Why do larger mothers produce larger offspring? A test of classic theory

Authors: Hayley Cameron, Keyne Monro, Martino Malerba, Stephan Munch and Dustin Marshall

Published in: Ecology, volume 97, issue 12 (December 2016)

Abstract

Across a wide range of taxa, larger mothers produce larger offspring.

Theory assumes that larger, more fecund mothers create higher local densities of siblings, and so larger mothers produce larger offspring to offset sibling competition. This assumption has been debated for over 30 years, but direct empirical tests are surprisingly rare.

Here, we test two key assumptions of classic theories that predict sibling competition drives maternal-size–offspring-size (MSOS) correlations:

  1. independent effects of offspring size and sibling density on offspring performance or
  2. as a product of an interaction between these two factors.

To simultaneously test these alternative assumptions, we manipulate offspring size and sibling density in the marine invertebrate, Bugula neritina, and monitor offspring performance in the field.

We found that, depending on the fitness metric being considered, offspring size and sibling density can either independently or interactively affect offspring performance. Yet sibling density did not affect offspring performance in the ways that classic theories assume.

Given our results, it is unlikely that sibling competition drives the positive MSOS correlation observed in this species. Empirical support for these classic theories remains lacking, suggesting alternative explanations are necessary.

Citation

Cameron H, Monro K, Malerba M, Munch S, Marshall DJ (2016) Why do larger mothers produce larger offspring? A test of classic theory. Ecology, 97: 3452–3459. PDF 430 KB doi:10.1002/ecy.1590

Unravelling anisogamy: egg size and ejaculate size mediate selection on morphology in free-swimming sperm

Authors: Keyne Monro and Dustin J Marshall

Published in: Proceedings of the Royal Society B, volume 283, issue 1834 (July 2016)

Abstract

Gamete dimorphism (anisogamy) defines the sexes in most multicellular organisms.

Theoretical explanations for its maintenance usually emphasize the size-related selection pressures of sperm competition and zygote survival, assuming that fertilization of all eggs precludes selection for phenotypes that enhance fertility. In external fertilizers, however, fertilization is often incomplete due to sperm limitation, and the risk of polyspermy weakens theadvantage of high sperm numbers that is predicted to limit sperm size, allowing alternative selection pressures to target free-swimming sperm.

We asked whether egg size and ejaculate size mediate selection on the free-swimming sperm of Galeolaria caespitosa, a marine tubeworm with external fertilization, by comparing relationships between sperm morphology and male fertility across manipulations of egg size and sperm density.

Our results suggest that selection pressures exerted by these factors may aid the maintenance of anisogamy in external fertilizers by limiting the adaptive value of larger sperm in the absence of competition. In doing so, our study offers a more complete explanation for the stability of anisogamy across the range of sperm environments typical of this mating system and identifies new potential for the sexes to coevolve via mutual selection pressures exerted by gametes at fertilization.

Citation

Monro K, Marshall DJ (2016) Unravelling anisogamy: egg size and ejaculate size mediate selection on morphology in free-swimming sperm, Proceedings of the Royal Society B, 283:1834 PDF 2.7 MB doi:10.1098/rspb.2016.0671

Global change, life-history complexity and the potential for evolutionary rescue

Authors: Dustin J Marshall, Scott C Burgess and Tim Connallon

Published in: Evolutionary Applications, May 2016

Abstract

Most organisms have complex life cycles, and in marine taxa, larval life-history stages tend to be more sensitive to environmental stress than adult (reproductive) life-history stages.

While there are several models of stage-specific adaptation across the life history, the extent to which differential sensitivity to environmental stress (defined here as reductions in absolute fitness across the life history) affects the tempo of adaptive evolution to change remains unclear.

We used a heuristic model to explore how commonly observed features associated with marine complex life histories alter a population’s capacity to cope with environmental change.

We found that increasing the complexity of the life history generally reduces the evolutionary potential of taxa to cope with environmental change. Our model also predicted that genetic correlations in stress tolerance between stages, levels of genetic variance in each stage, and the relative plasticity of different stages, all interact to affect the maximum rate of environmental change that will permit species persistence.

Our results suggest that marine organisms with complex life cycles are particularly vulnerable to anthropogenic global change, but we lack empirical estimates of key parameters for most species.

Citation

Marshall DJ, Burgess SC, Connallon T (2016) Global change, life-history complexity and the potential for evolutionary rescue, Evolutionary ApplicationsPDF 434 KB doi:10.1111/eva.12396

Metabolic rate covaries with fitness and the pace of the life history in the field

Authors: Amanda K Pettersen, Craig R White and Dustin J Marshall

Published in: Proceedings of the Royal Society B, volume 283, issue 1831 (May 2016)

Abstract

Metabolic rate reflects the ‘pace of life’ in every organism. Metabolic rate is related to an organism’s capacity for essential maintenance, growth and reproduction—all of which interact to affect fitness.

