یک سوال اساسی که ذهن مرا به خود مشغول کرده است مفهوم زیستی مرگ است. امروز دیگر مشخص شده است که پدیدههای «پیرشدن» و «مرگ طبیعی» درون ژنهای هر موجودی کدگذاری شدهاند و در زمان معین فعال میشوند. از سوی دیگر، انتخاب طبیعی در سطح فرد (از هر گونه) رخ میدهد و یا دقیقتر گفته شود در سطح ژن. یعنی کسب حداکثر سازگاری با محیط برای بیشترین احتمال بقا. مثلاً اگر رنگ پوشش خرگوش قطبی در زمستان سفید میشود برای استتار است و بقا. چون اگر در محیط برفی به همان رنگ قهوهای و یا کرمرنگ تابستانهاش باشد احتمال مرگش بسیار بیشتر میشود. این توانایی در طول دورههای طولانی تکاملی در خرگوش قطبی ایجاد شده است. حال، اگر مرگ و پیرشدن هم در ژنها کدگذاری شده باشد، چگونه با بقای فرد همسو میشود؟! اینکه نقض غرض است! یعنی اورگانسیم طی تکامل توانایی را در خود پرورده است که خلاف بقایش عمل میکند! چگونه ممکن است؟!
این سوال را با چند تن از استادان زیستشناسی و دیرینشناسی مطرح کردم و هرکدام بنا بر وقتی که داشتند توضیحاتی ارسال کردند. برخی مختصر و برخی مفصلتر.
در زیر متن سوال و جوابها را عینا به انگلیسی درج میکنم. سعی میکنم در اولین فرصت ماحصل پاسخها را به زبان فارسی و بهصورت ترکیبی در پستی جدا ارائه کنم.
Dear professor …
To me, death seems to be a perplexing invention of evolution.
As we know today, death is not accidental at all; it has been encoded in the genes of every organism, extinct and extant. They have been evolved to be perished someday. We also know that natural selection (as the axis of evolution) does its job at the level of individual organisms. If the utmost goal of genes (or the selfish genes, to borrow this unforgettable phrase from Richard Dawkins) is to survive, then why have they evolved “ageing” and “death” in the first place?!
It’s very hard to see how my death will benefit me or death of any other organism whatsoever on Earth will benefit those organisms. What will gain – say for example- a male praying mantis from being eaten headfirst by his female mate?
Best wishes
Abdolreza
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Dear Abdolreza,
You do ask some fascinating, and difficult questions.
I guess that, while death is encoded, basic Darwinian natural selection says that once an animal or plant had reproduced, it is not valuable any more - it has passed on its genetic code and that's enough. Also, I suppose, as we get older (and animals and plants), it gets harder to keep repairing the tissues and keeping all cells and organs working in top condition. For a species, maybe it is less costly to keep spawning new generations of fresh, functioning cells (so the need for reproduction) rather than to keep one generation alive forever...
Best wishes,
Mike
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Michael Benton
Professor of Vertebrate Palaeontology
School of Earth Sciences, University of Bristol, Bristol, BS8 1RJ, UK
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Great question,
The simple answer is we have death because we have birth, otherwise a population cannot be maintained by its environment due to an increasing population. Why we have aging is another matter that isn't well understood. You can view aging as the accumulation of cellular and other damage over time, but why damage accumulates is great unknown.
Don
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Donald Canfield
professor of ecology at the University of Southern Denmark
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Really interesting question.
I am working on the flip side of this right now - that is, how can some organisms evolve to have exceedingly long lifespans? There are bivalves, e.g., that life for many hundreds of years (look up Arctica islandica). Aging and death are the result of cell damage associated with metabolic processes and cell division - the more a cell divides, the shorter its telomeres get (pieces on the end of the DNA strand that get shortened each time the cell divides), and hence the closer it gets to dying, and the more free radicals accumulate in the cell as a result of metabolism, the more damage they cause. Things that grow fast tend to die young, while things that grow very slowly tend to have longer lifespans. So I think that senescence and death are not things that have evolved, but rather they are a necessary side consequence of being alive, and instead some taxa have figured out how to evolve to prolong it as much as possible. The tricky part is that long life seems to correlate with very slow growth and delayed reproduction - life history strategies that are not terribly successful for most organisms, and so those traits are generally selected against (thereby dooming the organism also to a shorter lifespan!). It's only in a few places and ecologies that such a strategy is a viable one. What I'm working on now is trying to understand those particular circumstances.
Linda
Linda C. Ivany
Professor of Geology
Director of Undergraduate Studies
Department of Earth Sciences
Heroy Geology Laboratory rm 214
Syracuse University
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Hello Abdolreza,
Interesting thoughts. Species are perpetuated when genes are continually propagated (i.e. continue to survive through subsequent generations) by reproductively healthy individuals of a population. These are often the young members of a population. If male praying mantis' are more likely to pass on their genes if they stick around long enough to have their heads eaten, then that trait will be maintained in the population. Of course they will not live to propagate again... I agree that such things do not sit well with our sentiments.
Best,
Karen Chin
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Karen Chin
Associate Professor and Curator of Paleontology
Geological Sciences and Museum of Natural History
University of Colorado
«زيستشناسي فقط در پرتوي تكامل است كه معنا مي يابد.»