large-scale evolutionary trends
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Large-scale evolutionary trends
Foraminiferal size, oxygen and photosymbiosis
Outline
• Cope’s rule• What are foraminifera?• Dramatic size increase in late Paleozoic
fusulinid foraminifera• Passive and driven evolutionary trends• Tests for analyzing the fusulinid size trend• Interpretation of results
Evolution of size
• Cope’s rule:o Tendency in animal groups to
evolve toward larger sizeo First articulated in 1870s o Size trends recognized in reptiles,
mammals, arthropods, mollusks
Cope’s rule: Traditional explanation
• The largest size class is always unoccupied. Therefore, over time the number of size classes will increase since the one at the top is always open and available to be filled.
absoluteminimum
size
If extinction vacates organisms in a givensize class, others from adjacent size classes
might increase or decrease in size in order to fill the void There’salways room
at the top
Increasing size
What are foraminifera?
• Living protists with fossil record dating back to Cambrian Period (500 myr)
• 5,000 living species; >100,000 fossil species• Marine, brackish and freshwater• Benthic and planktonic (20% of total modern
carbonate production)• Most studied group of fossils
~ 3 cm
Foram sculpture park (China)
live foram assemblage
semelparousreproduction
Fusulinid forams
• Originated ~330 Ma; became extinct ~250 Ma• Very abundant & diverse; “rock-building” protists• Many lineages achieved “gigantic” size
arrowhead made of silicifiedfusulinid limestone
fusulinid limestones
Fusulinid forams
Large specimens can reach 16 mm in length and 8 mm in diameter(volume = 500 mm3 surface area = 340 mm2)
Smallest specimen is 0.06 mm in length and 0.15 mm in diameter(volume = 0.01 mm3 surface area = 0.04 mm2)
dramatic size evolutionin fusulinids
McShea 1994 Evolution
Confining lower boundary;increases and decreases
equally likely
No confining boundary;increases more likely
than decreases (impliesselection for large size)
PASSIVE DRIVEN
“Passive” vs. “Driven” trends
McShea 1994 Evolution
PASSIVE DRIVEN
Minimum does not increase Minimum increases
Minimum test
Minimum test suggests adriven trend
McShea 1994 Evolution
Subclade test:Size distribution of parent clade is nearly always right-skewed.
Subclade from the tail of theparent clade’s distributionis right-skewed
Subclade from the tail of theparent clade’s distributionis not skewed
Fusulinid size distribution
Volume (mm3)
parent clade
Subclade test suggests a driven trend
Quantifying passive and driven components of large-scale trends
• Wang (2001) recognized that large-scale trends are unlikely to be entirely passive or entirely driven, but rather a combination of both types
• Analysis of skewness test determines the proportional influence of passive and driven mechanisms: Sums of Cubes
SCtotal = SCbetween groups + SCwithin groups + SCheteroskedacticity
Each subclade exhibits a normal distribution, butsubclade means are not normally distributed
about the parent clade mean
indicatespassive trend
Wang 2001 Evolution
Subclade means are normally distributedabout the parent clade mean, but each
subclade is right-skewed
indicatesdriven trend
Wang 2001 Evolution
Subclade means are normally distributed about the parent clade mean,and each subclade is normally distributed, but standard deviation is
greater for subclades near right tail of parent clade’s distribution
indicatespassive trend
Wang 2001 Evolution
Analysis of skewness(fusulinid dataset)
100.00305,758,497Total skewness
passive33.09101,161,672Heteroskedasticity skewness
passive-0.10-295,643Skewness between subclades
driven67.01204,892,468Skewness within subclades
Trend indicated%ValueCategory
Total skewness of fusulinoidean volume distribution as the sum of three components.
Interpretation of results
• Size trend in fusulinids is 2/3 driven and 1/3 passive
• Driven component likely reflects selection for large sizeo Large size as a result of photosymbiosis
• Passive component likely reflects relaxed constraints on sizeo Large size permitted by hyperoxia
Photosymbiosis in forams
• Early suggestions of photosymbiosis in living forams (1880s — 1950s)
• Lee et al. (1965) established first unequivocal evidence for photosymbiosis in living forams
• Photosymbionts now confirmed in 12 extant familieso Symbionts include diatoms, dinoflagellates, unicellular
green algae, unicellular red algae and cyanobacteria
Photosymbionts in live foram and coral
National Geographic
image courtesy of Pam Hallock
foram with photosymbionts
image courtesy of Pam Hallock
Symbionts cultured from live foram
image courtesy of Scott Fay & Jere Lipps
20 µm
Benefits of photosymbiosis
• Energyo Mixotrophic nutrition (feeding &
photosynthesis)• Calcification
o ATP energy for concentrating inorganic carbon; removal of ions that inhibit calcification
• Removal of host metabolites by symbionts
Characteristics of modern, symbiont-bearing forams
• Preference for tropical, oligotrophic habitatso Stable environment; protected from continental and
seasonal influences• Unique life history strategy
o Large size (delayed reproductive maturity)o Production of few, large embryons with low mortality
Giant embryons?
Fusulinid withspherical adult shelland elongate interior
Oxygen & size
• Availability of oxygen constrains maximum cell size
Surface Volume2/3
As the linear dimensions of an object increase by a factor of X, itssurface area increases by X2 while its volume increases by X3
Radius = 1Surface = 12.6Volume = 4.2
Radius = 2Surface = 50.3Volume = 33.5
Four-fold increase in surfaceEight-fold increase in volume
× 2
Late Paleozoic hyperoxia
Oxygen & size
size increase associatedwith equally dramaticincrease in atmosphericoxygen
p = 0.0002r2 = 0.41
Linear regression analysis
Conclusions
• Fusulinid size evolution was mostly a driven response to photosymbiosis (selection for large size)
• BUT, a significant part of the trend was passive size increase in response to increasing oxygen availability (increase in the upper bound to cell size)
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