What is the mechanism that keeps evolution in human beings alive?

[arun219kv]

With advances in medicinesin 1800s and now the avg life expectancy being higher than reproductive age what could be driving human evolution?
something like :
Hundreds of Human Genes Still Evolving
http://www.livescience.com/health/060308_human_evolve.html

I understand genes could change in individuals gradually.But what ensures that these slight variations in an individual's genetic makeup become a widespread change?

Before advances in technology we were unable to modify the surroundings and factors such as isolation of tribes humans,struggle for mates,survival of fittest in hostile environment and as we do would have forced a considerable number of individuals (something like african males having longer penises (thats the only one i can think of now ...im too sleepy)) .
But currently even if a change happens won't that be diluted due to the absence the factors listed above?

so can we conclude that the rate of evolution become almost zero in humans? (limit tends zero mathematically) unless we are faced with some unforseen accidents like a uncurable epidemic or something

 

[tiny-tim]

With advances in medicinesin 1800s and now the avg life expectancy being higher than reproductive age what could be driving human evolution?

uhh? that's very negative …

avoiding death (of the body) isn't the only thing driving evolution …

genes for positive traits, such as increased sexual attractiveness, or fertility (but also for traits less directly correlated with producing offspring), are also favoured.

 

[sylas]

I understand genes could change in individuals gradually.But what ensures that these slight variations in an individual's genetic makeup become a widespread change?

The fact that the changes are passed on to descendants.

Evolution is more than natural selection. Here are two very simple cases to think about.

(1) Evolution without selection

Speculate on a perfectly managed world. Every individual is required to mate, and have two children. No exceptions. Medical advances are so drastic that every child born survives and has their two children. No exceptions.

Or if you like, there can be some exceptions, but only a tiny number. (Caveat. There's actually always selection involved, in the race for sperm to reach an egg, or the success of an egg to make it all the way through the birth. I'm being a bit idealized for simplicity.)

The evolution of such a species will be dominated by genetic drift. Variations still arise in individuals, and still get passed on to children, but it is random. There are a lot of mutations to play with. In humans, for example, there are something of the order of 120 new mutations per individual. The main point is... you're a mutant.

Each mutation might be passed on to none, one or both your children. And so on after that. In the absence of selection, the rate at which new mutations become "fixed" in a population is independent of the size of the population, so the typical genome of the population undergoes a slow random walk. This is called genetic drift, and it is an evolutionary effect in good standing, which results in change to a species over a long period of time.

(2) Change to the selective pressures

This is what tiny-tim describes, and it certainly applies for humans. We are nothing like the ideal case I have given above. Medical advances don't remove selection. They only change the nature of the selective pressures.

Even if medicine is able to help people survive more often, there's still selective pressure associated if one consequence of the disease makes you even a little bit less desirable as a mate. And that's common. There's a lot of difference in the range of fertility of different individuals, and that gives ample scope for selection to apply. What selection is unclear.

And as a final aside... one other thing that strikes me in this common question. Most of the human population does not have access to good medicine. There have been great advances (eradication of smallpox is a classic example) but think about child mortality rates around the world. Don't just pick my wealthy neighbourhood... it is not representative. So although the question is a good one, the premise doesn't really apply as strongly as you might think for humanity.

Cheers -- sylas

 

[nitramR]

The fact that the changes are passed on to descendants.

and individuals differing in respect to their heritable traits (variation) and reproduction rates differing between individuals.

e.g. Without variation (e.g. if all human were genetic identical and homozygous) => no evolution.

If you have a reproducing population combining this three factors ... traits passed on to descendants, variability in inheritable traits and differences in reproduction rates in individuals => evolution is the inevitable outcome!

Add a mutation rate (A source of variation and modified inheritable traits).
Add a environment with a limited capacity for the respective organism (like our small world) ... you will clearly see genetic drift and fixation of inheritable traits in populations.
Add inheritable characters influencing reproduction rates (explaining selection and adaptation) and you have all the integrants that are needed to understand Darwin & Wallace.

