural selection and evolution?

Variation is crucial for evolution to occur. Adaptation would not be possible without variation because the environment changes over time. At any given timepoint, in a large population with a wide range of phenotypes, not all individuals will be optimally adapted to the current environment. However, some of those individuals will possess alleles well suited to different conditions. When the environmental conditions change, these individuals will survive and pass on their genes.

What happens when a population becomes too small?

In contrast, a small population with slight variation might not survive such changes. It may be that there simply isn’t an individual that possesses a phenotype that suits the new conditions.

Chance can also affect which alleles get passed onto the next generation in a small population; over time, some alleles may be lost or favoured randomly. Genetic drift is the gradual change in allele frequencies in a small population due to chance rather than natural selection. This is why smaller populations are much more vulnerable to sudden changes in their environment. The smaller a population becomes, the smaller its chances of survival.

Natural Selection - Key takeaways

  • Natural selection describes individuals’ differential survival and reproduction due to differences in their phenotype. This process is influenced by selection pressures, such as predation and competition.
  • Natural selection leads to changes in allele frequencies in a population, favouring the organisms that are better suited to survive.
  • Natural selection differs from artificial selection as it does not require human intervention.
  • Fitness is an organism’s ability to survive and pass its genes on to future offspring.
  • In small populations, natural selection and genetic drift influence the change in a population’s allele frequency.

Natural Selection

Charles Darwin is credited as the scientist who first offered the theory of evolution by natural selection. The theory was also conceived independently by Alfred Russell Wallace.

Natural selection is the mechanism through which evolution takes place. It leads to changes in the heritable characteristics of populations over several generations. 

Individuals who are well suited to their environments (i.e., individuals with high fitness) are more likely to survive and reproduce. 


It follows that individuals who are not well suited to an environment will not be able to survive and reproduce, and their frequency will dwindle. Over time, the population becomes fitter and more adapted to its environment. 

Over time, the selection pressures acting on a population can create changes in the allele frequencies of certain genes, leading to changes in the phenotype frequency. 


Natural selection is the differential survival and reproduction of individuals due to differences in their phenotype. 

  • Random mutations produce new alleles.
  • The new allele puts the organism at an advantage to survive selection pressures, such as predation and disease. 
  • The well-suited organism will survive and reproduce offspring which will inherit the advantageous allele. 
  • Over several generations, the advantageous allele will dominate the gene pool of a population.

Final Natural Selection Quiz

Question

Natural selection is the _____________ and ________ of individuals due to differences in their _________.



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Answer

differential survival; reproduction; phenotype



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These are environmental factors that affect an individual’s chances of survival.

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Answer

Selection pressures



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 _______ is an organism’s ability to survive and pass its genes on to its offspring.


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Answer

Fitness

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_______ is crucial for natural selection to act upon. Small populations have _____ variation than big populations.



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Answer

variation; less

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Question

True or false. When a population’s dominant phenotype changes, the population has evolved.



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Answer

false

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Question

Give two reasons why smaller populations are more vulnerable to extinction.



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Answer

  • Smaller populations have less variation

  • Genetic drift

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Question

Genetic _____ is the gradual change in allele frequencies in a small population due to chance, rather than natural selection.



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Answer

drift

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Question

True or false. Speciation occurs through evolution. 



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Answer

true

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Question

An imaginary population of moths has two morphs: early emergers, whose caterpillars hatch in July, and late emergers, whose caterpillars hatch in September. These caterpillars only feed on one species of flowering plant.


Suppose that historically, the plants are most abundant in July-August. Predict which moth morph is more likely to dominate in the population.



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Answer

Early emergers

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Question

An imaginary population of moths has two morphs: early emergers, whose caterpillars hatch in July, and late emergers, whose caterpillars hatch in September. These caterpillars only feed on one species of flowering plant.


Due to climate change, the host plant’s flowering cycle is starting later in the year. Predict which moth morph will dominate the population in the future.



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Answer

Late emergers

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Question

________ selection selects for the average phenotypes and against more extreme phenotypes.

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Answer

Stabilising

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 ________ selection favours one extreme phenotype and usually occurs when changes in the environment shift the optimum phenotype to the left or right of the original value.

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Answer

Directional

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________ selection, also known as diversifying selection, favours extreme phenotypes rather than intermediate phenotypes, and is the least common form of selection.

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Answer

Disruptive

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Question

What is polymorphism?


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Answer

Polymorphism is the existance of several phenotypes in a population. It occurs when there are many genetically distinct forms (morphs) of a species that still interbreed in the same population.

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Question

Which morph of the peppered moth was most abundant in industrial England?


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Answer

The melanic morph

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Question

Suggest which form of selection best describes the following statement.


Polar bears with a birth weight of less than 0.4kg have a significantly lower survival rate.

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Answer

Directional selection

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Question

Suggest which form of selection best describes the following statement.


This type of selection is involved in speciation.

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Answer

Divergent selection

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Question

Suggest which form of selection best describes the following statement.


Grey moths are less successful than white or black moths.

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Answer

Divergent selection

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Question

Suggest which form of selection best describes the following statement.


Pangolins with longer tongues are able to forage more effectively.

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Answer

Directional selection

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What is natural selection?

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Answer

The differential survival and reproduction of individuals due to differences in their phenotype.

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Why is genetic variation important?


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Answer

Because without genetic variation, a population wouldn't be able to evolve in response to the changing environment and, as a result, may have increased risk of extinction.

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Question

What are the three types of selection?

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-        Stabilising selection

-        Directional selection

-        Disruptive selection

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What is continuous variation?

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Answer

 A characteristic that changes gradually over a range of values shows continuous variation. For example, height and weight.

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What is discontinuous variation?

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A characteristic of any species with only a limited number of possible values shows discontinuous variation. For example, sex and eye colour

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What type of environment does stabilising selection occur in?


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Answer

An environment where environmental conditions remain constant for long periods.

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What are factors that affect a populations survival rate?

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Answer

Selection pressures

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Question

What are the steps of natural selection?

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Answer

  1. Random mutations produce new alleles of a gene.
  2. Under certain environmental conditions, the new alleles may be beneficial, meaning the organism is better adapted for survival. 
  3. The organisms will survive and reproduce to create offspring which will inherit the advantageous allele.
  4. Over several generations the advantageous allele will increase in frequency within the population. 

Show question

Question

What was the book written by Charles Darwin called?

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Answer

On the Origin of  species

Show question

Question

Give three examples of abiotic factors.

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Answer

  • Light
  • Water
  • Temperature

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Question

Give three examples of biotic factors.

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Answer

  • Predation
  • Competition for resources
  • Disease (e.g., parasites)

Show question

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