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"Two species that reduce one another's fitness on microevolutionary (short-term) timescales can increase each other's macroevolutionary (long-term) fitness"


Intuitive Examples:

  • Body Size - Predator-prey relationships or interference relationships (fighting, e.g. for territory) are often documented to put seletion pressures to grow in body size, often a positive feedback loop. In the short term the presence of the natural enemy in this case often leads to decreased population fitness (probability of being eaten/probability of fatal conflict over territory). But in the long term such large body sizes acts as an advantage over potential future enemies of the same kind. This is particularly relevant to marine ecosystems - where the size of fish often determine where in the food chain that fish is.

  • Speed/agility - Predators and prey that rely on speed/agility to
    capture (predator) or escape (prey) undergo an evolutionary arms race leading to positive feedback of speed/agility boosting morphologies. In the short-term the natural enemy of the prey acts as a burden to
    population fitness (probability of being eaten) but in deep time the prey has acquired morphological advantages over potential future
    natural enemies such as competitors for resources.

  • Venomousness -Why do venomous animals live in warm climates?

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up vote 3 down vote accepted

The question is interesting.

I am afraid about the use of the word "fitness" in you question. The fitness is usually defined as the number of offsprings an individual can sire in its lifetime. Most often we talk about relative fitness which is the relative number of offspring in an individual can sire in its lifetime compare to the individual in the population that sire the greatest number of offsprings.

How would you compare fitness of individuals between different species? Just by counting the number of offspring they left behind? Or did you mean that most fit species are actually those species that can outcompete others? If yes, then I would say that saying

Two species that reduce one another's fitness on microevolutionary (short-term) timescales can increase each other's macroevolutionary (long-term) fitness

can be restated

Two species that reduce one another's fitness on microevolutionary (short-term) timescales can increase each other's macroevolutionary (long-term) species competition ability

and it is actually just a special case of

One species that evolve under many strong selection pressure ends up being a very competing species.

The explanation is the following:

One species that undergoes one selective pressure, will see its genetic variance decreases. The weak individuals will disappear letting places to the strong individuals. Then newly arisen variance (mutation) will come up only in the strong individuals (as the other ones disapeared). This is what J.Gould meant by saying "Natural selection creates the fit". As a consequence a species that undergo many successive selection pressure will end up with purified and very strong genotypes while, in a species that were free to evolve without selection they would have evolved many different genomes with some good mutations nested in very poor genetic background.

You could as well use the concept of lineage selection and states that lineage that succeeds to outcompete others will thrive while species that suffer from competition with other species will get extinct. As a result, if you compare all species that were in competition with another species in the past with all species that were not in competition, you will find out that those that were in competition are more resistant to competition. But I guess this is not exactly what you meant when saying: "Two species that reduce one another's fitness on microevolutionary (short-term) timescales can increase each other's macroevolutionary (long-term) fitness"

Finally you might think of the ability of species to evolve better. This ability is called "evolvability" (see this book for example). You might say that species that undergo many selection pressures were selected to resist to newly arisen selection pressure. As a result, species that are under competition might end up with a greater evolvability than those that were not under selection. Resulting that species that were under competition will tend to increase to always increase their fitness in the long term.

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I believe I was not just addressing "competition". E.g. Having larger body size does not simply increase competitive ability, and may sometimes reduce it (increased resource requirements). The ramifications of having certain positive feedback loops in morphology change has a more general effect. Indeed fitness was a word I chose since I could not choose another - perhaps I mean "fitness relative to future potential phenotypes". Fitness is not just restricted to #offpspring, that is only the "life histories" definition isn't it? – hello_there_andy Nov 26 '13 at 12:11
The wikipedia link you provide says: "Fitness is a central idea in evolutionary theory. It can be defined either with respect to a genotype or to a phenotype in a given environment. In either case, it describes the ability to both survive and reproduce, and is equal to the average contribution to the gene pool of the next generation that is made by an average individual of the specified genotype or phenotype." and this wiki article does not use the word life-history. – Remi.b Nov 26 '13 at 12:20
Fitness is usually considered to be the (relative or not) number of offsprings one can produce in its lifetime (or per year). It is also often considered to be number of fertile offsprings or even the number of grand-children. This concept even if it is very central. Here is a discussion concerning the concept of fitness but the discussion is still open I think. – Remi.b Nov 26 '13 at 12:22
But anyway. What is the definition of the words you're using in "Two species that reduce one another's fitness on microevolutionary (short-term) timescales can increase each other's macroevolutionary (long-term) fitness"? – Remi.b Nov 26 '13 at 12:23
Yes, I agree with you definitions of micro- and macro-evolution. I don't think there are any different between fitness and reproductive success except the one that a particular person wants to make. Sometimes reproductive success is considered non-relative while fitness is relative. I still don't really understand what you call "life history definition of fitness" – Remi.b Nov 26 '13 at 12:31

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