Although thousands of measurements of metabolic rate have been made, the microevolutionary forces that shape metabolic rate remain poorly resolved. The relationship between metabolic rate and components of fitness are often inconsistent, possibly because these fitness components incompletely map to actual fitness and often negatively covary with each other.

Here we measure metabolic rate across ontogeny and monitor its effects on actual fitness (lifetime reproductive output) for a marine bryozoan in the field. We also measure key components of fitness throughout the entire life history including growth rate, longevity and age at the onset of reproduction.

We found that correlational selection favours individuals with higher metabolic rates in one stage and lower metabolic rates in the other—individuals with similar metabolic rates in each developmental stage displayed the lowest fitness. Furthermore, individuals with the lowest metabolic rates lived for longer and reproduced more, but they also grew more slowly and took longer to reproduce initially.

That metabolic rate is related to the pace of the life history in nature has long been suggested by macroevolutionary patterns but this study reveals the microevolutionary processes that probably generated these patterns.

Citation

Pettersen A, White CF, Marshall DJ (2016) Metabolic rate covaries with fitness and the pace of the life history in the field, Proceedings of the Royal Society B, 283: 20160323
PDF 548 KB doi: 20160323. doi:10.1098/rspb.2016.0323

Biofilm history and oxygen availability interact to affect habitat selection in a marine invertebrate

Authors: Marcelo E Lagos, Craig R White and Dustin J Marshall

Published in: Biofouling, volume 32, issue 6

Abstract

In marine systems, oxygen availability varies at small temporal and spatial scales, such that current oxygen levels may not reflect conditions of the past.

Different studies have shown that marine invertebrate larvae can select settlement sites based on local oxygen levels and oxygenation history of the biofilm, but no study has examined the interaction of both.

The influence of normoxic and hypoxic water and oxygenation history of biofilms on pre-settlement behavior and settlement of the bryozoan Bugula neritina was tested. Larvae used cues in a hierarchical way: the oxygen levels in the water prime larvae to respond, the response to different biofilms is contingent on oxygen levels in the water. When oxygen levels varied throughout biofilm formation, larvae responded differently depending on the history of the biofilm.

It appears that B. neritina larvae integrate cues about current and historical oxygen levels to select the appropriate microhabitat and maximize their fitness.

Citation

Lagos ME, White CR, Marshall DJ (2016) Biofilm history and oxygen availability interact to affect habitat selection in a marine invertebrate, Biofouling, 32:6 645–655 PDF 1.6 MB doi: 10.1080/08927014.2016.1178725

Propagule size and dispersal costs mediate establishment success of an invasive species

Authors: Rolanda Lange and Dustin J Marshall

Published in: Ecology, volume 97, issue 3 (March 2016)

Abstract

Bio-invasions depend on the number and frequency of invaders arriving in new habitats. Yet, as is often the case, it is not only quantity that counts, but also quality.

The process of dispersal can change disperser quality and establishment success. Invasions are a form of extra-range dispersal, so that invaders often experience changes in quality through dispersal.

To study effects of dispersal on invader quality, and its interactions with quantity on invasion success, we manipulated both in a field experiment using an invasive marine invertebrate.

Establishment success increased with the number of individuals arriving in a new habitat. Prolonged larval durations – our manipulation of prolonged dispersal – decreased individual quality and establishment success. Groups of invaders with prolonged larval durations contributed only a third of the offspring relative to invaders that settled immediately.

We also found an interaction between the quality and quantity of invaders on individual growth: only within high-quality cohorts did individuals experience density-dependent effects on growth.

Our findings highlight that dispersal not only affects the quantity of invaders arriving in a new habitat but also their quality, and both mediate establishment success.

Citation

Lange R , Marshall DJ (2016) Propagule size and dispersal costs mediate establishment success of an invasive species. Ecology, 97(3), 2016, pp. 569–575
DOI: 10.1890/15-1573 PDF 238 KB

Genetic compatibility underlies benefits of mate choice in an external fertilizer

Authors: J David Aguirre, Mark W Blows, and Dustin J Marshall

Published in: The American Naturalist, volume 187, number 5 (May 2016)

Abstract

Mate choice is a common feature of sexually reproducing species. In sessile or sedentary external fertilizers, however, direct interactions between reproductive partners are minimal, and instead mate recognition and choice must occur at the level of gametes.

It is common for some sperm and egg combinations to have higher fertilization success than others, but it remains unclear whether differences in fertilization reflect gamete-level mate choice (GMC) for paternal quality or parental compatibility.

