Interbreeding of species?

[Akamia]

I'm studying for a test on the natural sciences – a step toward an end goal of a degree in computer science – and the textbook I'm studying from had two statements that appear to conflict with each other.

Genetic drift occurs within finite separated populations, allowing that population to develop its own distinct gene pool. However, occasionally an individual from an adjacent population of the same species may immigrate and breed with a member of the previously locally isolated group. The introduction of new genes from the immigrant results in a change of the gene pool, known as gene migration . Gene migration is also occasionally successful between members of different, but related species. The resultant hybrids succeed in adding increased variability to the gene pool.

In order for a new species to develop, substantial genetic changes must occur between populations, which prohibit them from interbreeding. These genetic changes may result from genetic drift or from mutation that take place separately in the two populations. Allopatric speciation occurs when two populations are geographically isolated from each other. For instance, a population of squirrels may be geographically separated by a catastrophic event such as a volcanic eruption. Two populations (separated by the volcanic flow) continue to reproduce and experience genetic drift and/or mutation over time. This limits each population’s gene pool and produces changes in expressed traits. Later, the geographical separation may be eliminated as the volcanic flow subsides; even so, the two populations have now experienced too much change to allow them to successfully interbreed again. The result is the production of two separate species.

In the former statement, it appears to be saying that different species can successfully interbreed – with the caveat that the species are related – but the wording of the latter does not appear to allow for that.

Is it an oversight on the textbook editor's part, or am I missing something huge?


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[Nesslig20]

I'm studying for a test on the natural sciences – a step toward an end goal of a degree in computer science – and the textbook I'm studying from had two statements that appear to conflict with each other.

Genetic drift occurs within finite separated populations, allowing that population to develop its own distinct gene pool. However, occasionally an individual from an adjacent population of the same species may immigrate and breed with a member of the previously locally isolated group. The introduction of new genes from the immigrant results in a change of the gene pool, known as gene migration . Gene migration is also occasionally successful between members of different, but related species. The resultant hybrids succeed in adding increased variability to the gene pool.

In order for a new species to develop, substantial genetic changes must occur between populations, which prohibit them from interbreeding. These genetic changes may result from genetic drift or from mutation that take place separately in the two populations. Allopatric speciation occurs when two populations are geographically isolated from each other. For instance, a population of squirrels may be geographically separated by a catastrophic event such as a volcanic eruption. Two populations (separated by the volcanic flow) continue to reproduce and experience genetic drift and/or mutation over time. This limits each population’s gene pool and produces changes in expressed traits. Later, the geographical separation may be eliminated as the volcanic flow subsides; even so, the two populations have now experienced too much change to allow them to successfully interbreed again. The result is the production of two separate species.

In the former statement, it appears to be saying that different species can successfully interbreed – with the caveat that the species are related – but the wording of the latter does not appear to allow for that.

Is it an oversight on the textbook editor's part, or am I missing something huge?

These two statements aren't really in conflict with each other. It is known as the species problem. Because of evolution, lifeforms are fluid dynamical, constantly changing into new subtle varieties. Thus concepts like "species" that are trying to put life into distinct boxes often don't apply to everything.

The simplest depiction of how new species evolve from an existing species is when a species is split into two populations.

This is known as allopatric speciation, but a more frequent form is Peripatric where the isolated population is much smaller than the main one.
Right after the split, these two population are still the same species, but over time, unique mutations that get fixed in each population will drive them further apart, becoming increasingly more distinct as time goes on.

Gene flow inhibits this, by preventing the two populations to become different from each other. However, in this example for simplicity, we prevent gene flow from happening, so what will happen?

At first they will become different varieties of the same species that can still interbreed, such they are different subspecies. If enough change has occurred, it will be increasingly more difficult for the two populations to interbreed once more.

There can be no moment that you can point out as saying "right at this moment they are no longer able to interbreed". It is gradual, at first they still can interbreed but, they usually won't by factors of geographical isolation or different behaviors, such that they usually just won't mate even they come into contact with each other. And much later on, fertility becomes an issue when hybridization becomes increasingly more difficult. Eventually the hybrids themselves won't be even viable anymore.

There are many examples of this spectrum of degrees of interbreeding (gene flow) between different species. A very good example is the family of Ursids (i.e. bears)

Picture from https://www.allgrizzly.org/evolutionary-relations
Very interesting to read this page.

Pandas are bears that have diverged early one from all other extant bears. They are so far apart that no panda can produce any hybrid with any other species of bear. Conversely on the other end of this spectrum, the relation between brown bears and polar bears are very interesting. They are very closely related. So close that we now know based on their genomes that frequent gene flow has occurred between them. In fact, Polar bears might not even be a thing. Some consider them a subset of brown bear or a new species of brown bear that evolved from the now paraphyletic brown bear species. Whether the polar bear is a subspecies or a different species from brown bears, is still up for debate.
https://whyevolutionistrue.wordpress.com/2010/03/17/do-polar-bears-exist/

I hope this answers your question. Feel free to ask more.

 

[he_who_is_nobody]

It reads to me as if they are talking about the same thing, but left out time as a factor. The first example appears to be talking about a population that is separated, but every dozens or so generation, members from both populations are able to mix and share their genes. The second page seems to be talking about populations that were seperated for hundreds to thousands of generations before mixing. By that point, there probably is enough change in both population, just based on genetic drift, that the populations could not interbreed even if they wanted.