Cope’s Rule &

Passive versus Driven Trends• Key concepts

– Traditional explanations of Cope’s Rule commonly invoke “bigger is better” or the notion that new ecologic opportunities can be exploited by large organisms

– In fact, Cope’s Rule should be clarified to state that, more often than not, lineages evolve from a small size rather than toward a large size

• Most potential ancestors in a given clade are small

• Because small organisms are less specialized than large ones, they are more likely to give rise to a new clade

– Evolutionary trends can be either passive or driven (or some combination of both).

• As a clade diversifies passively the mean increases while the minimum is unchanged. In a driven trend both the mean and the minimum are expected to increase.

• A subclade from the tail of a right-skewed parent clade will be symmetrical if the parent clade has experienced a passive trend; the subclade itself will be right-skewed if the clade has experienced a driven trend.

Cope’s Rule &

Passive versus Driven Trends

• Key terms

– Cope’s Rule

– Right-skewed size distribution

– Similitude

– Passive and driven trends

– Minimum test

– Subclade test

Cope’s Rule

• Tendency in animal groups to evolve toward larger size

• First articulated in 1870s

• Size trends recognized in reptiles, mammals, arthropods, mollusks

Cope’s rule: Traditional explanations

• “Bigger is better:”

– Advantages associated with large size might

include:

• Improved ability to capture prey

• Improved ability to ward off predators

• Greater reproductive success

• Increased intelligence (with increased brain size)

• Larger size range of acceptable food

• Extended longevity

• Efficient body temperature regulation

Cope’s rule: Traditional explanations

• “Ecologic opportunity:”

– 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.

absolute

minimum

size

If extinction vacates organisms in a given

size class, others from adjacent size classes

might increase or decrease in size in order to fill the void There’s

always room

at the top

Increasing size

Cope’s rule: an alternative explanation

• For any evolving lineage there is an optimum body size for the niche being occupied…

• Whether body size increases or decreases over time depends on whether the mean size at the beginning of the lineage was smaller than or larger than the adaptive optimum…

• It turns out that most new lineages originate at a small size!

Steve Stanley

Alternative explanation for

Cope’s Rule (1 of 2)

• Typical size distribution within a higher taxon (e.g., mammals, birds)

• Size distributions are almost always right-skewed, meaning that most species are small and few species are very large

0 100 200 300 400 500 600

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Volume (mm3)

N = 268 species

mean size = 46 mm3

body size

num

ber

of specie

s

range midpoint

mean (average) size

median size (50th percentile)

Characteristics of a right-skewed

size distribution

body size

num

ber

of specie

srange midpoint

mean (average) size

median size (50th percentile)

In a right-skewed distribution,

more than half of the species

are smaller than the mean

size, and all but a few are

smaller than the range

midpoint!

Thus, if a new lineage

emerges from this clade, it is

more likely to originate from a

small ancestor than from a

large ancestor.

Characteristics of a right-skewed

size distribution

Alternative explanation for

Cope’s Rule (2 of 2)

• Within any higher taxon, specialization

(complexity) varies directly with body size

– Because of similitude, area-dependent processes

such as absorption of nutrients, respiration, muscle-force exertion and skeletal support are unable to keep

pace with mass as body size increases

– Therefore, surface area and cross-sectional features

must become more complex as body size increases

(a type of allometry)

– Larger species almost invariably are more complex

anatomically and physiologically than smaller species

Alternative explanations for

Cope’s Rule (2 of 2)

• Major adaptive breakthroughs, which might lead to the origin of a new lineage, are more likely to occur in small, unspecialized species.

– The structural/physiologic specialization of

large forms makes them unlikely to give rise

to dramatically different, new forms: it is

unlikely that elephants could give rise to an

entirely new order of mammals.

Cope’s Rule: Summary

• Cope’s Rule is best explained by the tendency of most groups to originate at small size relative to their optima

• This leads to a re-statement of Cope’s Rule:– Animal lineages tend

to evolve from a small starting size rather than toward a larger size

A diversifying clade in

which average size

increases over time. Much

of the size increase is

explained by the fact that

the clade originated with

an ancestor that was

smaller that the optimum

size for animals in this clade.(from Stanley 1973)

Evolution of

horses is

commonly cited

as an example

of Cope’s Rule

But, when examined phylogenetically,it turns out that certain subclades actuallyevolved to a smaller size than theirimmediate ancestor.

Different kinds of horses probably wereadapted to different niches withdifferent size optima.

Trend toward size increase in A, B, C

Trend toward size decrease in D–F

The nature of trends….

• Some trends might be mostly passive: e.g., diffusion away from a very small ancestor

• Other trends might be mostly driven: e.g., strong selection for a newly acquired trait

• How are passive and driven trends distinguished from one another???

Fusulinids

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 evolution

in fusulinids

McShea 1994 Evolution

Confining lower boundary;

increases and decreases

equally likely

Confining boundary;

increases more likely

than decreases (implies

selection for large size)

PASSIVE DRIVEN

“Passive” vs. “Driven” size trends

McShea 1994 Evolution

PASSIVE DRIVEN

In a diversifying clade, the

mean increases but the

minimum does not

In a diversifying clade, both

the mean and the minimum

increase

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 the

parent clade’s distribution

is right-skewed

Subclade from the tail of the

parent clade’s distribution

is not skewedExamine the sizedistribution in asubclade from theright tail of the parentclade’s distribution, farfrom the confining lowerlimit.

Fusulinid size distribution

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Volume (mm3)

N = 268 species

mean size = 46 mm3

parent clade

Five subclades: One from the right tail of the

parent clade is right-skewed. This suggests a

driven trend

Conclusion: size increase in fusulinids

was a driven trend

• Fusulinids are thought to have been “k-strategists:” They grew to large size in order to produce relatively few,

large offspring whose juvenile mortality was low (high

likelihood of survival)

• Certain other forams are “r-strategists:” They produce

many small offspring whose juvenile mortality is high (low likelihood of survival)