Here, we examine the mechanisms underlying GMC in an externally fertilizing ascidian. A manipulative mate-choice assay confirmed that offspring viability was greater in clutches where we allowed GMC than in clutches where we precluded GMC. A complementary quantitative genetic experiment then revealed that paternal quality effects were generally weaker than parental compatibility effects, particularly for the trait combination underlying the benefits of GMC.

Overall, our data suggest that gametes that are more compatible at fertilization produce more viable offspring than gametes that are less compatible at fertilization. Therefore, although the regalia we typically associate with sexual selection are absent in external fertilizers, mechanisms that allow females to bias fertilization in favor of some males over others produce significant fitness benefits in organisms reproducing via the ancestral strategy.

Citation

Aguirre, JD, Blows MW, Marshall DJ (2016) Genetic compatibility underlies benefits of mate choice in an external fertiliser. The American Naturalist, 187(5) DOI: 10.1086/685892 PDF 672 KB

Relative contributions of offspring quality and environmental quality to adult field performance

Authors: Rolanda Lange and Dustin J Marshall

Published in: Oikos, volume 125,  issue 2 (February 2016)

Abstract

Determinants of adult performance, such as growth and survival, are influenced by extrinsic, environmental and intrinsic, phenotypic factors. The relative importance of extrinsic and intrinsic factors, while ecologically relevant, is rarely estimated simultaneously.

We estimate the relative contributions of offspring size (intrinsic) and various environmental factors (extrinsic) on adult performance in the marine colonial bryozoan Watersipora subtorquata.

We used a variance partitioning approach for both new and previously published data, enabling us to examine the performance of over 1000 individuals in the field.

We found offspring size to explain relatively little variation in adult performance. Of the environmental factors taken to account, temporal variation and an environmental gradient had the strongest influences.

Citation

Lange R, Marshall DJ (2016) Relative contributions of offspring quality and environmental quality to adult field performance, Oikos 125: 210–217, 2016 PDF 1.3 MB doi:10.1111/oik.02473

Research Fellow position (Biological Sciences, Centre for Geometric Biology)

  • Level A, research-only academic
  • Up to $99K including 17% superannuation
  • Full-time
  • Two-year, fixed-term
  • Monash University Clayton campus
  • Applications are now closed

Prof Dustin Marshall is seeking an evolutionary ecologist to explore the dynamics of biological systems, specifically how size and shape affect the ways resources move through populations and communities.

As a postdoctoral researcher, you will explore how biological entities change their function as they change in size and shape, using empirical and/or theoretical approaches. The research will be conducted in collaboration with Prof Marshall as part of the Centre for Geometric Biology at Monash University.

You will further be expected to maintain consistently high research output in the form of quality publications, supervision of students, development and submission of grant proposals to external funding agencies, contribute more generally to communicating the research activities of the group, and participation in appropriate career development activities.

This role is a full-time position; however, flexible working arrangements may be negotiated.

Key selection criteria

  1. A PhD in theoretical biology, or empirical evolutionary ecology.
  2. Demonstrated experience in developing theoretical models in fundamental ecology or empirical research using cutting-edge quantitative approaches.
  3. Demonstrated ability to undertake outstanding research; with a high quality research publication record in recognised journals;
  4. Ability to solve problems by using discretion, innovation and the exercise of high level diagnostic skills within areas of functional responsibility or professional expertise;
  5. Excellent written communication and verbal communication skills with proven ability to effectively analyse information and produce clear, succinct reports and documents which requires interaction with others;
  6. Demonstrated planning and organisational skills, with the ability to prioritise multiple tasks and set and meet deadlines;;
  7. Demonstrated awareness of the principles of confidentiality, privacy and information handling.
  8. Demonstrated ability to effectively work independently and in a multidisciplinary team to make a contribution toresearch and scholarship.
  9. Experience of, or willingness to work on, marine systems (desirable.
  10. A demonstrated understanding of questions in fundamental ecology and/or evolution (desirable)

Enquiries to Professor Dustin Marshall on +61 3 9902 4449

For more information, or to apply, refer to the Monash University website.

How not to influence ecology: three things we have learned at Oikos

Authors: Dustin J Marshall and Dries Bonte

Published in: Oikos, volume 125, issue 1 (January 2016)

Here at OIKOS, we receive around five or six proposals for a Forum article every week. The Forum is a place where ecological ideas can be kicked around and examined from a different angle. Our hope is that some will provide a high level of synthesis in the field of ecology.

The Forum is a place where ecological ideas can be kicked around and examined from a different angle. Our hope is that some will provide a high level of synthesis in the field of ecology.

