On Natural Selection

Some people seem to be having problems with natural selection and how it can lead to the huge diversity of life on Earth.  I thought I would explain this as I did to my students.  This is a basic primer on natural selection, not a comprehensive explanation.

Charles Darwin came up with the idea of natural selection after his trip on HMS Beagle and observations of artificial selection.  Breeders of domestic livestock had known for a long time (centuries probably) that by breeding their best individual animals together, they tended to get more really good individuals.  That’s selection.

What is natural selection?

In the simplest terms, natural selection says that organisms that are more for fit for their environment tend to preferentially survive and pass their traits to their offspring. 

This is a simple concept.  If you have two lion cubs, one of which is large and strong, the other of which was born with a mangled leg, then, in a natural environment, the large, strong lion cub with be much more likely to survive.  The cub with the bad leg probably will not survive to reproductive age, much less be able to fight for and mate with a female. 

The gene that caused his mangled leg will not continue.  While the genes for size and strength will continue on in the offspring of the large, strong cub.

That’s it.  That’s all there is to it.

Now, if the environment changes, then the traits that are important change.  If you have an ice age, suddenly woolly mammoths are better able to survive than hairless mammoths.  Once the ice age ends, and global warming kicks in, then hairless elephants are better able to survive than heavily coated woolly mammoths.

Natural selection cannot work in a vacuum.  In fact, you might think of the environment to be analogous to the breeder of domestic animals.

Genes and Natural Selection

At this point, we should remember that genes carry the information needed to construct a protein.  An allele is a variant of a gene.  Some alleles are dominant over other alleles.

It is important to realize that natural selection does not, and cannot, act on what genes (alleles) are in the organism.  Natural selection can only act on how the organism looks (the phenotype).  This is why and how, sometimes harmful alleles get a free ride.

Let’s say that an organism has a gene for muscles.  There are two alleles one for ‘Lots-o-muscles’ (we’ll call this ‘L’) and one for ‘little-bit-o-muscles’ (we’ll call this ‘l’).   Now ‘L’ is dominant to ‘l’.  So if an organism has the genotype LL or Ll, then it has lots-o-muscles.  Only if the organism has a genotype of ll does it have a little-bit-o-muscles.

As far as the environment is concerned, LL is no different from Ll.  Both genotypes produce the same phenotype (lots-o-muscles).  Only ll is effectively different. 

Now, if we have two organisms a male with Ll and a female with Ll, then we should remember than 25% of their offspring will be ll (little-bit-o-muscles) and 50% will be Ll.  In the organisms current environment, lots-o-muscles is a good thing and they dominate the individuals. 

But if something happens in the environment, something where the energy needed to maintain lots-o-muscles is no longer available, then those guys with ll (little-bit-o-muscles) will be more fit than the lots-o-muscles individuals.  Those organisms will then tend to survive and the dominant allele will rapidly become a minority in the gene pool of the organism.

Natural Selection and Diversity

But how can simple changes in the gene pool, which are usually changes in the percentage of each allele, result in the massive diversity around us?

Remember that fitness is not a fixed concept.  If fitness was fixed and didn’t change, then all the organisms alive would be approaching a fixed point.  Everything that was less fit would die and everything on Earth would be tending to be exactly alike.

There are what we call, niches, areas of specialization if you will.  Just like our society has carpenters, and plumbers, and firemen, and programmers, and teachers.  The natural world has lots of ways for organisms to make a living.  Some will photosynthesize, some eat the ones that photosynthesize, some eat those, etc.  What you end up with is an ‘arms’ race between the species doing the eating and the species being eaten.

Take a look at cheetahs and Thomson’s gazelles.

Cheetahs tend to catch the slower gazelles.  Why chase a fast one when you can chase a slow one.  So, in classic fashion* the gazelles that are faster will tend to survive and breed offspring that are also faster.

Cheetahs have to be fast to catch the faster generation of gazelles, so only the fastest cheetahs tend to breed.  Which, in turn, means cheetahs that are faster.  Repeat over a few million years and you get what we have now.

But what if, there was a significant mutation in a cheetah line that resulted in the ability to catch a new source of food?  Well, the currently living cheetahs wouldn’t be competing with the new line, so they would continue on as always, but now we have this new line of cheetahs diverging.  What is fit for this new line has changed, it’s different from the speedy cheetahs.  Maybe it’s based on smaller size and being able to survive on rabbits and mice rather than gazelles.  Maybe they are better ambush hunters than runners, slightly stronger maybe, or no scent.  The possibilities are endless.

But the new line slowly changes over time to be less and less like the ‘normal’ cheetah that we think of.  Until the point we look at them and say, ‘That’s not a cheetah.  I don’t know what it is, but it’s not a cheetah.”

A creationist might say, “That’s a just-so story.  It’s not real.  It’s made up.”

Except it’s not…

Go ahead, explain to me that these two organisms are the same species without invoking the intermediates.

You see, dogs are the greatest example of selection.  The fact that it’s mostly artificial selection rather than natural selection has exactly nothing to do with anything.  The fact is that the species Canis familaris has such a massive amount of diversity, that for all intents and purposes they are different species.

Looking at the extremes, take any small, toy, or miniature breed, and try to breed it with a large, working, or Shepard breed and see what happens.  There is no way that the two individuals in the picture could mate.  Even they could physically get together, there is almost no way for the offspring to come to term.  Even if they did, they would probably not be viable for long as a mix of two such disparate organisms.

Dogs are well on their way to becoming different species.  In fact some might suggest (humorously) that it should be done for the same reasons I mention above.

But the point is natural selection and diversity of species.  Dogs actually exhibit a wider range of morphology than all other carnivora species combined.

Please keep in mind that all classification is purely human based.  There are no constraints on what is and what isn’t a species except for what we think. 

Dogs represent a cline.  That is individual small groups can interbreed, but the extremes cannot.  Like this:

A –> B –> C –> D –> E –> F –> G

A and B can interbreed and F and G can interbreed.  But A and G cannot for whatever reason (size, season, physical location, etc).

The extremes, if the middle wasn’t present, would be easily identifiable as different species.  Go ahead, without using any intermediates, tell me how the two dogs in the picture are the same species.  Keep in mind that, even different species can, occasionally, interbreed and make viable offspring (Ligers and Tions, for example).

Now, the big problem is that we don’t have all the intermediates for all the species we know of.  The fossil record isn’t 100% complete.  It can’t be.  I guarantee that there are species that existed in the past that we will never, ever realize even existed.  It’s the nature of fossilization.

But just because we can’t find those intermediates doesn’t mean that we don’t know they existed.  Look back at dogs.  We don’t invoke a designer for the difference between them.  We know that humans engaged in selective breeding.  We didn’t change the genes of one great Dane to create a poodles. 

It’s nothing but breeding and selection.  Which is where the creationist argument fails.  Yes, humans were involved.  But all humans did was SELECT, we did not do anything else.  So, if you have a natural mechanism for selection (and we do), you don’t need an artificial mechanism for creating species.

So you see, natural selection can result in the huge diversity of life around us.

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*The old joke is “When being chased by a tiger, you don’t have to run fastest.  You just have to run faster than your slowest friend.”