Dispersal & Dormancy

8.1 Delphinium nelsonii

The figure shows effects of crossing distance on 3 measurements of fitness of Delphinium nelsonii. Pollen was taken from plants that were 1, 3, 10 or 30 metres from the receptor plant.  Bars show one Standard Error of the mean.

Explain what phenomena these graphs are showing and why this happens.

Source: Begon, M., Townsend, C.R. & Harper, J.L. 2006. Blackwell Publishing, Oxford

Figure 6.9 redrawn with permission of John Wiley & Sons.

© 2005 Wiley Blackwell.

Data from Waser, N.M. & Price, M.V. 1994. Crossing distance effects in Delphinium nelsonii: outbreeding and inbreeding depression in progency fitness. Evolution, 48: 842-852


Model answer:

The graphs show that Delphinium nelsonii experiences both inbreeding and outbreeding depression.  The height of progeny at three years old, progeny lifespan and overall fitness were maximised when the receptor plant was crossed with plants growing at intermediate distances (3-10 m away).  When crossed with plants growing 1 m away, the progeny showed inbreeding depression. This occurs when the phenotype expresses recessive deleterious alleles; it is more likely to occur when breeding individuals are closely related.  Similarly, the progeny of plants growing 30 m apart are also reduced in height, lifespan and overall fitness.  In contrast, this is due to outbreeding depression and occurs when co-adapted combinations of genes are separated reducing adaptation to specific site conditions. Species which usually experience outbreeding are more susceptible to inbreeding, since inbred populations have generally lost all their deleterious recessive genes.  The results found here suggest that plants which are close together are more closely related, and that most seeds fall close to their parent plant.

8.2 Magicicadas

Magicicada spp. emerge synchronously as adults in vast numbers after 13 or 17 years underground as larvae.

It’s suggested that if the entire cohort emerges together, it will satiate prey and thus result in greater numbers of adults surviving to breed.  Does this graph support that idea?  Explain your reasoning.

Source: Williams, K.S., Smith, K.G. & Stephen, F.M.  1993. Ecology, 74: 1143-1152.

Figure 8 reproduced with permission of John Wiley & Sons.

© 1993 by the Ecological Society of America.

8.3 White butterfly triggerplant

Hufford et al (2012)  investigated germination success of Stylidium hispidum (the white butterfly triggerplant), a plant with a very restricted distribution in Western Australia.

Molecular marker differentiation showed there were 2 genetically distinct clusters which they described as ‘north’ and ‘south’.  They crossed plants in 4 locations (2 from each cluster) with plants from the other cluster (‘long distance’), from the other population within the same cluster (‘short distance’) and within each population.  They then measured the proportion of seeds that germinated.  Bars with a letter show significant differences between the types of cross.

What phenomenon is shown by these results and why might it occur?


Source: Hufford et al (2012). Ecology & Evolution, 2: 2262-2273

Figures 1 and 4 reproduced with permission of Blackwell Publishing.

©2012 The Author. Published by Blackwell Publishing Ltd.

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