Some of the best papers that we’ve had the privilege to handle as an editor have been Forum papers and we believe that the Forum plays a very important role in OIKOS, and a role in the field of ecology more generally. For that reason, we love the chance to handle great Forum papers. We really dislike the other part of our job: rejecting most of the Forum proposals we get each week. Here, we thought we should provide some insights into why most proposals get rejected.

We really dislike the other part of our job: rejecting most of the Forum proposals we get each week. Here, we thought we should provide some insights into why most proposals get rejected.

Necessity

While we get Forum pieces on many, many topics, a single feature unites the best ones: the author felt compelled to write it. The most effective Forum pieces are those that the author felt just had to be written, that the field was limping along under a misguided principle, that key principles were misunderstood, or despite what the field thought it knew, it actually didn’t. Those proposals where the author fiercely believes that ecology won’t be as effective as it could be with-out the Forum paper are the ones that are, in my view, the best. We remember one proposal in particular that basically said, “I really didn’t want to write this thing but the field keeps messing this thing up so I had no choice”. The

We remember one proposal in particular that basically said, “I really didn’t want to write this thing but the field keeps messing this thing up so I had no choice”. The resulting article was an excellent contribution that changed the field for the better. In contrast, many other proposals, while they have something to say, are basically tracking along with the field and it is unclear that the field actually needs a Forum piece in order to move forward. One issue here is the

One issue here is the pressure for graduate students to accumulate papers as quickly as possible, which leads to the submission of thesis summaries as Forum proposals. In our experience at OIKOS, many of these proposals are adept summaries of the field, but generally are not suitable as Forum pieces as they rarely provide synthesis.

Telling rather than showing

We get many, many submissions proposing a new framework / model system / approach where the purpose of the Forum article would be to draw attention to a particular thing that the author believes is valuable. We have published such papers over the years and

We have published such papers over the years and they almost universally fail to have much influence on the field. We suspect that the reason for

We suspect that the reason for lack of traction: there are lots of approaches / frameworks / model systems out there, an overwhelming number actually. In response to this overwhelming number, most researchers use approaches or model systems or frameworks that have been shown to work very well.

A model system becomes a model system because some brave soul shows the world the power of such a system through excellent empiricism. An approach is taken up more widely when researchers show that it allows unprecedented access to a particular problem via a series of experiments.

You may feel that your particular approach is superior, or that your framework is the absolute best for tackling a problem, you may also be right but scientists are rather conservative. Scientists will not take something up just because you tell them it is the right thing to do; instead you have to show them, through your own work.

One paper (however good) is unlikely to change a field’s whole approach to a problem, so you have to be patient. Through repeated demonstration in your own empiricism, show that your framework (or approach, or model system) is actually best.

The uptake of a new approach takes time, it is idiosyncratic, probably biased towards established academics, and unfortunately, probably biased geographically (we are guessing here but we suspect that if someone utilises a new method in California, it is more likely to catch on than if the same method was discovered in Peru).

So we see the appeal of writing a Forum piece to catalyse the uptake of some great new thing.

The problem is, we rarely see such an approach succeed. At the very least, ecologists are likely to be wary of a new thing until they see the proponents of the thing successfully tackle a range of problems themselves.

So, trust that science is an (imperfect) meritocracy, your great ideas will shine through and be taken up by the field if you persist with them.

Overall, we think this issue can be best summed up with a (paraphrased) Twain quote:

“Don’t say the person screamed. Bring them on and let them scream.”

They said something mean about my field or paper

As a scientist, it is infuriating when someone else misrepresents one’s work. It can also be very confronting to have key premises of a field challenged, or dismissed.

We receive many Forum proposals that seek to respond to such challenges or attacks.

We wrote about this problem around four years ago on the OIKOS blog and the issue has not gone away. We do not publish such exchanges in OIKOS as we have not seen much evidence that they are productive. We recognise this will be exasperating for many given that we occasionally publish heretical Forum pieces and then do not provide the opportunity for others to respond in kind. We note that no one has managed to obliterate a field via a Forum article so far and we doubt that they ever will. So, if you truly disagree with a Forum article, show through empiricism that it is incorrect. Hold your fire, instead use empiricism to make your point.

These three suggestions will seem opaque to some, offensive to others, and obvious to most. Nevertheless, we suspect that editors for other journals will recognise some of these same issues, and may agree (we would love to hear from them either way).

Our hope in writing this editorial is that authors avoid wasting their time with doomed proposals. Our key suggestion is for authors to ask themselves, “How would I react to my proposed Forum piece if it were not written by me? Would I change the way I thought about, or did ecology?” If the answer is yes, then go ahead. We look forward to reading it.

Citation

Marshall DJ, Bonte D (2016) How not to influence ecology: three things we have learned at Oikos. Oikos, 125: 1–2, 2016 PDF 472 KB doi: 10.1111/oik.03107 doi:10.1111/oik.03107