How could Selection by "Differential Fertility" be true?

Question

Natural Selection is supposed to work in two ways:

1) Differential Viability

2) Differential Fertility

The first is easy to understand: Variants that have a genetic

resistance to a pathogen will survive and reproduce while

those who don't will perish.

But evolutionary change is rarely ever like that. The development of ALL complex internal and external organs could not have been due to such environmental pressures. Any

changes must have lead to greater reproductiveness in the

phenotype: hence, "differential fertility".

But how can this be? It would mean that those further up the

evolutionary tree would be more reproductive than those further down: But the opposite is true.

So was any reproductive gain short-lived or what?

Answer 1

You are making an incorrect assumption about fitness in that you are equating higher fitness with higher rates in reproduction. This is not the case. Fitness is defined as the number of offspring surviving to reproductive age. There are two basic strategies to increase fitness: 1) Increased number of offspring or 2) Fewer number of offspring, but increased nurturing to ensure survival.

Many successful species employ the first strategy. For example Arabidopsis thaliana (a terrestrial plant) produces thousands of seeds per plant and is extremely successful from a evolutionary standpoint.

Other organisms tend to have fewer offspring, but spend more energy in care and nurturing. Humans use this strategy: little energy devoted to reproduction as compared to care and nurturing.

Also, this is not a binary decision with two polar opposite choices. Consider it a continuum with a 100% commitment to high reproduction on one end of the spectrum and 100% commitment to nurturing on the other. Many species will fall in the middle (Rats for example: high birth rate, and some limited nurturing).

In the end all that matters is survival to maturity, how you get there can vary. Or put another way: There’s more than one way to skin a cat.

I also suspect you are equating “complexity” with being highly evolved, which is overly simplistic. A good dose of E. coli 0157:H7 could bring you down in a week, even though poor little E. coli is single celled, has a simple biochemistry, still transcribes most genes in operons, etc. and you are a big, bad multicellular eukaryote with a highly evolved immune system.

Answer 2

I'm not really sure what you are trying to ask. But anyway, if you look at marsupials vs. placental mammals, you can see how fertility can be an advantage.

When marsupials give birth, their babies are basically still a fetus. The baby is extremely tiny, and has to crawl up its mother to get into the pouch. This is an extremely vulnerable time in its life and it is very easy for it to die.

Placental mammals never have to go through this. When they are born, they are at a size where its mother can easily take care of it. Many mammals are even born so developed that they can stand and walk within minutes. While they still rely on their mother for milk, they aren't quite as vulnerable as the extremely small marsupial babies.

Because of these differences, placental mammals have out-competed marsupials in almost every niche. The exceptions are the possum in the Americas (which has no placental competitors) and the marsupials of Australia. Australia became an island at a time when only marsupials lived there, so no placental mammals could set roots there until humans arrived. Now, there are invasive mammals there that are slowly edging out the marsupials.