S4: Theories of Ageing Flashcards
3 major views on why we age
- Wear and tear (we wear out).
- Adaptive evolutionary (evolved to age).
- Non-adaptive evolutionary (is a byproduct of evolution).
Describe Wear and Tear view on ageing
- This reason on why we age is simply that the organism, like a machine, starts to wear out due to accumulation of damage overtime. Eventually this becomes too much and we die. For example elephants have six sets of teeth and once these have been used it cannot get new ones and can’t eat its food and dies.
Weddel seals teeth also wear away over years eventually leading to its death. - However ageing doesn’t occur in all animals, sea anemone’s do not age. Their germline DNA can also be repaired. Some animals can also repair whole organs e.g. a salamander can regrow its limbs.
- Therefore wear and tear cannot be the whole answer because certain animals such as the ones described either do not age or can repair/replace organs that have aged or have impaired function.
Describe Adaptive Evolutionary view on ageing
- Another theory then came into light that was based on the idea that ageing has developed through the process of evolution and natural selection. In practice this means that ageing came about as it is selectively advantageous to a species as it prevents the old and worn out individuals (who have aged) competing with new young offspring. This theory was initially popular as it conformed to Darwinian principles.
- However when analysed further the argument didn’t hold, as natural selection is based on individual advantage and that being passed on to the next generation NOT population advantage which is what the theory was for ageing.
- Ageing is also rarely seen in natural populations of animals, this is because animals in the wild often die before they even start ageing.
- A final point that didn’t make sense was that it prevented old individuals from competing with young, because if people didn’t age they would always essentially be young and in their prime. Thus this point of the evolutionary theory is a circular argument and doesn’t hold.
What are the two non adaptive evolutionary theories and how did they arise?
With the adaptive evolutionary theory not holding ground, a move was made to theories that instead thought that ageing was in fact a byproduct of evolution. There are two theories here:
1. Mutation accumulation.
2. Antagonistic pleiotropic genes.
Plus a mathematical model that provides evidence (disposable soma theory).
Describe mutation accumulation theory in ageing
- This was the first non-adaptive evolutionary theory that came out and it essentially states that the powers of natural selection decline with age. Genes expressed early on in life will effect most of the population and therefore under selective pressure (keep good ones and get rid of the bad ones).
- However genes expressed later on in life, long after reproduction cannot be controlled by selection as they weren’t doing anything at the time of reproduction and were therefore just passed on.
- So it may be that ageing is due to a collection of late acting deleterious genes that are lost from evolutionary control so are continually passed on.
- However there is no experimental support for this theory.
Describe antagonistic pleitropic genes theory in ageing
This was the second theory that came through, this holds that certain genes have two effects that are opposite. A gene may have a good helpful effect early on in life (so is passed on to next generation) but then a bad effect later on contributing to ageing.
There is evidence for this in drosophilia (allow us to get through many generations in short period of time, rather than waiting 50yrs for humans) with alleles that reduce longevity but enable increased early reproductivity.
Describe disposable soma theory in ageing
- This is a theory based on a mathematical model that views an organism like a machine and the function of that machine is to transfer its free energy into its offspring. That is its success, to ensure survival of its genes into its offspring in an efficient way. It is called disposable soma theory because disposable means produced with a limited lifespan (organism is disposable) and soma is body (not germline). So your body is disposable, but germline needs to be passed on.
- Disposable soma theory holds that an organism takes in energy and then needs to decide how it uses that energy based on evolutionary purposes.
It has to use energy for food processing, for foraging for more food and for defence from other organisms these have to be done in order to actually survive. - However with using the energy for maintenance of the body and for reproduction there can be a bit of compromise in which way we go.
- Hence, we can see that the amount of energy expended on the various possible options will depend on the ecological niche occupied by that organism e.g. Cat and mouse will be different. As a result of this distribution, it results in species specific longevity (i.e. that distribution will lead to that organism living a certain amount of time).
For some organisms their ecological niche means they are far better off prioritising fertility while for other organisms they are better off maintaining their soma for longer
Disposable soma theory with cat and mouse as example
- For example a mouse could put a huge amount of energy into maintaining its body and therefore living for a long time. However this isn’t a useful way of distributing energy in evolutionary sense as mice are at the bottom of the food chain (highly predated) and it means more likely a cat or other animal will eventually find it and eat it and its germline has been lost. Alternatively what it can do (and what it does do) is to put its energy into reproducing early and as maximally as possible in order to ensure survival of its germline. That way, some of the mouse’s offspring will survive to then go on to reproduce. Investing in maintaining their body is not a good strategy in the long term for passing on the germline and maintaining the species.
- Cats on the other hand need to live for longer because they have to learn how to hunt and be parents etc. Therefore cats are better off putting energy into maintaining their bodies so they can learn how to be effective hunters and parents but are less fertile. However this is useful for the ecological niche that cats occupy.
Describe the physics of disposable soma theory
The second law of thermodynamics states that entropy increases, chaos occurs i.e. in our case we age and decay naturally.
However we resist this ageing process with our defensive, maintenance and repair processes but unfortunately these protective mechanisms against ageing will eventually fail. We will succumb to ageing and die.
However the rate that which we age is determined by how much we as an organism have invested into self-maintaining our body.
The more we invest, the slower we will see ageing changes. We are programmed to survive not to age, we try resist ageing!
How do we age (theories)?
- System level theories (based on whole body systems).
- Cellular/molecular level theories.
- Genetic theories.
- Genomic stability.
Describe neuroendocrine theory of how we age
- The neuroendocrine theory is a system level theory that holds that a functional decrease in neurones and their associated hormones (generally hypothalamic) is central to the ageing process.
- We know that the hypothalamic - pituitary - adrenal axis controls growth and development, so could it control ageing as well?There is some evidence for this, we see decreased pulsatile growth hormone and GnRH release in ageing rats.
- Very interestingly removing rats hypothalamus (hypothesectomy) and then replacing the hormones that would have been lost actually leads to a longer life span.
- A death hormone proposed has never been found so appears unlikely.
List cellular/molecular level theories of how we age
- Wear and tear and rate of living.
- Cross link formation.
- Heat shock proteins.
- Hayflick phenomena.
Describe wear and tear and rate of living as a cellular/molecular level theory of how we age
Some aspects of ageing seen do appear to be due to wear and tear/accumulated damage. Animals with a higher basal metabolic rate have a shorter lifespan.
Describe cross link formation as a as a cellular/molecular level theory of how we age
Many biological molecules develop cross linkage or bonds with passage or time this alters their physical/chemical properties. We see this with collagen and can be demonstrated by looking at how young peoples skin bounces back when pinched but older peoples skin can tear as the collagen has cross linked and the skin has become more brittle.
Describe heat shock proteins as a as a cellular/molecular level theory of how we age
Heat shock proteins are an important class of protein that are produced at times of cellular stress (e.g. increased temp!). These proteins help enable proteins to be produced properly (e.g. fold properly) despite stress and get rid of damaged proteins. We see that with age there is a reduced production of heat shock proteins and therefore understandably there will be a decreased ability to deal with (cellular) stress leads to ageing.
