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Posts Tagged ‘Natural Selection’

A news story has been circulating a fair bit in the past couple of weeks. This story has been picked up by numerous news and science outlets. How it is being reported and explained is just plain misleading and inaccurate.

Image result for aldabra rail

The Aldabra Rail is a subspecies of the White-throated Rail.

Here are a few titles that show how the subject is being covered.

Science Magazine – Evolution Brings Extinct Island Bird Back into Existence

Smithsonian Magazine – How Evolution Brought a Flightless Bird Back from Extinction

CBS News – An Extinct Bird Species Has Evolved Back into Existence, Study Says

From these titles, and from the bodies of the articles themselves, readers would think that the same species of bird existed at some point in the past, went extinct (as in died out completely), and then re-evolved!

That does not happen.

Here is what actually did occur.

The small atoll of Aldabra is a pretty spectacular spot. It is very remote. It is quite beautiful. It is home to a bunch of unique animals found no where else on earth. It has one of the longest fossil records on any island in the Indian Ocean.

That fossil record includes a lot of the animals that have called the atoll home over the past few million years. One of those animals was the Aldabra Rail. This rail was a small flightless bird that was probably found hunting through reed beds along the edges of water. The Aldabra Rail went extinct about 136,000 years ago at about the same time that global sea level was rising and submerging oceanic islands like Aldabra. After a few thousand years, sea level dropped and Aldabra became an exposed island once more. Not long after that fossils of a rail on Aldabra start showing up again.

There are a couple of possible explanations. One is that some remnant population of the Aldabra Rail hung on, some how, and did not die. These were flightless birds, so it is not clear how this might have happened, but perhaps a small population managed to survive on a floating raft of vegetation long enough to reach an exposed bit of land. This seems like a very long shot. It is much more likely that the Aldabra Rail simply died out completely. It went extinct.

The other possible explanation is much more likely and widely understood and accepted, and it is this: the Aldabra Rail went extinct when the atoll went under water. Then after it re-emerged, a group of birds likely from the same parent stock of the original Aldabra Rail re-colonized the atoll (quite probably from Madagascar). This new group of colonizers eventually became flightless and filled the same, or very similar, ecological niche as the original Aldabra Rail.

This is a process called iterative evolution and it is pretty rare. The definition of iterative evolution is: the evolution of similar or parallel structures in the development of the same main line.

But iterative evolution does not produce the same species twice. It may produce similar species, but to produce the same species twice would require starting with the same gene pool twice. The group of birds that first colonized Aldabra, and became the Aldabra Rail 1.0, had a unique combination of genes to work with. The group of birds that later colonized Aldabra, and became the Aldabra Rail 2.0, had a unique combination of genes to work with. Those two combinations of genes may have been similar, but they were not the same. Therefore the decedents of those two groups would not be the same.

I really think that the implications of how this story is being reported is really misleading and possible even damaging.

Misleading because they imply that a species can evolve twice. To go back to the definition of  iterative evolution, it the evolution of “similar or parallel structures…” Similar or parallel structures are not the same as identical species. Two rails that evolved at different times in the same place and that are both flightless, are not the same species.

Damaging because there is weight to the idea of extinction. Extinction is forever. It means that an entire evolutionary lineage has ended, and any potential future that that lineage may have had is gone. If the idea of extinction becomes an impermanent one, it looses its urgency and tragedy. People may well not worry about extinction as that species can just re-evolve. No harm, no foul.

Again, no species can ever occur twice. Once a species goes extinct, that is it for that evolutionary lineage. Even if some other lineage emerges that is close, it will not be the same and will not have the same evolutionary trajectory or potential.

When reporting on science, I feel strongly that the ideas behind the science should be accurately represented. I think it is especially distressing when the sources of the misrepresentations are otherwise reputable sources for science.

I hope the current Aldabra Rail has a long future filled with descendants, and I mourn the loss of the previous rail of Aldabra and the lineage it might have left behind, but never will.

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Charles Darwin (1809-1882) is famous for developing a way for evolution to occur, natural selection.  It should be pointed out that when Darwin was alive there was no question that species evolved.  Scientists in general all agreed that the species that were alive around them did not remain fixed forever and ever, but rather changed over the course of long spans of time.  The problem was that no one could figure out how this took place.  Darwin made observations of the natural world and noticed four simple features that would result in species changing in response to the natural environment.  The process of change that Darwin proposed occurs as an inevitable consequence of these four conditions, and does not require any divine influence.  The four conditions that Darwin elucidated were variation, heritability, superfecundity, and non-random mortality.

Variation means that that each individual in a population is unique.  These differences may be very minor, but they are always there.  This is so obvious a fact that it almost does not need to be spelled out.  You are a unique individual who has never occurred before and will never occur again, and the same is true of every other species.

