Friday, July 29, 2011

Natural Selection and Population Growth

christmas island crabs
The yearly population explosion of crabs on Christmas Island was possibly due to the extinction of a native predator; the recent introduction of the yellow crazy ant has started bringing this population back down

By this point in our walkthrough of Dobzhansky’s biological classic, Genetics and the Origin of Species, we have seen how much raw phenotypic variation exists in the wild, and how this variation is generated by several different mechanisms, including mutations.  And we have discovered that some of this phenotypic variation, namely, genetic variation, is passed from parents to offspring.  Without heritable genetic variation, there can be no evolution.  But what drives evolution?  As we saw last week, evolution can be driven by mutation pressures and genetic drift.  But usually when we think of evolution, we are thinking of evolution by natural selection.  Dobzhansky devotes chapter six to this important topic. 

For selection to work, we need two other things besides heritable variation:

1. We need the varieties to not be made equal.  That is, some varieties need to perform better than others in some environment, with performance (called fitness) being generally measured as the number of offspring (and presumably great-offspring) produced by the variety.  Sometimes fitness is measured as survival, but it is not necessarily true that longer-lived individuals produce more offspring.  There are great complexities when one tries to define fitness.  For instance, a mother that has more children may be less able to care for those children, resulting in higher infant mortality or lower reproductive performance of her offspring, whereas a parent with fewer children may end up with more grand-children because they invested more energy into caring for their children.  So the number of offspring, their survival to reproduction and (ideally) the offspring’s reproductive success should all be factored in to determining fitness.  We won’t get into the details here (such as absolute vs relative fitness, or how fitness is not a product of an individual but is a probability for a genotype); what is important is that, if each variety has the same fitness in all environments, evolution by natural selection cannot proceed.

2. There needs to be more individuals produced than can possibly survive.  It is this second point we will focus on today.

It might seem obvious today that nature is unkind to the very young.  Infants of any species are prone to death by disease, inclemental weather, predation and the like.  We have all seen videos of the thousands of turtles that hatch at the same time and desperately crawl towards the water while birds ruthlessly hunt them down.  And we have seen how those that make it often swim right into the mouths of waiting fish.  It is not easy being young, and many more young are produced than can possibly survive.

But in Darwin’s time this fact was underappreciated.  In Victorian England’s concept of the benevolent world, there was no place for waste.  It was up to Darwin to change this perspective.  Says Dobzhansky, ‘With consummate mastery Darwin shows natural selection to be a direct consequence of the appallingly great reproductive powers of living beings.’

Consider this: In Darwin’s 1858 paper that he published jointly with Alfred Russell Wallace, a simple calculation is made concerning birds.  Darwin imagines that we begin with eight pairs of birds, but every year only four of those pairs are able to breed, and they successfully raise four young a piece.  If the offspring reproduce at the same rate, then after seven years we are left with 2048 birds from an initial sixteen.  ‘As this increase is quite impossible,’ writes Darwin, ‘we must conclude either that birds do not rear nearly half their young, or that the average life of a bird is, from accident, not nearly seven years.  Both checks probably occur.’

If organisms lived up to their reproductive potential, we would be swarming with every type of organism.  The fungus Lycoperdon bovista produces 7x1011 spores; a single salmon lays up to 7500 eggs per season; American oysters release 114 million eggs per year.  And yet salmon are endangered in many rivers, and the American oyster fishery recently faced collapse.  How could species producing millions of offspring be facing extinction?  The answer, according to Darwin, is competition and changing environments.  The environment simply cannot handle a huge population explosion – there is not enough food or shelter to go around – there are predators actively seeking food – the environment can suddenly shift without warning – all of these things conspire together to keep populations down.

Populations, then, are not small because there is a lack of offspring.  Populations are small because a large proportion of offspring die.

Consider the American oyster with its 114 million eggs.  If even 98% of those eggs fail to find sperm, that still leaves over 2 million fertilized eggs from one female.  Imagine the number of mutations that could arise in 2 million embryos!  Imagine how many different recombinants could be produced!  The possible variation in 2 million larval oysters is staggering.  Many of these mutants will likely have developmental problems and die.  However, if some variants are better able to survive and reproduce than others, they will contribute correspondingly more to the gene pool of the next generation than those variants that died young or produced few offspring.  There will therefore be more of those successful genes in the next generation than there were in the parental generation.  The genetic composition of the population will have thereby changed.  This is evolution by natural selection.

If there were no difference in how varieties survived and reproduced, or if the environment was so fertile that there was sufficient room for every organism to flourish, there would be no selection.  Death would be random with respect to genotype.  Evolution by natural selection can only occur if: (1) there is competition or a struggle to survive; (2) some varieties are better able to exploit their resources than others, and (3) these varieties are able to pass their variation on to their offspring.     

Let me close today with some hypothetical examples to make this clear.  Imagine that we had a population of salmon, but we were rearing them in tanks.  We controlled their spawning, ensuring that they received plenty of food, and opened up new tanks when old ones got crowded.  What would be the result?  Having seen this in action, I can tell you: the result is the survival of fish that would have ordinarily died in the wild.  Of course, there is nothing you can do about embryos that die before hatching.  But those that hatch with deformities (such as bent tails, missing fins, in some cases two heads) can be successfully raised, sometimes to adulthood.  In the wild this would never happen.  They can survive here because we have relaxed selection. (On a side note, in reality salmon farms relax natural selection, but introduce artificial selection as the farmers only breed those fish with marketable traits).

We could imagine another scenario, this time in a river, in which I sadistically cut off all of the fins of a random sample of young salmon within a population and then released them back into the wild.  They would have a difficult time swimming, and would likely get picked off by predators or succumb to parasitic infections.  Since only those salmon with fins would breed, would the population evolve?  No.  Natural selection has weeded out those inferior finless fish, but it has not caused evolution.  Why?  Because finlessness in this case was not a genetic, heritable trait.  None of those fish died because they had a finless gene.  They died because the environment (me) altered their bodies, irrespective of their genetic material.

As a counterexample, suppose that I captured only the slowest swimmers and then cut off their fins.  Would the population evolve?  Most likely.  Why?  Because finlessness in this case was associated with swimming speed, which likely has a genetic basis.

Next week we will look at natural selection and adaptation in slightly more detail.

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