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Evolutionary Biology
Speciation 4
| Question | Answer |
|---|---|
| How does diversification occur? | To try and understand reasons why some species speciate rapidly and others do not - we should try and understand how species diversify |
| Phyletic gradualism | Speciation events are not associated with increased rates of evolutionary change within a lineage |
| The punctuated equilibrium model | They observed that the fossil record gives a different picture for the evolution They claim that there were long periods of stasis (4-10 million years) involving little evolutionary change Then occasional rapid formation of new species |
| Punctuated evolution | All the character change is directly associated with cladogenesis |
| Gradual evolution | All the character change is within lineages (anagenesis) |
| Three major hypotheses for stasis: (1) Internal genetic or developmental constraints | - would require either no genetic variation, or strong correlated evolution between characters |
| Three major hypotheses for stasis: (2) Stabilising selection for a constant optimum phenotype | a variable optimum in a changing environment? E.g. Pleistocene glaciations? |
| Three major hypotheses for stasis: (3) Ephemeral local divergence | - adaptational shifts do occur in local populations, but do not spread throughout the species range, and disappear through local population extinction or introgression of the initial morphological form back into the main population |
| Measuring speciation and diversification rates | Time for speciation - the time required for reproductive isolation to evolve after a split Biological speciation interval - the average time between speciation events |
| Speciation rates | The diversity we see for any taxonomic group is the difference between speciation and extinction Extinction rate is an important metric Extinction will result in the underestimation of speciation rates |
| Using phylogenetic trees to investigate diversification | Speciation intervals Patterns of clade richness Temporal changes in diversification |
| Phylogenies and studying speciation rates | Phylogenies can provide two sources of information relevant to the study of diversification rates: - the topological distribution of species diversity across branches tree shape - the temporal distribution of branching events through time |
| Topological methods - Tree shape | Trees with equal numbers of species in each clade are said to be balanced Trees with large differences in the number of species in clades are unbalanced |
| Topological methods - Tree shape | Topological methods compare the observed difference in species diversity between two (or more) groups descended from a common node to a distribution of diversity differences generated under a stochastic model of diversification |
| Temporal methods - timing | Adaptive radiations would be expected to lead to high rates of diversification (probably high rates of speciation) early, and declining diversification through time |
| Temporal methods - timing | Temporal methods compare the distribution of branching times (based on branch length/duration estimates) to that generated under a null model of random diversification |
| Temporal variation in diversification | Temporal variation in diversification that effects an entire clade could be attributed to several factors |
| Temporal variation in diversification | Adaptive radiations would be expected to lead to high rates of diversification (probably high rates of speciation) early, and declining diversification through time |
| Lineage Through Time plots - an apparent increase in speciation rate towards the present, is found in the Ellipsoptera group of tiger beetles. This can be caused by: | 1) an actual increase in the speciation rate, or (2) an illusory increase caused by a constant background extinction rate, d, that is high relative to the (constant) speciation rate, b |
| Traits that increase diversification rates | No evidence that sexual selection promoted divergence in female plumage traits, or in male traits related to foraging and locomotion |
| Traits that increase diversification rates | Strong evidence that female choice and male–male competition are dominant mechanisms driving divergence during speciation in birds, potentially linking sexual selection to the accelerated evolution of pre-mating reproductive isolation |
| What determines species richness? | Across all clades and taxonomic ranks no cases where species richness was explained by clade age Many comparisons showed a significant negative relationship between richness and clade ages (kingdom, phylum, class) |
| What determines species richness? | Variation in diversification rates explained species richness among clades in many groups and ranks |
| Is speciation rate related to phenotypic change? | Some groups e.g. African cichlids, have rapidly diversified into a vast number of species and functional forms |
| Is speciation rate related to phenotypic change? | This exceptional diversity contrasts with the many groups that contain few species and/or harbour comparatively little phenotypic or ecological diversity |
| Diversification rates across vertebrates - authors ask two general questions about the patterns of diversification across the vertebrate tree of life: | (i) What is the background tempo of vertebrate diversification; and (ii) which, if any, vertebrate lineages have patterns of extant richness that are too species-rich or -poor to be outcomes of the background diversification rate? |
Created by:
reub8n
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