Heritability means that each individual will tend to pass on the variations it has to its offspring.  In this way, the variations that are present in a population will tend to be passed down through the generations.  In other words, short individuals will tend to have short offspring, etc.  This heritability is not perfect, in most traits, because there is mixing between the traits of each parent.

Superfecundity means that more young are produced than can possibly survive.  Each individual strives to pass its genes on into the future.  To accomplish this, the more offspring produced the better and since all organisms use this strategy, a great many offspring are produced.  This leads to competition among unique individuals for a limited number of available resources needed for life.

Non-random Mortality means that how dies matters.  Many organisms die, and this is especially true of young organisms.  This is driven,in part,  by the competition mentioned above and, in part, by factors such as harsh weather conditions and other environmental factors.  But while it is a foregone conclusion of superfecundity that some individuals will die, it is the fact that these deaths do not occur at random that allows for population-level changes to occur.  In other words, the survivors survive for a reason, and the reason is that they have some advantage, however small, over those that did not survive.  The survivors are then able to pass their advantages, whatever they are, on to the next generation.

After generations and generations of this combination of conditions, populations of individuals become evermore adapted to the environments in which they live, and so evolution by means of natural selection occurs.

 

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Vocalizations, like any other phenotypic trait, can change over time.  These changes can have several causes which can be generally divided into drift (the random accumulation of mutations that are generally neutral in terms of fitness) and selection (changes that are under some directional pressure and tend to increase fitness).  The forms of drift and selection fall into five major categories.  In any given population, these categories can act independently of one another, they can act in concert, or they can oppose each other.  The five categories are Cultural Drift, Genetic Drift, Cultural Selection, Natural Selection, and Sexual Selection.

Cultural Drift is the process where changes in vocalizations occur by chance.  These changes can come from imitation errors as young individuals attempt to copy the sounds produced by adults.  They can also arise in the form of innovations where an adult incorporates a new sound element into its vocalization.  The accumulation of these changes can eventually lead to the formation of new vocal types.

Genetic Drift is the random accumulation of mutations at loci that regulate sound production.  Ass these mutations accumulate, the physiological and mechanical abilities of an organism to make sounds may be altered.  Due to this, genetic drift is likely to have a greater effect on the evolution of vocalizations when the mutations happen to effect the limits of performance for an animal.

Cultural Selection occurs when there is differential propagation of vocalizations across generations.  This can occur in the form of vertical transmission from parents to offspring, horizontal transmission between peer groups or siblings, or oblique transmission from adults to unrelated young.  Unlike the following two mechanisms, cultural selection is not directly driven be fitness.  Instead, variations in vocalizations can spread through a population for other reasons.  One example is because of a dominant individual using one particular variation and not others.  Another example is when a particular frequency transmits through a habitat better than others, such as how low frequency sounds travel through dense foliage farther tan high frequency sounds.  This would lead more young individuals to be exposed to low frequency sounds and so learn to imitate them.

Natural Selection can influence vocalizations directly, because of some fitness benefit that a particular vocalizations give the signaler, or indirectly, by altering some physical structure that is used is sound production (changing bill morphology adapting to different seed sizes, for example).  The most commonly discussed role of natural selection in vocal evolution is through the process known as reinforcement.  Reinforcement is where two populations have diverged to the point where hybrids between the populations are less fit than pure bred members of either population.  This might be because the two populations have split to use foods of two different sizes.  A hybrid might not be good and consuming either food size, and so be less fit.  If such hybrid disadvantage exists, natural selection is expected to favor individuals of each population that tend to avoid mating with individuals of the other population.  Vocalizations are frequently the first from of contact that two individuals have, and so they are in a unique position to moderate interactions and will tend to evolve towards greater species-level specificity.

Sexual Selection can take the form of intersexual selection or intrasexual selection.  Intersexual selection can drive the evolution of vocalizations by the preferences of one sex (usually the female) for particular vocalizations of the other sex (usually the male), by sensory bias where one sex (usually the male) uses a vocalization that the other sex (usually the female) is predisposed to respond to, when a display can only be produced by individuals of high fitness, when the production of a display carries some fitness cost such as increased risk of predation, or when a vocal display can inform the receiver as to their likely genetic compatibility with the sender.  Intrasexual selection on vocalizations come ins the form of members of same sex (usually males) using vocalizations to compete with one another.  Here, the evolution of vocalizations can occur when vocalizations contain information about the sender.  This information can be in the form of the senders size, strength, willingness to fit, social status, etc.  Facets of vocalizations that are often favored include increased vocal complexity, high amplitude, low frequency, and high calling rate.

These mechanism for the evolution of vocalizations are most thoroughly studied in bird songs.  However, bird calls may be susceptible to all of these types of evolution as well.  This would be particularly true of calls that are learned, as opposed to innate, for which more and more examples are being discovered.

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