caravelair

it is clear that modern technology and medicine is affecting natural selection in humans. but what exactly is the effect? is this actually slowing evolution in humans, or just changing the direction we are evolving in?

MrDarwin

I'm not sure that's clear at all. Perhaps in developed countries, but I suspect that a large majority of the world's population has little or no access to (or can't afford) "modern technology and medicine".

Non-praying Mantis

Interesting question - Hardy Weinberg Equilibrium to the rescue!

The Hardy Weinberg Equilibrium is a mathmatical model of populations of organisms that can determine if evolution is occuring, and possibly how fast evolution is occuring. When all of the assumptions of the HWE apply, a population is not evolving. These are the five assumptions:

1) Population size is large
2) Population has no immigration or emigration
3) No mutation occurs (at least over the time period of interest)
4) No non-random mating is occuring
5) No natural selection - i.e. no differential in production of offspring

Given all of these assumptions apply to a population, there will be no change in a gene pool over time (i.e. evolution has stopped) Now, note that no natural population will ever fit into the HWE, so all natural populations are evolving. However, you can compare how much certain populations violate these assumptions, to get an estimate as to how fast a population will evolve compared to another population.

So let's assume that all of human population on the earth is a single population (not very good assumption, but it allows us to consider the human species as a whole, instead of taking account of different populations of humans around the earth). Let's compare the status of the human race today with that of 1000 years ago:

1) Global human population 1000 ya was approximately 1 billion individuals - Today population is about 6 billion. These numbers are approximately the same (on the same order of magnitude, at least), and really the "Population size is large" restriction is only important for very small populations, so we can assume no difference in evolution from this effect.

2) Since all of the human race is being considered one population, there is no immigration or emigration today or 1000 years ago (it would have to come from off-planet!).

3) Mutation, of course, occurs naturally at low levels in any population, but what is important to the HWE is that no beneficial mutations appear in the compared populations during the period being observed. Unless we take into account things today such as increased UV radiation due to decreased ozone layer, or the reduction of smoke (which has lots of mutagens) in modern times (fires were used for cooking and heating then, so even considering cigarette smoke today, we have less from this source), we can assume that the mutation rate is probably similar for both time periods.

4) There are two main sources for non-random mating in populations: a) from mate selection, and b) from lack of internal mobility of the population.

a) Mate selection of course is found in many organisms. Peahens select peacocks for their brightly colored tails, for instance. Humans also choose their mates from a pool of possible selections based on certain features such as hip/waist ratio (for women) and intelligence (for both sexes), for instance. What is important for the HWE is whether certain genes make it more likely for an organism to be selected as a mate. This is obviously true for many populations, including our own.

It all boils down to two points: one, no population of mate selecters can ever stop evolving completely, and two, is there a significant difference between mate selection today, compared to 1000 ya? Probably not.

b) Internal mobility is important for HWE to take effect, because if the population is limited in mobility, genes that appear at one location in the population (from a mutation, for instance), are likely to stay near that location for some time. Therefore, it is more likely that organisms that have those genes will mate with each other.

Here is the first big difference between today and 1000 ya. Today, with the internet to help us find mates across continents, and significantly increased travel from one place to another, there is much more internal mobility today of the human species than there was 1000 ya. We have the possibility of mating with someone born on the other side of the world today, which was practically impossible 1000 ya (even about 500 ya).

So, the rate of evolution due to internal isolation is much less today than it was 1000 ya (although it is still quite a significant factor).

5) The rate of natural selection in a population comes from a differential in the amount of offspring produced from each individual. It can involve many factors including death, differences in fertility, success at finding a mate, and number of offspring that can be supported in an environment.

Today, our death rate is lower, in that a greater percentage of people reach adulthood. Our fertility can be increased by using certain technologies that allow what would have been infertile couples 1000 ya a chance to reproduce. Increased communication allows for easier mate finding. The average number of offspring produced per couple is much lower than the limit imposed by environmental constraints (at least compared to 1000 ya), and we have less variation in the number of offspring produced by each couple, at least in the western world.

So, yes, I would say that the rate of evolution today is less than it was 1000 ya, and is probably at an all-time low since our species first evolved.

That is a much harder question to answer, since evolution does not usually have a direction!

NPM

Peez

Hi, NPM!It is certainly a useful thing to look at in this context, but note that while a population that is evolving is not in HWE, a population that is not in HWE is not necessarily evolving.The assumption is actual an infinite population, though in practical terms a very large one is a good approximation.This is a reasonable approach for a WAG, but note that evolution may proceed very rapidly with a relatively minor violation of one of these assumptions, but may also proceed very slowly with rather large violations of several of them.Are you sure of that? I was under the impression that the estimated world population of humans was closer to 250 million at that time.I would agree that genetic drift is weak in populations of greater than 250 million, and can probably be ignored for the purposes of this topic.What, you don't believe that aliens are having sex with earthlings?! That depends on what one calls "low levels" (just about everyone is born with a few brand new mutations).Hmmm, I didn't think that we were comparing populations, but in any event "beneficial mutations" are not required for evolution to proceed.That is an interesting hypothesis, but we should be cautious.I am not aware of any evidence that campfire or cigarette smoke influences the relevant mutation rates (in the germ line).Mate selection does not necessarily lead to evolution, though of course it sometimes can. I would also suggest that mate selection might easily be based on different criteria now than 1000 years ago, but the differences (if they exist) might not be very noticeable.Actually, I suspect that air travel is much more significant than the internet in allowing such matings.None of that necessarily suggest less potential for natural selection. The relevant value is the variance in reproductive success, not the average.This is probably true of the Western world, but if we consider the world population of humans it might not be (though I suspect that it is, perhaps to a modest degree).Perhaps, but then again there are some alleles (genes) that do seem to be increasing in frequency in the world population of humans, and at a rather high rate. Just pick an area of the world, one where the people are reproducing much faster than elsewhere. Find a few alleles that are more common in this sub-population than elsewhere: voila! Evolution.


Peez

caravelair

very good point. what does this imply about the evolution of 3rd world populations relative to those in more wealthy nations?

caravelair

i did not mean to imply that there was a set direction. but the features that evolve generally result in the species becoming better adapted to their environment, correct? therefore if we change our environment, we would expect different features to evolve than if we had not. make more sense?

KC

The 'Mile-High Club' effect also encourages gene flow between far-flung human populations:

Mile-High Club Airline Accomodations

KC

callmejay

Well the biggest thing that sticks out to me is BIRTH CONTROL!!! The most educated and most wealthy (and least religious, btw) are having the FEWEST children, so if we do change significantly as a species, I'd guess we'd end up less intelligent (I'm assuming there is SOME correlation between intelligence and education) and more religious!