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 ancestor-descandant 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

ComplexityIncreasing 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 log-lenghts

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?