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Unformatted text preview: Examples of unequal rates of morphological evolution Shrew Sengi- not shrews Afrotheria Tree From: http://www.peabody.yale.edu/exhibits/treeoflife/afrotheria.html Cladograms can be rotated at any node without changing the relationships between sister taxa. What matters is which lineages (taxa) descend from which particular nodes. For any speciation event on a phylogeny, the choice of which lineage goes to the right or left is arbitrary. Is this tree correct? Which of these trees is not like the other two? A B 1 C A B 2 C B C 3 A Macroevolution `Creeps and Jerks in the fossil record' Can microevolutionary processes explain macroevolutionary patterns?
s What are macroevolutionary patterns?
x Broad-scale patterns of change, diversification and extinction in the fossil record
3 Patterns of change over time Anagenesis- A 3 Patterns of diversification Cladogenesis- B 3 Crossing the big boundaries origin and consequences of new body plans A B What are macroevolutionary patterns?
s 1. Gradualism x Classic Darwinian view x Generally slow, constant change x Distinction among fossil species fairly arbitrary x Appeal to sketchiness of fossil record to explain gaps x Does not claim that all change will be gradual, only that this is a predominant pattern Gradualism What are macroevolutionary patterns?
s 2. Punctuated equilibrium x Emphasize periods of stasis interspersed with periods of "rapid" change during speciation (geologically speaking) x "Stasis is data" Stephen J. Gould Punctuated equilibrium What is "rapid" change?
s Origin of new species and characteristics over a time period that is short relative to the period of stasis. s Does not say that speciation/change are instantaneous - merely that they happen too quickly to be generally captured in the fossil record.
x Depending on resolution of fossil record, may still be a million years! Can microevolutionary processes explain macroevolutionary patterns?
s Punctuated equilibrium:
x Emphasizes that most change occurs at speciation3 Why would this be so? 3 Why don't we see the change in the fossil record?
x Ex: Allopatric speciation in peripheral isolates Allopatric speciation in peripheral isolates
s Punctuated equilibrium: s Small isolated population (isolates) is more likely to change to new species than large populations s Border populations (peripheral) probably have different gene pool than parent B B A B T I M E A A What is stasis?
s Long periods of little evolutionary change
x E.g. Coelacanths, Horseshoe crabs, Amphioxus, gingko, etc. (cockroach?) x "Living fossils" are pretty common What causes stasis?
s Not always clear. Probably includes x Stabilizing selection keeping the species from changing x Variable directional selection that keeps the species fluctuating around a mean x Genetic/Developmental constraints (pp. 498500)
3 Retention (maintenance/preservation) of primitive features in the absence of appropriate variation or directional selection. Mosaic Evolution in Monotremes What causes rapid diversification?
x May allow or s 1. promote evolution of new characteristics
3 E.g. Cambrian explosion Section 27.3 What causes rapid diversification?
s E.g.: Cambrian explosion
x "Sudden" appearance of every animal phyla within about 50 million years
3 Actually, diversification probably began much earlier, judging from molecular data 3 "Explosion" represents the appearance in the fossil record of many divergent phyla What causes rapid diversification?
x Why the sudden appearance? Factors suggested include:
3 Increased O2 levels supporting larger body size What causes rapid diversification?
s 2. Ecological opportunity - Extrinsic factors
x Provide opportunities to occupy previously unavailable niches (place/position/slot/function/role) Adaptive radiation What causes rapid diversification?
s E.g. Radiation of Mammals
x Pretty ordinary for 140 million years x Dinosaurs, the dominant land vertebrates, were wiped out by a meteor impact What causes rapid diversification?
x Suddenly, many new s E.g. Radiation of Mammals ecological opportunities for mammals x Rapid (< 20 million year) origin of most mammalian orders (e.g. bats, whale, etc.)
3 This was probably also a factor in the Cambrian "explosion" of Bilateria etc.,etc. What causes rapid diversification?
s E.g.Ecological opportunity Intrinsic (fundamental/basic/essential) factors
x Key innovations - Characteristics that open up new opportunities.
3 E.g. Flowers, wings, etc. 3 Novel characteristics Summary
s The fossil record shows both gradualism and punctuated equilibrium s Gradualism
x Emphasis on gradual diversification s Punctuated equilibrium
x Emphasis on divergence of peripheral isolates at speciation, followed by stasis x Mechanism not that different from gradualist view x Stasis maintained by stabilizing and fluctuating selection, and by developmental constraint Summary
s Large scale patterns of diversification can be facilitated by:
x Environmental change x Ecological opportunity - New niches open x Ecological opportunity - Key innovations ...
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