Download L567 lecture 23

Lecture 23: MacroEvolution First some quotes. Nothing in
Biology makes sense except in the light of Evolution
(Dobzhansky) Nothing in Evolution makes sense except in
the light of Population Genetics (Lynch, IGERT talk 2006)
Nothing is Evolution makes sense except in the light of
_____________________ (fill in the blank).
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The following is from Coyne and Orr (1998) (see web
page). Some have argued that Speciation is a by-product
of conventional evolutionary forces, like selection and
drift… Thus, The origin of species is simply an
epiphenomenon of normal population genetic processes.
But Under the biological species concept (BSC), the origin
of species requires joint consideration of two species, and
usually an interaction between their genomes. The
distinctive feature of the genetics of speciation is therefore
epistasis
1. Necessarily true for post-zygotic isolation
2. Usually true for pre-zygotic isolation True? And
Is speciation a by-product? (And why does epistasis only
figure into conventional population genetics when it comes to
speciation? See M. Wade’s paper on a gene’s eye view of epistasis
and speciation)
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In the fossil record: can the patterns of rapid
morphological change (evolution), followed by long periods
of stasis be explained by conventional evolutionary forces?
Neo-Darwinian view
New view from
Gould & Eldridge
(plot
morphology against time for both views) (Remember Thomas
Huxley?) Quotes: 1. Is a new and more general theory of
evolution emerging? 2. Marco-evolution is effectively
decoupled from micro-evolution 3. NeoDarwinism is dead.
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Theory of macro-migration (from R. Dawkins) Macroevolution (species or clade selection) Remember the
equation for density-independent population growth?
dN = (births! deaths)N = rN
dt
dN = (b! d)N = rN
dt
What if b = number of new
species “born” And d = number of species “extinctions”
Could different clades have different values for r? Factors
affecting Speciation (birth) rate
Extinction
rate 1.
1. 2.
2. 3.
3. 4.
4.
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From Jablonski on larval ecology and macroevolution
Consider the following cases for marine invertebrates 1.
Planktotrophic larvae (i.e., they eat plankton)
many
small larvae produced (r selected) widely dispersed (low
isolation)
2. Non-planktotrophic larvae (they eat yolk)
few, large offspring produced (K selected) dispersal
is more local, leading to isolated pops. Expectations 1.
Clades with Planktotrophic larvae Large geographic range
Low speciation and extinction rates 2. Clades with
Non-planktotrophic larvae
Smaller geographic range
Higher speciation and extinction rates. Shifting
Balance?
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Note that the non-planktotrophs have (as expected): 1.
Higher extinction rates and 2. Smaller geographic ranges.
From: Extinction of geographic ranges of late cretaceous
gastropods (Jablonski, D. 1986. Bull. Mar. Sci. 39: 565-587)
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N=
Non-planktotrophs P= Planktotrophs Note that nonplanktotrophs have higher rates of speciation, independent
of trophic group. *per species per million years.
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“The causal mechanisms for the observed patterns…
probably lie at the hierarchical levels above the traditional
neodarwinian one of the individual organism. I am most
emphatically not invoking traditional group selection, but
species selection in the strict sense. Differences in genetic
population structure and geographic range – species level
traits that are not reducible to the organismic level – are
responsible for the observed patters.” From: Extinction of
geographic ranges of late cretaceous gastropods (Jablonski, D.
1986. Bull. Mar. Sci. 39: 565-587) What does this all mean?
Are lineages selected to speciate? _______ What it
macroevolution? ______________ Is it decoupled from
microevolution? ___________ What is the “currency” of
microevolution? _______ What is the “currency” of
macroevolution? _______
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Speaking of extinction… How do we explain the
strikingly constant rate of extinction in the above graphs?
(Data from van Valen 1973) Write answer here:
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From Coyne and Orr, page 288 (a) What is novel about speciation?
Some of our colleagues have suggested that speciation is not a
distinct field of study because --as a by-product of conventional
evolutionary forces like selection and drift -- the origin of species is
simply an epiphenomenon of normal population-genetic processes.
But even if speciation is an epiphenomenon, it does not follow that
the mathematics or genetics of speciation can be inferred from
traditional models of evolution in single lineages. Under the BSC,
the origin of species involves reproductive isolation, a character
that is unique because it requires the joint consideration of two
species, and usually an interaction between the genomes of two
species (Coyne 1994). The distinctive feature of the genetics of
speciation is, therefore, epistasis. This is necessarily true for all
forms of postzygotic isolation, in which an allele that yields a
normal phenotype in its own species causes hybrid inviability or
sterility on the genetic background of another (see below).
Epistasis also occurs in many forms of prezygotic isolation. Sexual
isolation, for example, usually requires the coevolution of male
traits and female preferences, so that the fitness of a male trait
depends on whether the choosing female is conspecific or
heterospecific. These complex interactions between the genomes of
two species guarantee that the mathematics of speciation will differ
from that describing evolutionary change within species, and
suggest that speciation may show emergent properties not seen in
traditional models.
From Wade 1922, page 338 Speciation theory is a good example
of how the epistasis can be minimized in microevolution and yet be
essential to macroevolution. Genetic epistasis causes a reduction in
fitness of interpopulation hybrids (macroevolution), yet it plays no
role at all in the microevolution within populations preceding the
speciation event (microevolution) (Charlesworth et al., 1987;
Coyne et al., 2000; cf. review by Johnson, 2000). The ‘meaningful’
simplicity of ‘beanbag genetics’ suffices for within-population theory
(see recent defence by Crow, 2001), but the beans become magic,
enchanted with epistasis, when it comes to explaining the origin of
species. Why so?
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