Cope’s Rule & Passive versus Driven groves/CopesRuleandTrends.pdf Cope’s Rule &...

Click here to load reader

  • date post

    08-Jul-2020
  • Category

    Documents

  • view

    1
  • download

    0

Embed Size (px)

Transcript of Cope’s Rule & Passive versus Driven groves/CopesRuleandTrends.pdf Cope’s Rule &...

  • 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

    All

    0.0

    0.1

    0.2

    0.3

    0.4

    0.5

    0.6

    0.7

    0.8

    0.9

    0

    50

    100

    150

    200

    250

    fr e

    q u

    e n

    c y

    Volume (mm3)

    N = 268 species

    mean size = 46 mm3

  • body size

    n u m

    b e r

    o f s p e c ie

    s

    range midpoint

    mean (average) size

    median size (50th percentile)

    Characteristics of a right-skewed

    size distribution

  • body size

    n u m

    b e r

    o f s p e c ie

    s range 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 actually evolved to a smaller size than their immediate ancestor.

    Different kinds of horses probably were adapted to different niches with different 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 a driven 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 skewed Examine the size distribution in a subclade from the right tail of the parent clade’s distribution, far from the confining lower limit.

  • Fusulinid size distribution

    0 100 200 300 400 500 600

    All

    0.0

    0.1

    0.2

    0.3

    0.4

    0.5

    0.6

    0.7

    0.8

    0.9

    0

    50

    100

    150

    200

    250

    fr e

    q u

    e n

    c y

    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)