Meeting 3.5 – Senescence and Life History in Biology

On 23 October 2014, the group met for the fifth time this semester.  We talked about ideas of senescence (aging) in biology and whether aging, death, and reproductive cessation can ever be “adaptive”; are their any circumstances under which they are favored by natural selection

READING

Kirkwood TBL and Melov S (2011) On the Programmed/Non-Programmed Review Nature of Ageing within the Life History. Current Biology 21:R701-R707.

Early 19th century perspectives on evolution focused on the survival and reproduction of individual organisms, leading to questions about death. If longer survival and more reproduction are – by definition – more fit, why don’t organisms live longer and longer? Death cannot be selected for if selection only acts on individuals.

More recent perspectives have focused on genes as types, instantiated in token alleles within individual organisms. More abstractly, we can speak of families in which predecessors opt out of competing with their own offspring resulting in better long term fitness. Such views mean both reproductive cessation (e.g., menopause/andropause) and death could be adaptations. Debate continues as to whether they are.

THEORIES
(Lucas’ four categories for speaking of senescence in evolutionary theory.  Note that they are not mutually exclusive.)

Unopposed Decay or “Non-Adaptive Theories of Aging”: After a certain age, selective forces are too weak to extend life.

(Mutation accumulation a la Medawar 1952. Diminishing returns a la Hamilton 1966.)

Fitness Trade-Offs: Traits selected for earlier in life have secondary effects later in life.

(Antagonistic Pleiotropy a la Williams 1957. Disposable Soma a la Kirkwood and Holliday 1979.)

Competition with Kin: Earlier death means more resources are available for offspring and other kin. Includes behavioral traits, such as terminal reproduction.

Cooperation: Cells, even individuals, may sacrifice themselves for collective reproduction.

Note that in multicellular organisms only a small number of cell lineages (the germ line) have the potential to reach the next generation. All other cells (soma) fail to reproduce. Similar (but not identical) relationships appear in the cast structure of eusocial organisms.

KIRKWOOOD and MELOV 2011

Argue against senescence and death being adaptive. In particular, they do not see how ‘programmed’ death could be possible, that is intrinsic mechanisms for ending a life.

Weissman 1891 made a species level selection argument. An individual dies, having made the maximal contribution to the good of the group. It is unclear how this would work, as intra-species selection should swamp any inter-group forces. [K&M note that this might apply in rare circumstances with negligible migration between groups.]

Some species do not appear to age: Hydra (and many if not most unicellular asexual species).

Semelparity: life-histories with only one reproductive episode before death (antonym: iteroparity. K&M argue that this is not active or programmed death, but passive decay having actively favored some reproductive act [a germ/soma conflict theory].

Menopause (Andropause): cessation of reproductive function in females (males). K&M attribute post-reproductive lifespan to intergenerational cooperation.

Adaptive Aging?

Possibilities requiring further study include

Biological Altruism: dying to free resources for nearby close kin

Evolvability: dying to make way for better adapted later generations

One intriguing example: FoxO, genes for fork head transcription factors, regulate metabolism, growth, division, and antioxidant response. It has been suggested that the FOXO3 protein plays a role in programmed cell death and human longevity.

K&M end their argument by saying that genes cannot tell time. Any absolute measure of time for aging seems improbable. [They do not speak to environmental factors and clocks as are present in development.]

Discussion

Do you agree with K&M?

As a group we leaned toward the idea that natural selection could favor, within populations, genes causing traits that lead to death.  Whether one calls these “intrinsic programs”, “biological trade-offs”, or “competition” depends upon which units of selection/transmission you use relative to internal/external divisions. Similarly, what you consider an adaptation as opposed to a by-product or spandrel depends on school of thought.  Despite these terminological debates, for almost all purposes we agreed with the observed and theoretical picture presented in the article.

We noted life histories where parents regularly die in producing offspring (e.g., sexual cannibalism, matriphagy). Whether you consider the death a primary effect (and therefore an adaptation) or a secondary effect (and therefore a trade-off), the behavior patter and life history appears to be favored by natural selection.

Is human death desirable from a fitness standpoint?

What dies and what persists?

(Can a body be maintained indefinitely? Can a person be maintained indefinitely? Do they necessarily go together?)

In light of our increasing ability to maintain living Homo sapiens tissue, we thought these questions were relevant to both our understanding and our morality of death. Can a Homo sapiens body persist without a human person?  Can a human person exist without a Homo sapiens body?

The “evolution of death” bears on whether bodies are intrinsically and evolutionarily programmed to die, if they have natural life spans, and if they are active in their own demise. The intrinsic/extrinsic questions here also bear on the relation of human to community and environment.  Does death come from our community? Our surroundings?

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One thought on “Meeting 3.5 – Senescence and Life History in Biology

  1. Pingback: “Life” Work | Lucas's Weblog

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