Reproductive Isolation

Model of an Island. Getty/Roger T. Schmidt

Evolution is the defined as the change in species over time. While there are many different definitions for the generic term "species", a biological species is most often described as a group of similar individuals that can interbreed in nature and produce viable and fertile offspring. These species arise through a process called "speciation" that relies on the reproductive isolation of a group of individuals from another similar population of the same species.

In order for the speciation to occur and different lineages to diverge from a common ancestor, these populations must remain isolated from each other so they cannot get back to together to reproduce.

There are several ways a population can be reproductively isolated from its former larger population. These isolations can be physical barriers that keep the populations apart, or they can be on a more behavioral level having to do with reproductive rituals or fertile times. Finally, the reproductive isolation may be a product of a difference of chromosomes on a micro level that no longer are compatible with the other population's chromosomal structure.

Physical Isolation Leads to Reproductive Isolation

In Geology, the Theory of Plate Tectonics says that at one point in the history of the Earth, all land masses were together as one giant supercontinent called Pangaea. During this time, individuals were able to walk from continent to continent with little to no problems or obstacles.

As the continents started to drift apart, large oceans and other types of bodies of water began to form between the land masses and walking from one continent to another was no longer possible. These very large bodies of water physically separated what were once the same species so they could no longer interbreed.

Over time, this separation, along with mutations at the DNA level and adaptations for the new environments, caused the species to diverge and become classified as new species since they were no longer compatible.

Physical isolation does not have to be on that big of a scale as a drifting continent. Something as simple as a tree falling in a forest can physically isolate different populations of the same small species, such as a snail or slug. Human interference can also physically separate populations and drive speciation. For instance, building a dam in the middle of a river may separate once interacting fish populations that will then cause them to change and diverge as new speices.

Reproductive Isolation on a Chromosome Level

A chromosome is a strand of DNA wrapped around a histone protein core that carries all of the genetic information in an individual. During meiosis, chromosomes are split and separated into gametes that can then be used during sexual reproduction to pass down the genetic traits to the offspring. Individuals whose chromosomes do not match up, or who have different numbers of chromosomes, are not able to produce offspring. Even if the alleles that code for certain traits are found on different chromosomes or different places on the same chromosome, the individuals will not be able to reproduce a viable and fertile offspring.

Therefore, individuals of different species are reproductively isolated from other species on a chromosome level.

Once a population has been isolated for a long period of time, many random mutations produce new alleles that are advantageous for their environments and become a part of the DNA that is passed down to offspring. Those new alleles are no longer compatible with the older make up of the genes in the original population and speciation has occurred. While in some cases it may be possible for these now new species to produce a hybrid offspring if they mate, those offspring will not be viable or fertile so the two populations would then be considered different species. This is known as the Dobzhansky-Muller Model.

There are also instances of the size of chromosomes to change through mutations over time.

Some smaller chromosomes can fuse with other smaller chromosomes to make a larger chromosome with a new mix of genes in various locations. This reduces the chromosome number so the new species is no longer able to sexually reproduce with an old population of what used to be the same species because they do not have the same number of chromosomes. This process of chromosomes coming together is known as centric fusion and is rare, but has happened in some species of bats.

Mistakes in meiosis can also lead to speciation. Gametes of sexually reproducing individuals should be haploid, meaning they should only contain one set of chromosomes. Then, when fertilization occurs and the gametes fuse into a zygote, the zygote is then diploid and has the necessary two sets of chromosomes. However, if a mistake happens when making those gametes, then the fusion occurs and could produce multiple sets of chromosomes. This process is known as polyploidy. It actually happens frequently in plants, but it usually a lethal mutation in animals.

Reproductive Isolation Due to Behavior

Behavioral isolation can also lead to reproductive isolation. Many different species have different mating rituals that must happen in order to reproduce. This could be a certain mating call, an elaborate display or dance, or even a type of courtship that can last for quite some time. If the mating rituals of individuals do not match up, they are no longer sexually compatible and therefore reproductively isolated from others who have a different mating ritual. Even small changes in the mating ritual, such as the pitch of a frog's mating call, can cause a formerly interbreeding population to no longer interbreed.

Another way for reproductive isolation to occur within a species is the location of mating. Especially in species that lay eggs to reproduce, the location of those eggs is very specific. If a change happens on a molecular level that changes the preferred location of where a female lays its eggs, then males the female once was able to breed with are not able to fertilize the eggs due to their location.

This is an especially important mechanism for species that undergo external sexual reproduction. The female will lay her eggs in water and the male will come and fertilize those eggs. Since it happens in water, some of the sperm from the male gets carried away with the movement of the water. If any sperm could fertilize any egg in this situation, there would be a enormous amount of hybrids swimming around in the water. Since these types of species are not compatible, then only the sperm of the same species as the egg can penetrate and create offspring.