SHORT-TERM EVOLUTION IN THE SIZE AND SHAPE OF PEA APHIDS.

Area of Science:

• Evolutionary Biology
• Population Genetics
• Insect Ecology

Background:

• Phenotypic evolution is typically observed under novel selective pressures causing rapid changes.
• Cyclical parthenogens offer unique opportunities to study adaptive evolution due to yearly cycles of phenotypic change.
• Morphological traits in insect herbivores are often targets of natural selection for resource exploitation.

Purpose of the Study:

• To estimate the genetic variability of morphological traits in the pea aphid (Acyrthosiphon pisum).
• To test the hypothesis that size and shape evolve through clonal selection during parthenogenetic reproduction.
• To investigate the role of natural selection in driving short-term morphological evolution in pea aphids.

Main Methods:

• Estimated genetic variation of morphological traits using laboratory-reared descendants of field-collected pea aphid clones.
• Assessed clonal heritability for overall morphology, size, and shape traits.
• Estimated genetic correlations between size and shape variables and compared phenotypes between early and late season collections.
• Measured demographic and morphological variables on individuals under field conditions to estimate selection on size.

Main Results:

• Significant clonal heritability for morphological traits was observed early in the season in both sampled years.
• Qualitatively similar changes in multivariate morphological phenotypes were observed between early and late season collections.
• Natural selection acting primarily through body size likely explains the observed evolutionary changes in pea aphid morphology.
• A shift to smaller size between late 1988 and early 1989 collections suggests persistent yearly cycles of morphological evolution.

Conclusions:

• Pea aphids exhibit significant potential for morphological evolution driven by clonal selection during parthenogenetic reproduction.
• Natural selection, particularly on body size, is a key factor in short-term morphological evolution in this species.
• Yearly cycles of morphological evolution may occur in cyclical parthenogens due to recombination costs or seasonal selective forces.