Download Persistent directional changes in higher taxa spanning significant

Macro-evolutionaire trends toenemende complexiteit
Volgens McShea D.W. Door Dams J.R.M.
Large scale evolutionary trends
• “Persistent directional changes in higher taxa
spanning significant periods of geological time”
• Trends are large in scale
– In a large group of species, a clade, rather than a
single lineage,
– Relatively long span of time, enough time that they
incorporate a number of speciation events
• Trends in:
– Size
– Complexity
– ..
Driven or passive?
• Driven
– The mean increases on account of a force
that acts on lineages throughout the space in
which diversification occurs
• Passive
– No pervasive force
– “Diffusion within a structured design space.”
Fisher (1986)
 Boundary!
Driven or passive?
Driven or passive?
• Tests
– Minimum test
– Ancestor-descendant test
– Subclade test
Minimum test
• Passive process
– The distribution minimum ought to move to
the boundary and stay near it
• Driven process
– The minimum is expected to increase
significantly
Ancestor-descendant test
• If phylogeny is known
– A bias in the direction of change may be
detectable in a random sample of ancestordescandant comparisons
 Passive process
– Increases and decreases in the state variable
should occur about equally often
↔ Driven process
Subclade test
• A trend in which a clade’s
distribution becomes skewed
• The skewness of a subclade
drawn from the tail of that
distribution will tend to reflect a
local regime of constraints or
selective forces or both
• The clade as a whole will
reflect a global regime
Subclade test
• If:
– Parent distribution is skewed
– Mean skew of a sample of subclades drawn
from the tail is significantly positive
The system is probably driven
• Requires only two distributions
– An early or ancestral distribution and a
skewed terminal or descendant distribution
Complexity
Increasing complexity in evolution?
Complexity
• Historically
– Complexity should increase in evolution
– Rensh (1960)
• Complex organisms are mechanically more
efficient
– Waddington (1969)
• As diversity increases, niches become more
complex, and more complex niches are filled by
more complex organisms
– Saunders and Ho (1976)
• Component additions are more likely than
deletions, because additions are less likely to
disrupt normal function
Defining complexity
• Löfgren (1977); Papetin (1980, 1982)
– The length of the shortest complete
description of the system
• Kolmogorov(1965)
– The length of the shortest algorithm that will
generate it
 No broad definition that is both
operational and universal
A narrower view
The complexity of a system is some
increasing function of the number of
different types of parts or interactions it
has
• Purely structural
– Complexity depends only on a number of
different parts and interactions
– Not on their functionality
Complexity
• Hierarchical ↔
nonhierachical
structure
– Hierarchical object
complexity is the
number of levels of
nestedness of parts
within wholes
Complexity
• Object ↔ process
– Objest complexity
refers to the number of
different physical parts
in a system
– Process complexity
refers to the number of
different interactions
among them
Complexity
• Only morphology will be
considered under the heading of
object complexity
• Only development will be
considered under process
complexity
• Nonhierarchival Morphological Complexity
– The number of different elements it contains
• Nonhierarchival Developmental
Complexity
– The number of independant interactions, or
factors, controlling form
• Hierarchical Morphological Complexity
– The number of levels of nesting of parts within
wholes
• Hierarchical Developmental Complexity
– The number of links or levels in the causal
chain, or the average number where causal
nodes are disjunct
Causes
•
A: Driven – no boundary – strong
bias
•
B: Passive – Lower boundary –
No bias
•
C: Weakly driven - No boundary –
Weak bias
•
D: No trend – Upper and lower
boundary – No bias
•
E: Driven (at the large scale) – No
boundary – Strong bias (but
invoked only occasionally)
•
F: No trend – No boundary- No
Bias
Limits on complexity
• Selection might oppose greater complexity
when added parts begin to interfere with
proper function
• Increase might be limited if highly complex
systems are replaced by more simpler
ones
• Overly connected systems might tend to
behave chaotically and thus to be unstable
What is complexity?
• The complexity of a system is an increasing
function of the number of its parts or
interactions
Internal variance
• As the parts of an organism accumulate
variations in evolution, they should tend to
become more different from each other
The variance among the parts (internal
variance) will tend to increase spontaneously
• Internal variance = complexity
Why should modern organisms be
more complex than ancient ones?
• Possibilities
– Natural selection has favored complexity
along with functionality
• Functional improvement maybe required more part
types
– Evolutionary increases in body size demand
greater complexity for functional reasons
Why should modern organisms be
more complex than ancient ones?
“Organisms are expected to accumulate
variations spontaneously as they evolve,
with the result that their internal parts
become more differentiated”
McShea (2004)
Internal variance is an aspect of
complexity
• The internal-variance principle generates a
vector in evolution toward increasing
complexity
– The vector is a generative tendency, or a bias
in the production of variants
– Selection could reinforce the internal-variance
vector, act neutrally, or oppose it
The internal-Variance Principle
• Initially all parts are
identical
• In the time steps, random
heritable variation is added
to each part, so that its
length increases or
decreases by the same
factor
• Internal variation is
measured as the standard
deviation among loglenghts
Large-scale trend
= A long-term directional
change
• Mechanism
– The pattern of change
in the variable in
question
• Cause
– Is the drive or
boundary the result of
selection or of
constraint?
Large-scale trends of the first kind
• Passive with selection as the underlying
cause
• Increases and decreases occur equally
frequently
• Selective lower boundary
Large-scale trends of the second
kind
• Passive with constraint as underlying
cause
• Eukaryotes
Large-scale trends of the third kind
• Driven with selection as the driving foce
Large-scale trends of the fourth
kind
• Driven, so that increases occur more often
than decreases among lineages, with
constraint producing the upward tendency
• Discussion
– Its seems to justify interpreting certain
evolutionary results as maladaptive
– Loss of larval swimming ability
Side notes
• Parasites tend to lose complexity
• Simpler species have more ecological
opportunities
Complexity a trend?