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
Random mutations produce new alleles of a gene.
Under certain environmental conditions, the new alleles may be beneficial, meaning the organism is better adapted for survival.
The organisms will survive and reproduce to create offspring, which will inherit the advantageous allele.
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.
Fig. 1 - Process of natural selection
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.
Fig. 2 - Change of predominant phenotype over time due to natural selection
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:
More organisms are born than can survive
Characteristics vary within a species
Variations are inherited
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.
Fig. 3 - Exponential growth
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:
Examples of biotic factors include:
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