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Organisms that are well-adapted to their environment are more likely to survive and reproduce. However, natural selection does not lead to perfectly adapted organisms. Several factors constrain natural selection.
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Natural selection influences the frequencies of particular alleles and phenotypes within populations in several different ways. Primarily, natural selection can be directional, stabilizing, or disruptive. Directional selection favors one extreme trait and shifts the population towards that phenotype while selecting against individuals displaying alternate traits. Stabilizing selection favors an intermediate trait with a narrow range of variation. Deviation from the optimal phenotype towards an...
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Fluctuating survival selection explains variation in avian group size.

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Area of Science:

  • Ecology
  • Evolutionary Biology
  • Behavioral Ecology

Background:

  • Most animal populations exhibit wide variation in group sizes.
  • The persistence of this variation is unexplained, despite certain group sizes conferring higher fitness.

Purpose of the Study:

  • To investigate the drivers of persistent group size variation in cliff swallows.
  • To determine if selection pressures on group size fluctuate over time.

Main Methods:

  • A 30-year mark-recapture study of colonially breeding cliff swallows (Petrochelidon pyrrhonota).
  • Analysis of survival data in relation to colony size across different years.

Main Results:

  • Survival advantages of different colony sizes fluctuated annually.
  • Colony size experienced both stabilizing and directional selection, with regular reversals in directional selection.
  • Drought conditions partially predicted directional selection, favoring larger colonies in wetter years and smaller colonies in drier years.

Conclusions:

  • Fluctuating survival selection, driven by environmental conditions, maintains wide group size variation in cliff swallows.
  • This study presents the first evidence of regularly oscillating fitness advantages for different group sizes in a natural vertebrate population.
  • The findings highlight the dynamic interplay between sociality, environmental factors, and population dynamics.