A.J. Van Noordwijk

Genetic variation in the timing of reproduction in the Great Tit

SummaryAbout 40% of the population variation in the initiation dates of first clutches within years is genetically determined. The onset of laying, which is determined by the female, is not detectably influenced by spatial heterogeneity of the study area.There is a variable selection favoring early, middle, or late laying in some years. Over the study period as a whole there is a slight net selection for laying relatively late.The implications for a potential rapid evolutionary change are discussed. The conclusion is reached that the population mean might change with rates of up to one week per five generations, which is approximately a decade.

Heritability of body size in a natural population of the Great Tit (Parus major) and its relation to age and environmental conditions during growth

We have analysed data on weight and tarsus length collected during a long-term study of natural populations of Great Tits to evaluate the relative importance of genetic variation in body size. Some of our data were collected over a 25-year period, and therefore include a relatively large sample of naturally occurring environmental conditions. An overall heritability estimate calculated from the uncorrected mean weights of breeding birds amounts to 0-5. This estimate is unlikely to be influenced by resemblance in environmental conditions between relatives. Heritability estimates based on the size of fledglings vary between zero and the value for adults, depending on the environmental conditions during growth. If the feeding conditions for the nestlings are poor, no resemblance between parents and offspring is observed. Selection against small nestlings acts strongly on the environmental variance. This is concluded from the higher heritability estimates in the same cohorts after survival for at least three months after fledging, compared to measurements on nestlings. Such selection acting differentially on the genetic and environmental components of the phenotypic variance has important consequences for our ability to make predictions of phenotypic change from measured natural selection. Nevertheless, the amount of genetic variation would allow rapid response should selection on adult size occur.

Genetic and Environmental Variation in Clutch Size of the Great Tit (Parus major)

An analysis is made of genetic and non-genetic components in the number of eggs in the completed first clutch of the Great Tit. 1) It is doubtful whether a causal relation between density and clutch size exists. 2) There is hardly a systematic effect of age on clutch size. 3) There is an annual variation in clutch size with similar changes in individual females in the same population, but it is hardly correlated between populations. This emphasizes a lack of genotype-environment interaction. 4) Within populations there is no detectable variation in clutch size that can be attributed to differences in habitat quality. 5) About 40% of the total phenotypic variation in clutch size is genetic variation. Several ways of eliminating a possible resemblance through correlated environments yield the same result. 6) Selection for clutch size is demonstrated in several years. 7) The implications for rapid evolutionary change in mean clutch size are discussed.

Genetic variation in egg dimensions in natural populations of the Great Tit

Length and breadth of eggs were measured in ringed populations of the Great Tit. During a part of the study volume and weight were also measured, but this did not give additional information, viz. variation in specific weight of fresh eggs and deviations from calculated volume were within the limits of precision. Only in small eggs are length and breadth positively correlated.In two populations, a major part (60–80%) of the variation in the clutch means of egg length, egg breadth, shape index and egg volume is only found between clutches of different females. The absence of correlation between different female partners of one male and the similarity of female repeatability to heritability estimates based on daughter-mother regression lead to the conclusion that 60–80% of the variation in egg dimensions is genetic.The implications for a potential rapid response to selection resulting in a micro-evolutionary change are discussed.

Dispersal, kinship and inbreeding in an island population of the Great Tit

We investigated the kinship structure of an island population of the Great Tit (Parus major). Kinship of birds could be inferred by comparing their family trees. Dispersal was also studied to explain the observed pattern of kinship.

Genetic background, and not ontogenetic effects, affects avian seasonal timing of reproduction

Avian seasonal timing is a life‐history trait with important fitness consequences and which is currently under directional selection due to climate change. To predict micro‐evolution in this trait, it is crucial to properly estimate its heritability. Heritabilities are often estimated from pedigreed wild populations. As these are observational data, it leaves the possibility that the resemblance between related individuals is not due to shared genes but to ontogenetic effects; when the environment for the offspring provided by early laying pairs differs from that by late pairs and the laying dates of these offspring when they reproduce themselves is affected by this environment, this may lead to inflated heritability estimates. Using simulation studies, we first tested whether and how much such an early environmental effect can inflate heritability estimates from animal models, and we showed that pedigree structure determines by how much early environmental effects inflate heritability estimates. We then used data from a wild population of great tits (Parus major) to compare laying dates of females born early in the season in first broods and from sisters born much later, in second broods. These birds are raised under very different environmental conditions but have the same genetic background. The laying dates of first and second brood offspring do not differ when they reproduce themselves, clearly showing that ontogenetic effects are very small and hence, family resemblance in timing is due to genes. This finding is essential for the interpretation of the heritabilities reported from wild populations and for predicting micro‐evolution in response to climate change.