According to Darwinian evolutionary theory, the fitness of an organism or the species as a whole depends on the number of offspring that are left behind by themselves that survive and are capable of reproduction thus furthering their genes and continuing the survival of the species population as a whole.
With respect to that, organisms or species used to be classified in between two extreme types of species in evolutionary terms - the r selected species and the K-selected species. The r-selected species are the species that produce a higher number of progenies or offsprings but offer less parental care i.e. less individual parental investment, whereas the K-selected species produced less offspring or number of progenies but providing more parental care or parental investment to each individual.
Many species lie in-between the two extremes. But this theory in recent years has been replaced by the life-history paradigm which seeks to explain these factors of quality vs quantity based on several factors such as surroundings, etc., although it derives several important themes from the r/K selection theory.
A Brief Explanation of R-Selected Species
In the r selected species, the r can be said to be a reference to reproduction. The r strategist species are the ones that are said to be dependent on quantity over quality for the propagation of their genes over generations. In the evolutionary processes, the r strategist finds an advantage in the survival factor because of sheer numbers. If the numbers are more, then even under adverse conditions that can lead to the deaths of many offspring, some of them will survive and thus being capable to reproduce, they will pass on the genes to the next generation. Thus, this concept is completely based on the advantage of the quantity of offspring that an organism can produce for the survival of the species even while facing high mortality rates.
But in this process, there is a significant impact on the parental investment in the offspring of the r selected population. Here, parental investment means, parental care and protection, nurturing and providing nutrition to the offspring for a longer period of time, teaching to the offspring the methods of survival like hunting in wild animals, etc. Since in r strategist species there are a large number of children it is difficult for the parent or parents to invest and focus on each individual. Hence, because of this, the offsprings or progenies in an r selected population even mature early as compared to the K selected population. In some of the r selected species examples, it is observed that after the birth of progenies or laying of eggs, the parent usually leaves them off and goes on to live their lives accordingly. These are the cases that lie towards the r selection extreme of the spectrum of selection regime or quantity of population vs the quality of the population.
These are the two important characteristics of r selected species that provides the explanation of the concept. Other characteristics of r selected species are given as follows:
R selected species are the ones that are prominently involved in high growth rates.
They usually exploit less-crowded areas and ecological niches, each producing many offspring. Less crowding provides less competition for the resources.
The offsprings have less probability of surviving to adulthood owing to several environmental conditions.
The characteristics of r selected species prove them dominant species in the unpredictable or unstable environment because of their ability for rapid reproduction.
Although high in number, the r selected population are less likely to adapt rapidly to the changing environment and hence their survival depends on the number of offspring.
The traits that are important for the characteristics of r selected species include high ability to reproduce i.e. high fertility rate, small body size, early onset of maturity, short time of generation, and the capability to disperse off the progeny widely.
R strategists are usually the primary colonisers, especially in areas of major ecological disruption such as regions surrounding volcanic eruptions. They are followed by a succession of increasingly competitive flora and fauna.
R Selected Species Examples
There are many r selected species examples among which the most common ones are the rodents or rats. They are the most common and relatable species that is best to explain the concept of r selected species. Rats produce a number of progenies and then after tending them for a few days they disperse off their offspring. Also, they have the qualitative traits that define the characteristics of r selected species such as small body size, high ability to reproduce, early maturity and shorter time of generation as compared to any other species such as humans. This example also demonstrates the variation in the spectrum observed among a particular phylogeny as both rats and humans are mammals.
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Other R Selected Species Examples include the Following:
The most common species of organisms that can be found almost anywhere on Earth is bacteria. A bacterial species is the best example of the r selected species.
Diatoms are a group of algae or more specifically microalgae, and are found in oceans, waterways, and soils of the world. They are also r selected species.
In arthropods, insects are one of the best examples of r strategists. They lay eggs in huge numbers and this is the biggest reason for the survival of these species. Even with the development of modern pest control methods such as insecticides, they are able to survive because they lay huge numbers of eggs around a potential nutrient source.
Amongst, the plants of the different species of grasses are r selected. Even though they are trampled by and destroyed by large mammals such as elephants, they tend to survive because of their number. Also, they prove a basis for the development of more competitive species of exclusive plants, trees and other vegetation, which mostly are found in a particular region, unlike grass.