A species describes a group of organisms with similar morphological, physiological and behavioural features that can interbreed to produce fertile offspring and are reproductively isolated from other species.
As different populations diverge, they may become isolated from one another and become genetically distinct. Eventually, these populations (or subsets) may lose the ability to interbreed.
Speciation refers to the evolution of new species from existing species, where the new species are genetically different and reproductively separated from the ancestral species. Speciation is the reason for the incredible diversity of life we can observe on Earth today.
Speciation: the evolution of new species from existing species, where the new species are genetically different and reproductively separated from the ancestral species.
How are new species formed?
Speciation primarily occurs when two populations undergo reproductive separation, followed by genetic changes due to natural selection.
Reproductive separation is the inability of individuals to interbreed with others in the same population due to drastic changes in their alleles.
Suppose a hypothetical population of mice lives and freely breeds in a large grassland area. A large new development project divides this grassland in two, effectively separating the population into two new populations that cannot easily intermingle. The two populations are now reproductively isolated.
Suppose these two areas have slightly different environmental conditions, resulting in different selection pressures. Over time, both populations respond to the selection pressures in their respective habitats. Their gene pools begin to change as alleles that facilitate adaptation to their local environment become more frequent than less beneficial alleles.
Eventually, the two populations become so genetically distinct that even if they were allowed to intermingle again, they could no longer interbreed successfully. At this point, they would be considered different species.
Small populations and genetic drift
Smaller populations tend to experience genetic drift. Smaller populations have less genetic diversity; they possess a smaller variety of alleles than large populations.
Due to genetic drift, new mutations might randomly become fixed in the population much more quickly than in large populations, causing speciation to occur more quickly.
Genetic drift is less likely to affect larger populations, as any random variations in allele frequency will spread out across the whole population.
Genetic drift is when chance affects which individuals in a population survive, breed and pass on their alleles.
Alleles: different versions of the same gene.
What are the types of speciation?
To split into two or more species, populations must first become reproductively isolated. This isolation can occur if the populations become geographically separated, known as allopatric speciation. Another way is if the population remains in one area, but becomes separated by other means, for example, isolation mechanisms. This phenomenon is known as sympatric speciation. In parapatric speciation, a species is spread out over a large geographic area.
Allopatric speciation
In allopatric speciation, populations become geographically separated and live in different places.
Allo- is derived from the Ancient Greek allos, meaning other, while -patric is derived from patris or fatherland.
Fig. 1 - Allopatric speciation
Geographical separation comes about in several ways. For instance, a physical barrier that prevents two populations from intermingling with one another might appear. Barriers can include bodies of water like oceans and streams as well as landforms such as mountain ranges. Some barriers, such as mountains and oceans, might form over several millennia. Other barriers, such as man-made dams and roads, form relatively quickly.
What counts as a barrier also depends on the size and mobility of the species. For instance, a large river might be impossible to cross for small mammals and terrestrial insects but aren't an obstacle for fish and reptiles. Plants cannot uproot themselves and move to a new location, while some birds can migrate for thousands of kilometres.
If the environmental conditions on either side of the physical barrier are different, then natural selection will influence the two populations in different ways over several generations.
Mountain ranges create different environments on both sides. Besides acting as a barrier for organisms, mountain ranges also act as a barrier to the flow of air. The leeward side of the mountain is sheltered from the wind, creating a dry, warm climate. In contrast, the windward side of the mountain will be cold and wet.
Populations on opposite sides of the mountain would therefore be exposed to very different selection pressures and could very well become different species.
Sympatric speciation
In sympatric speciation, populations remain in the same place but become separated through mechanisms of isolation. A new species develops spontaneously.
Sym- is derived from the Greek syn meaning together.
It is thought that sympatric speciation occurs due to different organisms occupying different environmental niches, known as ecological separation.
Ecological separation: populations are separated because they live in different environments within the same area.
Niche: the area in which an organism resides (including abiotic and biotic factors) and the organism's role.
Fig. 2 - Sympatric speciation
Sympatric speciation frequently occurs in bacteria because bacteria can exchange genes with other individuals in a process known as horizontal gene transfer. Sympatric speciation has been observed in Bacillus and Synechococcus species of bacteria.
The horizontal gene transfer of bacteria describes the exchange of genes with other bacteria that are not offspring from binary fission. The three mechanisms include conjugation, transformation and transduction.
Parapatric speciation
Parapatric speciation occurs when a species is spread out over a large geographic area. Members of the species can mate with each other; however, individuals will only mate with those in their geographical region. The species are separated by the differences within the same environment, which occurs by an extreme change in the habitat.
Fig. 3 - Parapatric speciation
Parapatric speciation can occur when an environment is polluted. For example, mining will leave large amounts of metals in the soil. Grasses, such as buffalo grass, can tolerate the metals. Buffalo grass is a unique species from the grasses that grow non-polluted areas. Buffalo grass seeds pass on their characteristics to their offspring.
Example mechanisms of isolation
Isolating mechanisms describe the characteristics that prevent different species from interbreeding successfully.
Different niches
Populations can inhabit different niches within the same area, making it so that members of different populations rarely interact.
For example, insects usually lay eggs within the type of fruit in which they were born; as a result, individuals will specialise in feeding and mating on particular fruits over time. This phenomenon leads to less gene flow between different fruits, making them reproductively isolated.
Temporal separation
Populations can also become separated through time. For instance, a mutation might arise that causes some members of the population to start breeding earlier in the year. Eventually, when the breeding seasons of different groups are far enough apart, they do not overlap, and the groups will not be able to interbreed.
Behavioural separation
In many species, mating is preceded by courtship. Same-species recognition is a crucial part of this process, as individuals will have the highest chance of producing viable offspring if they mate with others of their species.
Mutations that cause noticeable changes in the appearance or behaviour of an individual, such as changes in the colour of their feathers or the sounds of the mating call, can prevent others from recognising them as potential mates and thus avoid reproducing with them.
We've read examples of animals and organisms undergoing speciation, but how about humans?
Humans are still evolving, just like every other organism on Earth. However, theoretically, if different human populations become geographically separated and did not intermingle, and their habitats possess different selection pressures, they might accumulate enough genetic differences over many generations.
However, given our present level of globalisation and technology, this is highly unlikely!
Speciation - Key takeaways
Speciation refers to the evolution of new species from existing species, where the new species are genetically different and reproductively separated from the ancestral species. This separation occurs when two populations undergo reproductive separation.
In allopatric speciation, populations become geographically separated and live in different places.
In sympatric speciation, populations remain in the same place but become separated through some mechanism of isolation.
In parapatric speciation, populations live in the same geographical area but possess different niches, which prevent interbreeding.
There are many mechanisms of isolation that prevent the interbreeding of different species. Examples include spatial separation, temporal separation, or behavioural separation.
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