Chapters
In this article, we will explain that natural selection takes place because populations can produce multiple offspring that compete for resources. We will also explain that the individuals that are best adapted are most likely to survive, reproduce and pass on their alleles to the next generation.
Within a species, each organism can contribute to the growth of the population by reproducing and having offspring. The population will grow exponentially if the offspring produced by an individual survives and reproduces further after reaching reproductive age.
Exponential Growth
Exponential growth only takes place in the absence of environmental factors or population checks on the population. For instance, an example of one such condition is having adequate resources and no disease. In this case, the population has the potential to experience exponential growth.
Example of Exponential Growth
One of the rare and popular examples of exponential growth in a population dates back to the 1800s when 24 European rabbits were introduced to Australia. These rabbits enjoyed plentiful resources, little to no competition, and a secure environment, i.e. absence of predators. These conditions helped them to grow their population rapidly and eventually, they became a major pest.
In real world scenario, individuals in a population do not have ideal conditions as many environmental factors inhibit each individual in a population to reach adulthood and reproduce.
Environmental Factors Affecting the Growth of a Population
Environmental factors inhibit the growth of the population whenever it attains a specific size. These environmental factors can be either biotic or abiotic.
- Biotic factors: Biotic factors include other living organisms. Examples of these factors include predation, competition for resources, and disease.
- Abiotic factors: Abiotic factors include non-living components of the environment. Examples of these factors include the availability of light, supply of water, and pH of soil.
The presence of biotic and abiotic factors inhibits every individual within a population to reach adulthood. For instance, if the food is scarce, few animals within a population will not get sufficient food and will starve to death. The majority of the populations in the wild produce a large number of offspring however, very few individuals survive.
In the next section of the article, we will explain the environmental factors that limit the growth of the population through the example of African lions.
Environmental Factors that Limit the Population Growth of African Lions
Many environmental factors inhibit the growth rate of the population of African lions living in the wild. These environmental factors are discussed below:
- Competition for Food
The supply of prey is not enough because other lions and carnivores are also looking after the same prey. If the lion cannot hunt and feed, then it will starve to death.
- Competition for a Reproductive Mate
Within a population, the number of female lions is often more than the number of male lions. It implies that the male lions in the population compete with each other to find a female mate. When a single male lion fights with another lion for a mate, then one or both lions can get injured or killed. The lion losing the fight won’t be able to mate with a female lion and produce offspring.
- Water Supply
The arid African habitat during the dry season is another environmental factor that limits their population. During the dry season, the water resources for the lions can be several miles away. In this weather, lakes also dry up, which can further kill many lions because of dehydration.
- Temperature
The African habitat is characterized by its extreme heat. Lions living in this habitat sometimes cannot tolerate this heat and eventually die. This heat also prevents them to hunt for longer periods during the day which means that they also experience low availability of food.
All the environmental factors discussed above play their roles to decrease population growth as only a few individuals can survive and reach adulthood.
In the next section of the article, we will discuss survival and natural selection.
Survival and Natural Selection
Within a species population, variation exists which implies that few individuals within a population have different phenotypes. Because of the genetic variation in the alleles, members of the same species will possess the same genes.
Environmental factors impact the probability of the survival of an organism. Hence, these factors act as selection pressure. Selection pressures boost the chance of individuals with a certain phenotype to survive and reproduce as compared to others. In other words, the individuals possessing the favoured phenotypes are highly fit and have a greater chance of survival. We can define the fitness of an organism in this way:
The ability of an organism to survive and transfer its alleles to offspring defines its fitness
The organisms that are highly fit have adaptations that make them suitable for their environment. The selection pressures acting over many generations of a species affect the frequency of alleles within a population via natural selection. We can define natural selection as:
The process by which individuals possessing the favored phenotype have a greater chance to survive and pass on their alleles to the offspring so that the beneficial alleles increase in frequency with time and after generations are known as natural selection.
In the next section of the article, we will discuss how natural selection occurs in rabbits.
How Natural Selection Occurs in Rabbits?
Within the populations of rabbits, there is a variation in their fur colour. A dominant allele at a single gene locus produces a brown-coloured fur. On the other hand, a recessive allele in a homozygous individual produces white-coloured fur. For rabbits, natural predators like foxes exist which act as a selection pressure. Rabbits having white fur are unable to camouflage as well as rabbits having brown fur. It implies that the predators can see white fur rabbits easily while hunting.
Consequently, the chance of survival of white fur rabbits is low as compared to brown fur rabbits. Because the rabbits with brown fur possess a selection advantage, hence they have a greater chance to reach the reproductive age and pass on their alleles to their offspring. Over several generations, the frequency of alleles for brown fur increases, and the allele frequency for white fur decreases.
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