You are sure to know that humans developed from apes millions of years ago. But how did we become so different from our distant relatives, the bonobos and chimpanzees? Many changes that persist in species develop by random mutations and stay within the species because they are advantageous at a given point. They give the individuals with said traits an upper hand when it comes to survival and reproduction. This phenomenon is known as natural selection.
Although the reasons are not definite, it is most likely that natural selection is behind why humans lost their tails, learnt to walk upright and lost most of their hair. The hypotheses are quite diverse: for example, not having a tail could have aided with walking on the hind legs, which in turn helps with carrying tools, a key to humans' survival. However, it could be that walking upright was just an advantage initially because we transitioned from living in the jungle to the savannah. What seems clear, though, is that there must have been some reason as to why the individuals that lost their tails and their hair, and could walk upright passed on their genes more frequently to such a point that now all humans walk upright and have no tail. Hairiness varies, but most of us are far from having such a thick coat as a chimpanzee!
- What is natural selection?
- The process of natural selection
- Stages of natural selection
- Conditions for natural selection
- Limited population growth
- Selection pressures
- What is fitness in biology?
- Natural selection example
- Evolution by natural selection
- Why is variation crucial for natural selection and evolution?
- What happens when a population becomes too small?
What is natural selection?
Charles Darwin was the English naturalist who first developed the idea of natural selection after a five-year voyage, during which 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 ideas of evolution and natural selection to the world’s attention. The theory was also conceived independently by Alfred Russell Wallace.
The case of Darwin's finches is the most well known. Darwin observed that there were over a dozen species of finches on the Galapagos Islands, each one with their own characteristics, most notably beak size and shape.
Each species seemed to have a beak that was well suited for eating the food the bird's species ate. However they also all seemed to have some similarities that pointed that they had derived from the same original bird species, i.e. that they had a common ancestor. Darwin, therefore, hypothesized that the birds that are now known as "Darwin's finches" all proceeded from that common ancestor and that they had changed, i.e. that they had adapted, to the environment they lived in to form different species. He also hypothesised that this happened by natural selection.
Natural selection is the process by which the environment influences the survival and reproduction rates of organisms within a species.
For example, in the case of Darwin's finches, the common ancestor landed on the Galapagos Islands with a general set of characteristics. The competition between individuals would have been so fierce, that only the best-suited organisms could survive in each niche of the environment. The birds with a bigger and curved beaks would have been able to feed from big seeds, whilst those with thinner and pointed beaks would have gone for insects. The differences in beak structures would have become enhanced as the organisms with those features survived and reproduced better, until two species formed, one with a big and curved beak and another with a thin and pointed one.
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. By 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.
Evolution: a change in heritable characteristics of a population over time.
The process of natural selection
Genetic variation is key in the process of natural selection. Genetic variation exists in populations due to the presence of different alleles, i.e. different variants of the same gene.
For example, because different alleles exist to code for eye colour, humans can have very varied eye colours. The same happens with genes that are more relevant for survival, like genes that predispose or protect against diseases.
The individuals with alleles that grant them an advantage over factors that affect survival (selection pressures) will reproduce and survive better. Therefore, the alleles that they carried will increase in frequency within the population. As a result, the phenotype frequency also changes. This phenomenon is natural selection. Over time, we can expect the frequency of advantageous alleles to increase in a population and the frequency of deleterious alleles to decrease.
Stages of natural selection
Let's take a closer look at the process of natural selection and it's stages:
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 and reproduction.
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, and thus the advantageous traits will also appear more frequently.
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.
Conditions for natural selection
There are 4 conditions that lead to natural selection. These are:
More organisms are born than can survive
Characteristics (alleles and traits) vary within a species
(Some) variations are heritable, i.e. they can be passed from the parent generation to the offspring via their genes.
There are differences in the reproduction and survival rates of individual organisms within a species due to these variations.
Limited population growth
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.
Exponential growth is the type of growth where the number of individuals increases proportionally to the population's size. I.e. if there are two individuals at an initial time point, there will be a multiple of 2 in the next generation (4, 6, 8, etc.) and then a multiple of that number in the next.
Fig. 2 - Exponential growth of a population
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 pressures
Selection pressures are environmental conditions that affect a population or organism’s chances of survival. Selection pressures can be either abiotic (non-living factors) or biotic (living factors).
Examples of abiotic factors include the availability of:
Examples of biotic factors include:
Predation
Nutrient/food availability
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 only 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 that are not.
Natural selection example
As a simple example of the natural selection process, let’s imagine a small population of mice that live on a dark sand beach.
The light mice are more vulnerable to predators, and as a result of this predation, their population 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. 3. Change of predominant phenotype over time due to natural selection
Evolution by natural selection
Natural selection is not an isolated process, but rather it combines with genetic variation and environmental changes to contribute to the overall changes in a species' characteristics through time, i.e. to a species' evolution.
In other words, evolution is a consequence of genetic variation and natural selection. To understand this further, let's consider the example of the brown and white mice again.
Because originally dark mice have a higher chance of survival among the dark beach sand, the majority of mice have dark fur. However, when human intervention changes the sand colour from dark brown to white (which constitutes a change in the environmental conditions for the mice), white mice will be best suited to survive and reproduce.
Over time, the population of dark mice will decrease. It might decrease so much that every dark brown mouse that is born is quickly picked by predators so that, eventually, only white mice live long enough to reproduce. Then, after some time, it could be that only white mice are born and reproduce because the animals with brown fur do not live long enough to contribute their genes (their dark fur alleles) to the next offspring generation.
Eventually, the "beach mouse" will have turned from a dark to a white species. I.e. the beach mouse will have evolved from a dark fur species to a white fur species.
Why is variation crucial for natural selection and evolution?
Variation is crucial for evolution to occur, as natural selection cannot happen if all individuals are the same within a population (there will be no better-suited group for survival, i.e. there won't be two groups from which to "select" the one with the highest chances of survivals).
Adaptation to environmental changes would not be possible without variation because there would only be one variety of animals that would either react well or badly to a change in the environment.
In the case of the beach mice, if all animals had dark fur at the beginning of the example when the sand is dark too, they would all survive well. However, when humans change the sand from dark to white, all individuals would stand out against the new environment, and predators would easily pick out all of them. This could mean that the population decreases significantly. The mice would need to develop other survival strategies, like living mostly underground, to avoid predators, or they could go extinct.
A species extinction always affects other species in the same environment. If the mice were to go extinct, their predators would also have to adapt: they would need to find other prey to feed, or their population numbers would also go down. On the other hand, the plants or insects from which the mice fed would increase in number if there are no other species to feed from them and control their population.
At any given time point, in a large population with a wide range of phenotypes (caused by genetic variation), 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. Without this variability, a species is very vulnerable to environmental changes.
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.
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