Why Does Divergent Evolution Occur?
Divergent evolution usually occurs when two or more separate species evolve differently from a common ancestor i.e. speciation in simple words. The newly evolved species are now better adapted to withstand the environment and its impact. These new species have similar looking body structures that perform different functions, proving their common ancestry.
Divergent evolution is a type of evolution in which an ancestral species over the course of evolution adapts various changes which leads to the formation of many descendent species.
It occurs when a group from a specific population develops into many new species populations.
Diverge means to separate from one point to many different points. And so, divergent evolution simply means that one ancestral species evolve into many descendent species.
It is also important to note that, adaptive radiation is actually a type of divergent evolution because it occurs when two or more separate species evolve differently from a common ancestor.
In Divergent evolution, different body structures evolve for different functions but are similar in anatomical structure. Natural selection may mold different body structures of the same ancestral species in ways that perform a lot of different functions, but these body structures may look similar.
The organs having common origin but performing different functions are known as homologous organs and the phenomenon is called homology. Homology is actually the result of divergent evolution.
Divergent Evolution occurs due to the following broad reasons:
1. Changes in abiotic factors
Divergent evolution may occur as the response of an organism to changes in abiotic factors, such as a change in environmental conditions, or when a new niche becomes available.
Change in the environment causes an alteration of the normal state of an ecosystem. It can be a result of natural causes like earthquakes, volcanic eruptions, etc. or various other human causes.
The new environment can have various different abiotic factors such as temperature and rainfall that vary from the previous environment based on latitude and elevation.
And so, after the environment is changed, this can create a challenging situation for the organism to survive and reproduce with the different abiotic factors which the organism isn’t used to.
And, this may force the species to try to adapt to its new environment which may lead to divergent evolution.
The concept that abiotic factors are relevant to the evolution and adaptation of living beings has been present since the early days of modern biology.
2. Changes in biotic factors
Divergent evolution may also take place in the response of an organism to changes in biotic factors, such as increased or decreased pressure from competition or predation.
Biotic factors are the living organism that shapes its environment. There’s always the occurrence of competition amongst the various biotic factors in an ecosystem for survival.
As the biotic factors of an environment change, the organisms living there get new competitors, predators, and co-ordinators. So, the organisms need to compete and adapt themselves with their new biotic factors for the ease of their survival.
And, the environment demands that species are well and fit enough in order to compete for its survival and reproduction. Those less fit and unfit organisms are destroyed over time.
And so, in order to survive and reproduce, all living organisms must adjust to the conditions imposed on them by their biotic factors. This makes them better predators, or mutually associated organisms and so better survivors overall.
Thus the organisms are better able to survive in their new environment with the various other biotic factors that are also available there. And so, slowly over time divergent evolution can occur.
3. Microevolution & Macroevolution
Any type of evolution can take place on a small-scale or large-scale. Small-scale evolution is called microevolution, and large-scale evolution is called macroevolution.
Microevolution is just a small evolution like changing a few genes leading to the creation of new alleles within a population. Microevolution just leads to the change in allele frequencies in a gene pool.
Whereas, macroevolution is a very large-scale evolution that results from the sum total of all of the microevolution that took place within a population. This can lead to the formation of new species i.e speciation.
Divergent evolution or adaptive radiation is a type of macroevolution because it leads to speciation that is the formation of new species.
In such an evolutionary process, the ancestor species shows various small adaptations to its environment and gradually becomes different from the ancestor species after achieving many of such small adaptations altogether.
For example: The finches (small black birds) in the Galapagos Island shows that those species that eat large seeds tend to have large-tough beaks, while those that eat insects have thin-sharp beaks. All have originated from their ancestral seed-eating finches through large-scale macroevolution.
4. Mass Extinction
Mass extinction is the massive destruction and wipe-out of a large number of species on earth within a relatively short period of geological time.
Large-scale destruction of habitat, unfavorable environment conditions, massive natural disasters, increase of predators and competitors, etc. are some of the causes of mass extinction.
Due to mass extinction, the organisms that were best adapted to the environment survive and the unfit or upadapted ones get destroyed and become extinct.
Divergent evolution or adaptive radiation is a universal occurrence after mass extinctions because the surviving species gradually evolve themselves over time and try to acquire the available resources that once the extinct species had utilized and used.
In doing so, they try to dominate over other species while at the same time adapt themselves to better withstand the environment.
For example: Fossil record indicates that mammals underwent dramatic divergent evolution/adaptive radiation to form thousands of present-day mammalian species after the extinction of terrestrial dinosaurs, about 65 million years ago. This is a perfect example of Adaptive Radiation due to mass extinction.
5. Genetic Drift
Genetic drift is a mechanism of evolution in which allele frequencies of a population change over generations due to chance.
It can result in the loss of some alleles (including beneficial ones) and the fixation, or rise to 100% frequency of other alleles.
For example: In the mouse population, the allele frequency of the dominant B allele (black fur) is 40% and the allele frequency of the recessive b allele (brown fur) is 60%. Now, if we see that the frequency of the B allele drops to 30% we can conclude that the population of the mouse is evolving.
Due to genetic drift, one population can slowly accumulate genetic differences thus causing evolution.
And over time due to large-scale evolution, that population can become so different that the members are eventually considered different species that once shared a common ancestor. Thus, leading to divergent evolution.
However, divergent evolution into many closely related species is difficult to explain by genetic drift alone. So one needs to consider other factors like gene flow, natural selection as well.
6. Gene Flow
Gene flow is the movement of genes from one population to another population. This can happen if a population of one place migrates and reproduces with a population of another place.
This will change the allele frequencies of the gene pool of a place due to the flow of genes by the population of another place.
It’s when the members of a population interbreed, they share a common group of genes called a gene pool. And if any new genes enter the gene pool new changes appears in the offsprings.
Due to new changes, divergent evolution can also occur within the sub-population if it totally becomes isolated from the rest of the population.
Once the isolation occurs, natural selection and genetic drift operate on the subpopulation independently, producing different divergent evolutionary outcomes leading to speciation.
For example: A domesticated Reindeer sub-population in western Alaska escape and mate with a migrating Caribou sub-population, adding the alleles for short legs and tame behavior in the gene pool of the Reindeer sub-population. Now if the domesticated reindeer sub-population in western Alaska becomes completely isolated and only reproduces strictly amongst its own members then divergent evolution will occur over time.
Divergent Evolution leads to Speciation
All of the above-mentioned causes lead to the formation of many species over time and this timing may be between several thousand to millions of years of evolution.
Speciation is how many new kinds of plant or animal species are created from a single ancestor. It occurs when a group within a species separates from other members of its species and develops its own unique characteristics.
Speciation is one very important result (consequence) of divergent evolution and adaptive radiation is a type of divergent evolution which highly supports it.
But, it is also to be noted that the occurrence of more than one adaptive radiation in an isolated geographical area with different habitats leads to convergent evolution and not divergent evolution.
There are 4 major ways speciation can occur from divergent evolution. These are:
- Allopatric Speciation: It occurs when a species separates into two separate groups that are isolated from one another.
- Peripatric speciation: It is a form of allopatric speciation that occurs when populations that have become isolated have very few individuals.
- Parapatric speciation: It occurs when a population is continuously distributed within a geographic area without any specific barriers to gene flow.
- Sympatric speciation: It is the evolutionary process whereby species are formed from a single ancestral species while inhabiting the same geographic area.
