It has long been known that patterns of phenotypic variation within and among related populations or species are similar. This observation has sometimes been interpreted to mean that evolutionary change is biased toward the direction of greatest variation. An alternative explanation is that the same factors that favor evolution in a certain direction also shape patterns of variation within populations. There has never been a satisfactory answer to the question of how Gmax, the multivariate direction of greatest additive genetic variance within populations, and Z, the direction of multivariate evolution between them, become oriented in the same direction.
This problem has puzzled workers in fields ranging from quantitative genetics to paleontology since. The proposed research will exploit the element of time in the fossil record to help resolve this stubborn problem. The basis for this study is an extraordinary deposit of fossil threespine stickleback fish, Gasterosteus doryssus, from the late middle Miocene, middle member of the Truckee Formation is west-central Nevada. Abundant, beautifully-preserved specimens exhibit striking patterns of stratigraphic variation for size, armor, and fin ray number in a quarry that has been used in several previous studies by the Principal Investigator. The deposit itself has special properties that offer ideal conditions to make large stickleback samples at regular time intervals over thousands of years. Not the least important of these properties is the presence of uninterrupted annual laminations (varves). Furthermore, paleoecological evidence and analogy with extant threespine stickleback show that samples from the fossil sequence represent populations with ancestor-descendant relationships (i.e., a lineage). This is exactly what is needed to examine the link between variation within populations and the direction of evolution.
Morphometric data obtained from this fossil lineage will be used to examine the relationship between the multivariate direction of greatest phenotypic variance within fossil samples, P, and the direction of multivariate evolution, Z, between successive samples. The directions of P and Z will be estimated using the morphometric data from 11 to 24 large samples spaced at 1000-year intervals. Angeles between P and Z will be compared to see if P within a sample tends to bias the direction of evolution (Z) between it and the next sample, or whether P tends to swing around to match the direction of Z between it and the previous sample. Both effects may occur.
Use of a fossil lineage will allow a critical analysis of the cause for the often-observed similarity between the direction of variation within and among populations. The proposed research will be the first such analysis that utilizes the crucial element of time provided by the fossil record. Not only will the proposed research be the first such analysis, but it will also contribute to methodological developments in morphometrics and the growing literature on stickleback evolution.