Human differ in many traits and children are similar to their parents in many of these traits (so there are variable and inheritable traits) furthermore individuals differ in reproduction rates (also obvious) => outcome: there is evolution in human.

 

[sylas]

and individuals differing in respect to their heritable traits (variation) and reproduction rates differing between individuals.

Strictly speaking, these latter two are not necessary, as my first example shows.

I am not denying the importance of the effects of reproductive fitness for adaptive evolution; but for the specific question posed -- "what ensures that these slight variations in an individual's genetic makeup become a widespread change?" -- the requirement is simply that changes are heritable.

Even maladaptive mutations can become fixed in a species, which shows that reproductive advantage is not a necessary precondition. Adaptive changes are of course much more likely to be fixed, and even a slight detrimental effect makes it very likely that a mutation will be lost rather than fixed. But there's a heck of a lot of change that gets fixed in a population despite having no effect at all. Population genetics deals with the maths of this.

For fixing change, the only necessary requirement is heritability.

You are of course completely correct that there is evolution in humans.

Cheers -- sylas

 

[nitramR]

Strictly speaking, these latter two are not necessary, as my first example shows.

...

For fixing change, the only necessary requirement is heritability.

...

Cheers -- sylas

Hi Sylas ... I see that "arun219kv"s question already includes the assumption of variation. So it wasn't necessary to mention variation in my answer, although it is essential to evolution.

I totally agree with you: Without inheritability => no fixation of traits and no adaptation => this means no evolution (at least not the kind of Darwinian-evolution we are used to). If there is a new trait in a population, it requires at least one thing to get fixed: inheritability.

I also like your example above very much. It discusses "same reproductive success" as one aspect of "same reproductive rate". ( “rate” is important: consider a subpopulation of humans having 2 children per couple at the age of 20, and another subpopulation that starts reproducing at the age of 40 (2 children per couple). Which subpopulation will dominate the population in a few years?). I also agree with you, that as we human are sexual and diploids, we have some additional possibilities to change genetics over generations, even if reproduction rates were identical between individuals - this is what you explained clearly above. However remember that sexual diploids are an exemption here, as populations of haploids [sexual or not] or asexual [haploids, diploids, triploids or whatever] will not evolve if reproduction rates are identical between individuals.

What I wanted to add is, that If you observe all three factors "variation" + "inheritability" + "different reproduction rates" in a population (And these things are obvious and often easy to observe ... so if these three are there, irrespective of what species the population consists of, and irrespective of reproductive mode and genetic makeup) => than you can be very sure to have evolution. This is a simple rule that can be applied to plants, bacteria, viruses even computer-programs and humans.

 

[sylas]

What I wanted to add is, that If you observe all three factors "variation" + "inheritability" + "different reproduction rates" in a population (And these things are obvious and often easy to observe ... so if these three are there, irrespective of what species the population consists of, and irrespective of reproductive mode and genetic makeup) => than you can be very sure to have evolution. This is a simple rule that can be applied to plants, bacteria, viruses even computer-programs and humans.

Yes; those three are the foundation. As you say, variation was already assumed in the OP. The second gives you evolution, and the last is essential for adaptive evolution, without which you could never have the intricate functional complexity of living things, nor the various practical applications of synthetic evolution in various engineering or other tasks.

Cheers -- sylas

 

[edpell]

I understand genes could change in individuals gradually.But what ensures that these slight variations in an individual's genetic makeup become a widespread change?

Only if the genetic difference leads to a difference in reproductive success. For example if there were a gene that led some to study and stay in school so much that they had less success finding mates and had less money and less focus on family life so they produced few children that gene would die out over the generations.

Do you know anyone who has zero children? Then whatever genes are contributing to that decision are dieing out. Do you know anyone who has four or more children? Then whatever genes are contribution to that decision are becoming dominate in the future gene pool.

 

[edpell]

p.s. reproductive success also has a time dimension. A gene that leads to three kids by the age of 30 (on average) is more successful than a gene that leads to three kids by the age of 45 (on average). Just do the math the faster exponential wins.