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Why Did Evolution Puzzle Newton? New Model Explains Mating Selection

Why do some animals have showy ornaments such as deer antlers, peacock feathers and horns on dung beetles? Charles Darwin couldn’t figure it out, but now a Northwestern University research team has developed a mathematical model to explain the puzzling phenomenon of evolution.

The mathematical model has made a surprising prediction: In animals with ornamentation, males will evolve out of the tension between natural selection and sexual selection into two distinct subspecies, one with flashy, “costly” ornaments for attracting

A new mathematical model developed by Northwestern University researchers explains why two distinct subspecies evolve in ornamented animals: one with flashy, "costly" ornaments for attracting mates and one with subdued, "low-cost" ornaments. Here, two male taurus scarab beetles, a type of dung beetle, illustrate the subdued subgroup (left) and the showy subgroup (right). CREDIT: Douglas Emlen, University of Montana

A new mathematical model developed by Northwestern University researchers explains why two distinct subspecies evolve in ornamented animals: one with flashy, “costly” ornaments for attracting mates and one with subdued, “low-cost” ornaments. Here, two male taurus scarab beetles, a type of dung beetle, illustrate the subdued subgroup (left) and the showy subgroup (right).
CREDIT: Douglas Emlen, University of Montana

mates and one with subdued, “low-cost” ornaments.

“Ornamentation does persist in nature, and our quantitative model reveals that a species can split into two subspecies as a result of the ornamentation battle that occurs over time,” said Daniel M. Abrams of the McCormick School of Engineering.

The researchers studied available data from 15 species. It was deer antlers that first made Abrams, an applied mathematician, wonder why some animals spend precious energy to grow and carry around something that could compromise life. It’s not unusual for male deer and their antlers to get stuck in trees or die in a fight.

“Animals with extravagant ornaments are showing just how fit and strong they are — that they can overcome the costs of these ornaments — and this attracts the opposite sex,” said Abrams, who led the study.

The study was published in the journal Royal Society Proceedings.

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