Within species, individuals in wild populations are often phenotypically diverse, exhibiting differences in many complex traits. While genetic variation undoubtedly explains some portion of phenotypic diversity in the wild, its effects can be obscured by environmental factors, which often also play a large role in determining individual phenotype. For traits relating to life history, which are highly associated with fitness and often greatly influenced by environmental conditions, understanding the complicated relationships between specific environmental factors and phenotype is essential to understanding how populations will respond in an age of rapidly changing climate. Further, understanding the role of environment will help to disentangle the effects of genetic variation from other complex influences on life history such as parental effects–the influence that parents have on their offspring through a parentally determined environment, which is itself a product of parental genotype and environment. Distinguishing and quantifying the influence of the complex suite of factors that determine individual life history traits is essential to understanding how selection may act on these traits and thus, the potential of wild populations to evolve in response to the shifting selective pressures associated with climate change.

In stark contrast with the observable phenotypic diversity within populations are ‘living fossils’, lineages thought to have undergone very little phenotypic change over millions of years of evolutionary history. Thus, the existence of living fossils poses an interesting question: Given the combined influences of genetic and environmental variation, how is it possible that any phenotype is maintained over many generations across such long periods of time? However, the challenge to answering this question, and indeed testing the validity of ‘living fossils’ lineages in the first place, lies in the fossil record. Only through the description of extinct ancestors and comparisons with their hypothesized ‘living fossil’ descendants, can we understand the extent to which phenotype has been conserved.

To address these questions, I combine a fine-scale approach to a singular, phenotypically variable populations of extant fishes, with a broad-scale approach that looks at phenotypic variation within an ancient order of fish on a 100-million-year evolutionary timescale. In my first two chapters, we harness a long-term dataset on a population of smallmouth bass, Micropterus dolomieu–which includes genetic, climate, and reproductive data–to elucidate the complex drivers of variation in reproductive phenotype, an important aspect of life history in this economically and ecologically important species. In chapter three, we describe a new species of extinct polypterid fish from fossils dating to the Cenomanian Age (100.5 million years ago – 93.9 million years ago) and leverage this exceptionally rare data point to test the longstanding hypothesis that extant polypterids are morphologically similar to their ancestors.

In Chapter 1, we investigate variation in the seasonal onset of reproduction in smallmouth bass (M. dolomieu) using a 10-year dataset. The onset of reproduction can be constrained in many systems by a need to first accumulate energetic reserves. We find that temperature patterns early in a season have differential impact on the reproductive timing of males of different sizes, suggesting that the onset of seasonal reproduction is constrained by basal metabolic rate and that large individuals can escape these size-associated energetic constraints. Thus, we reveal a more complex relationship between environment and phenotype than previously recognized, which is critical for understanding how climate change will influence population dynamics of seasonally reproducing fish species like M. dolomieu.

In Chapter 2, we construct a pedigree spanning the decade-long dataset of M. dolomieu to disentangle the complex sources of variation in first reproductive phenotype. We find low levels of additive genetic variation (and thus low levels of heritability) underlying most aspects of reproductive phenotype, along with effects of reproductive year, birth year, and parental identity. We further identify one specific measure of parental environment–the temperatures experienced in an offspring’s first year–which drives variation in age and size at first reproduction. Thus, our results elucidate a complex pattern of inheritance for reproductive phenotype, where parents shape offspring reproductive traits both through genetics and parental effects, an understanding of which is essential to predicting how populations of M. dolomieu will change across generations.

In Chapter 3, we broaden our scope to consider patterns of phenotypic diversity within an ancient order of fish, Polypteriformes, on a 100-million-year evolutionary timescale. We describe a new species of extinct polypterid from the fossil record of the Cenomanian and leverage this exceptionally rare data point to test the longstanding hypothesis that extant polypterids are morphologically similar to their ancestors. We identify marked similarities in the fish’s scale and tooth structure between our fossil and its living relatives, placing our specimen securely within the order Polypteriformes. However, three-dimensional shape analysis revealed that the morphology of scales and jaw bones–the dentary and premaxillary–did not overlap with the morphological variation of these components in extant polypterids. Thus, while extant polypterids may indeed exhibit some ancestral characters, they fail the first direct test of morphological similarity with an extinct ancestor. Moreover, we find that this new species of ancient fish is likely the largest species ever described within this lineage by an order of magnitude, which results in a new perspective on the evolution and ancient ecology of the Polypteridae, a family of ray-finned fishes that occur in freshwater habitats in tropical Africa and the Nile River system, typically in shallow swamps, floodplains and estuaries.

Overall, understanding patterns of phenotypic diversity both within a species or within a lineage across time are important goals in evolutionary biology. Research investigating how environmental and genetic variation translates into phenotypic variation will help us to understand how species will respond to climate change, both at an individual level, and across generations, while research investigating the patterns of phenotypic change across evolutionary history will help us to understand how the diverse species of today came to be.