Survival of the smallest?

[dichotomy]

Am I correct in saying that the smaller a life form is, the better probability it has of surviving by evolving to suit the newly emerging environments? As well as a sudden, or, rapidly changed environment?

 

I speculate that humans and other megafauna of the present, will paint themselves into a corner, like the megafauna of the Jurassic period eventually did. This cycle will repeat itself with the physically smaller species evolving into the new megafauna of the future. I’m sure this has been speculated at some point, probably by a guy with a beard . Of course, I'm interested in what thoughts others may have.

[CDarwin]

Am I correct in saying that the smaller a life form is, the better probability it has of surviving by evolving to suit the newly emerging environments? As well as a sudden, or, rapidly changed environment?

 

I speculate that humans and other megafauna of the present, will paint themselves into a corner, like the megafauna of the Jurassic period eventually did. This cycle will repeat itself with the physically smaller species evolving into the new megafauna of the future. I’m sure this has been speculated at some point, probably by a guy with a beard . Of course, I'm interested in what thoughts others may have.

 

You're tapping into a field of evolutionary theory known as density dependent selection. It assumes that there are two factors controlling how big a population is: r, the natural reproductive capacity of a species, and K, the carrying capacity of the environment. In any given environment, one of these will be dominant over the other. In stable environments with enough resources but without a super-abundance, you get K-selective regimes; in unstable environments where there's a lot of change in a "boom or bust" sort of cycle, you get r-selective regimes.

 

Now animals like small mammals (aphids provide a really good example of this, too) tend to have very high r values and are subject to r-selective regimes. They can breed rapidly to exploit temporary and super-abundant resources but then experience mass die-offs when the resources disappear. There isn't much competition for resources in an r-selective regime, it's either everyone lives or everyone dies, so offspring aren't given much of a competitive edge.

 

In K-selective regimes competition for resources is very important, so it encourages more specialization and longer maturation, which in many cases means larger sizes. This and the fact that the K limits how many resources can productively be spent on making new offspring lowers the r of these species and makes them more vulnerable to big changes.

 

That's a pretty bad muddling of r/K-selection theory but I hope that made sense. The Oxford Encyclopedia of Evolution has a good article on it if you're a member of a library that has access to it. Elsewise, there's always the wiki.

[Mr Skeptic]

Smaller critters have shorter life cycles and larger numbers; these allow quicker adaptation be it via mutation or natural selection.

 

I speculate that humans will inevitably either destroy themselves or take over the galaxy.

[dichotomy]

I speculate that humans will inevitably either destroy themselves or take over the galaxy.

 

Or, other natural events could do this for us - virii, bacteria, super volcanos, comets. Of course, we could always take over the galaxy and then destroy ourselves.

[CDarwin]

Smaller critters have shorter life cycles and larger numbers; these allow quicker adaptation be it via mutation or natural selection.

 

That's a much easier way to say everything I just did.

 

Although it's not mutation or natural selection, and mutation/recombination and natural selection (and genetic drift and blah blah blah; stochastic things).

[dichotomy]

That's a pretty bad muddling of r/K-selection theory but I hope that made sense. The Oxford Encyclopedia of Evolution has a good article on it if you're a member of a library that has access to it. Elsewise, there's always the wiki.

 

This makes sense. And it's a solid theory IMO. Thanks.

[PhDP]

About r/K selection. It's derived from this equation;

 

[math]

\frac{dx}{dt} = rx\left(1-\frac{x}{K}\right)

[/math]

 

With x being population size, K the carrying capacity and r the intrinsic rate of growth. It's really one of the most fundamental equation in ecology. However, the r/K selection theory is deeply flawed for a simple reason; r generally correlates with K, they are not opposed (an interesting explanation is provided by the metabolic theory of ecology). It's probably why you won't find much about this theory in modern books on life history evolution. Density-dependence/independence is a major issue in evolutionary ecology, but the approach now is very different.

 

About the advantages of being small. I'm going to give you another equation, but this time I won't explain it, haha ! I just hope you'll want to investigate it;

 

[math]

\frac{\partial \Psi}{\partial t} = M \frac{\partial\Psi}{\partial p_0} + \frac{V}{2}\frac{\partial^2\Psi}{\partial p_0^2}

[/math]

 

This beauty is the Kolmogorov backward equation, it combines the different forces of evolution (mutations, drift, selection, ...). It can be used to find the probability of fixation of alleles in different contexts. By looking at the behavior of the solution with large and small population, you'll see that small populations are very vulnerable, deleterious mutations can easily reach fixation. This is bad. Population size is strongly related (negatively) with body size, so yes, being small is a great thing.

 

Some scientists (i.e.:Michael Lynch) believe the genome of eukaryotes is messy in part because of that, we're large creatures with small population size. In short, small population = bad for at least two reasons; #1, they are often affected by inbreeding. #2 drift is too powerfull, it will kill diversity very fast and deleterious mutations will reach fixation.