These two species of turtles are believed to have one common ancestor, but one species has gone up in size and the other vice versa.
American evolutionary biologists have questioned the theory that "evolution indulges mediocrity."
It is believed that evolution is averaging: the most successful individuals in terms of natural selection have an average body size and a moderate speed of development. The most common example is that overweight or underweight newborns are less likely to survive than normal ("average") babies.
It is by this selection that evolution proceeds; cases of winning the evolutionary race by individuals who deviate significantly from the norm are rare. The share of separating selection, when “deviations from the norm” turn out to be the winner, there are rare cases of speciation: large and small individuals diverge and found in their own kind.
In the work of evolutionary biologists at the University of North Carolina at Chapel Hill, the opposite is true: more is better. Joel Kingsolver, one of the study's co-authors, says stabilizing selection that indulges "mediocrity" is not as common in evolution as is commonly believed. Scientists have analyzed over a hundred species of birds, lizards, snakes, insects and plants; for each species, it was known how the appearance and behavior changed over several generations, in other words, in which direction natural selection acted. It turned out that large and rapidly growing and maturing accelerates - those that begin to mate, bloom and bear fruit earlier - have a greater chance of surviving and leaving offspring.
True, another question immediately arises before us: if more is better, then why do we not live in a world of giants? The explanations for this can be as follows: firstly, not everything that is good for reproduction is good for survival, and vice versa. For example, in fish, a brightly colored large male can attract both females and predators with greater success. Second, not everything that is good today will be as good tomorrow. The authors explain this with the example of finches: birds with large beaks can eat large seeds, but if plants with small seeds are born next year, large finches will have a hard time: their beak is not able to work with small food. And thirdly: enlargement of the body cannot go beyond reasonable proportions and is subject to "engineering" considerations. For example,in flying insects, the greatest flight efficiency is achieved with large wings and a small body.
However, the researchers admit that they cannot find obstacles to temporary acceleration. It is clear what exactly acts against general selection in favor of large forms. But it is not clear why the world has not yet been captured by fast-growing and early maturing individuals.
This work was published in the March issue of American Naturalist.
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