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Friday, January 31, 2014

Natural Selection and Evolution

This blog was triggered by an item I heard on National Radio here in New Zealand.  An ornithologist was talking about banding birds and how they band the common house sparrow.  Apparently they band them, both to learn about sparrows but also to give their people practice in banding small birds before they tackle rare and endangered species.  In the talk, he mentioned that with multiple clutches, a pair of sparrows can easily produce 20 young each summer season.  He also mentioned that the natural life span of a sparrow is about 10 years.

This got me to thinking how quickly a population of sparrows could grow.  Start with one pair and by the end of the year you have 11 pair*.  Next year these 11 pairs breed and you have 121 pairs.  In the third year these breed and you have 1331 pairs.  Let's go to 10 years which is the life span of the first pair.  You get up to `259,374,246 pairs.  Over half a billion individual sparrows in a decade.

*The 10 pair they produce plus themselves.

Now clearly, the only time that such an expansion could occur would be when the sparrow was introduced into a new environment in which there was abundant food, no natural diseases, no predators and abundant nesting sites.  Even then, this sort of expansion would be unlikely to continue for more than a few years.   No matter how large the food supply, the sparrows would soon exhaust it.

We actually observe that in any area where sparrows have been extant for a while, the sparrow population remains pretty much the same.  There are good sparrow years and bad sparrow years but overall the population varies around a  mean.  Let's assume for the sake of the argument that with the various factors working on the population, the average life span of the first successful breeding pair is actually 5 years.  We are being pretty generous here.  What this means  for the successful breeding pair, is that on average,  of the 100 babies that they produce over their life time (at 20 per year for 5 years), only one pair will survive.  If more survived the population would grow, if less, it would shrink.  That is a 98% death rate or more to the point, 98% don't get to pass on their genes to the next generation*.  This is natural selection.  It is huge.  Talk about survival of the fittest.  Sure some will succumb to random accidents that have nothing to do with their fitness but consider the sparrows that will survive to pass on their characteristics.  A surviving sparrow will likely have:

*In evolutionary terms, not passing on your genes is 'death'.

*Better disease resistance
*A beak that is adapted to cracking the seeds that exists in the region.
*An ability to utilize other food sources
*The same colouring as its fellows so that it doesn't attract the attention of Kestrils
*A great ability to choose safe nest building sites and to build good nests
*A very strong inclination to look after it's young.
*A flexibility in choosing nesting material and, nowadays,  being able to utilize human products.

and on and on the list goes.

Here we come to one of the main characteristics of Natural Selection.  Once an animal is pretty well adapted to it's environment, and the environment is unchanging, Natural Selection has much more to do with keeping the animal the same than it has to do with producing new varieties.  Most variations will be less successful than the standard and with the huge reaping of the young, only the best will make it.  Paliantologists have been puzzled by punctuated evolution*.   Here, at least, is the explanation for the  opposite; the long periods in which no apparent evolution occurs.  Let's look at predation for example.

*Punctuated evolution - An animal or plant staying the same for a long time and then suddenly radiating into a bunch of new forms.

The Kestrel is known colloquially as the sparrow hawk for a very good reason.  It hunts sparrows and it hunts them on the wing. Our Kestrel is stooping on a flock of sparrows.  One of them is white.  How much easier it is for the Kestrel to keep it's eye on one individual bird if it is different.  If it keeps shifting it's focus from bird to bird, it is always chasing a fresh bird rather than the one it initially locked on to.  If all the sparrows were white then a dun coloured bird would be at a disadvantage but as long as the majority are dun, the white one will have a very limited life.  Think of Lions and Zebra's as another example. 

However if one beneficial mutation does occur which doesn't have a down-side, you can see how fast it could spread through the population as the carriers of this new gene out-compete the standard sparrow.

It's an interesting thought that the greatest enemy to a sparrow is another sparrow. We see the population staying more or less the same so  this enmity between sparrows is not apparent to our eye.  One sparrow does not predate another sparrow.  However they compete for exactly the same resources.  They compete for nesting sites, food, mates and so forth.   The successful sparrow passes on his genes, the unsuccessful one does not.

Even more so, if a huge number of ecological nitches are left empty after, say, a meteor strike such as occurred 65m years ago, evolution can go wild.  And as each new variety changes, it changes the environment for a range of species around it.  An environment is not only the trees and bushes and physical features where you live.  It also includes all the other animals in the vicinity that you prey on, the animals that prey on you and those that you compete with for food.There is an arms race such as occurs with humans during a protracted war.  Look at all the developments in a wide range of fields that occurred because of the first and second world wars.  Not only did we have the classic better shell, better armour, better shell better armour but also developments in medicine, radar, communication, aircraft technology and so forth.  The same happens amongst the animals and plants when they face a new environment. If they are sufficiently adapted to survive at all, they then undergo rapid evolution which better suits them to the new environment.  Darwin's finches are one of the best documented examples of this process.

Now let's assume that blown on the wind or brought on the feathers of a migratory bird, some seeds arrive.  These seeds are for a plant that has seeds that our sparrow could eat but only with a stronger beak.  The plant flourishes because nothing is eating it and eventually, with genetic variability, a sparrow is born that can use these seeds.  With a new food source that only he can utilize, his survival potential and his potential to produce surviving offspring who can also use this new food source is huge.  Remember, the potential to increase your population if there is a new food supply is huge.  The population of this new model expands very rapidly and since it competes with the older variety in every other aspect of its life, it rapidly replaces the original type.  All we notice is that the beaks of collected birds of this species increase in size over a period and we puzzle over why.  In fact there has been a huge unobserved mortality amongst the standard variety.

We come now to the question of how much natural variability is available for natural selection to work on.  We study various populations over our life time and through the museum and even the fossil record and rarely see an animal that varies much from the norm so we assume that "sports" are rare.  We, however, only have to look at our domestic plants and animals to realize how wrong we are.  Look at all the dog breeds we have.  We replaced selection by nature with human selection and we can keep alive whichever varieties we desire.  In a few thousand years, starting with the wolf, we have produced all the breeds of dogs you see today.  The wolf hasn't changed much over this period.  He is constrained by natural selection to stay in the already well adapted form that he has had for thousands of years.  Our conclusion??

Given a change in the environment that favors a different type of animal or plant, evolution can occur explosively.  There will be lots of sports to choose from and the selection process is relentless.  The expansion potential for an animal or plant that can exploit a new opportunity is vast. This may be the explanation of what geneticists call punctuated evolution.  It is most likely to occur when something has changed an environment.  This can include:

*A disease or human intervention that wipes out a species, especially a key species such as the beaver, or the honey bee.

*A cataclysm such as a large meteorite or mega volcano that wipes out the animals of an area.  But note here that some pretty large events can occur with no apparent effect on the make up of plants and animals of an area.  As long as some populations of each component of the ecology still exist, then after a bit of argy bardgy, they will pretty well re-establish the original ecology. 

*The introduction of a new species of plant or animal into an environment.  This can be causes geologically, when a continent such as India bumps into the under side of Asia (55m years ago) or when man brings a new animal or plant to an island or continent where it didn't exist before.

Something curious that emerges from this, very incomplete list, is that man can  have (and has had) a much larger effect than a major meteorite strike or a mega volcano erupting.

On a  time scale, much shorter than geological,  Natural selection / evolution has huge potential to create new varieties given the right set of circumstances.  In a well established environment, thought, it mainly works to keep everything the same.

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