"PRINCIPLES OF PHYLOGENETICS: ECOLOGY AND EVOLUTION" Integrative Biology 200B Spring 2009 University of California, Berkeley B.D. Mishler March 31, 2009. "Speciation" (Diversification) As discussed previously (3/10), there are two fundamental kinds of things in evolutionary theory (Hull, Dawkins). Both of these form nested hierarchies. replicator = things of which copies are made -- a succession of replicators forms a lineage interactor = things that interact in a common environment such that replication is differential Four things can happen to lineages: 1. Origin 2. Extinction 3. Divergence 4. Reticulation For today, will focus on divergence, which leads to diversification of lineages. The total number of lineages in a clade is diversity. Constraints: Why is morphospace not filled in completely? Adaptive landscapes vs developmental landscapes Need to go back to "species concepts" a bit (see notes from 1/27/09) GENERAL CLASSES OF CAUSAL FACTORS ADVANCED TO EXPLAIN COHESION/INTEGRATION OF "SPECIES": 1) GENE FLOW 2) STABILIZING SELECTION -- ECOLOGICAL CONSTRAINTS 3) DEVELOPMENTAL CONSTRAINTS THE NULL HYPOTHESIS: RANDOM WALK; ACCIDENTS OF HISTORY Reproductive isolation: 1. Prezygotic: 1) habitat isolation, 2) temporal isolation, 3) behavioral isolation, 4) mechanical isolation, 5) gametic incompatibility Anything that prevents mating and fertilization is a prezygotic mechanism. Prezygotic mechanisms fall into two broad categories. Habitat isolation, that is, preferring different habitats, is likely to have evolved because of natural selection. In this case, reproductive isolation is a byproduct of changes occurring for other reasons. In other cases, characters associated with mating also are isolating mechanisms. Differences between lineages could evolve in order to prevent hybridization. In some cases, such as the time of mating, it is difficult to tell. Lineages could be active at different times of the day or breed at slightly different times of the year, because of food preferences or because selection favored reduced hybridization.2. Postzygotic: 1) hybrid inviability, 2) hybrid sterility, 3) hybrid breakdown Postzygotic barriers prevent the hybrid zygote from developing into a viable, fertile adult. Reduced viability or fertility of hybrid offspring all are known to be postzygotic isolating mechanisms. Lack of ability to survive in intermediate habits is an ecological barrier. Often differences in chromosome number or arrangement of genes on chromosomes result in genetic barriers. Geographic modes of diversification: 1. Allopatric: Diversification by geographically separated populations. Either a barrier can form, separating a single lineage into two isolated ones, or a lineage can colonize a new area. In both cases, gene flow is highly reduced. The formation of a barrier is sometimes called a vicariant event. How large a barrier has to be depends on dispersal capacity. A river may be a barrier for a snake but not a bird. There are two important subtypes of allopatic diversification: I. Initial population divided into two large halves (the "dumbbell" model) II. Initial population divided into one large half an one small, marginal half (the peripheral isolate model) 2. Sympatric: Diversification between lineages with overlapping geographic ranges In plants polyploidy can lead to immediate reproductive isolation. Chromosomal mutations in self-fertile plants can lead to instant speciation. The genus Clarkia provides several examples of allopolyploid and autopolyploid species. Many domesticated plants are polyploid, including oats, wheat, barley, potatoes, bananas, tobacco. It is likely that polyploid individuals were used early in the domestication of plants, both because they had unusual properties and because they were reproductively isolated from their wild relatives. Implications for studies of "speciation": The study of speciation involves an interplay between empirical research and concepts of species (the units of speciation). As emphasized before, the converse is true in that an understanding of processes impacting on species can influence species concepts for a group. Studies of pattern (i.e., phylogenetic reconstructions) and process (underlying causal mechanisms) are related to each other in a reciprocal manner (Hull 1988). We must ask not just what species are and how they are to be defined, but also how they came into being and how they are maintained. General evolutionary theories about species, their origin and nature, can (and should) be examined in such a manner. The process of studying divergence should always start with a cladogram down to as fine a level as is possible to resolve. The next step is to examine the characters changing on the branches (morphological, functional, geographic, reproductive, etc.), looking for common denominators (possible causes/constraints). How do we study adaptive radiations in a rank-free world view? A sampling problem: how many "basal" lineages are
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