5/31/17. Week 10; Monday MEMORIAL DAY NO CLASS. Page 88

Transcription

1 Week 10; Monday MEMORIAL DAY NO CLASS Page 88

2 Week 10; Wednesday Announcements: Family ID final in lab Today Final exam next Tuesday at 8:30 am here Lecture: Species concepts & Speciation. What are species? Species are historically considered to be the basic unit of biodiversity, but what does this mean? Ever since Darwin, Biologists have been debating the nature of species, even debating the reality of species. Darwin was uncertain about whether species represented anything qualitatively different in nature Species also is a taxonomic rank in our classification system. Speciation The evolution of a new species from an existing species. Well now, I guess that will depend on what your concept of a species is To say that volumes have been written on the topic of speciation would be a gross understatement. Species concepts through time A species is what a competent taxonomist says it is (quote attributed to Cronquist) Typological species concept - Aristotle - Linneaus - pre-darwin - species were created in a perfect form - variation = environmental imperfection - "type" specimen and "type" species remain in taxonomy today. Speciation in a typological species concept entails special creation! Morphological species - post-darwin -species concept based entirely on morphology, including variation - the range of variation is measured and species boundaries are defined by gaps or morphological discontinuities. - variation is genetic and environmental; there is no perfect type - Numerical taxonomy is one manifestation of this concept. - The so-called evolutionary species concept is an outgrowth of this concept Page 89

3 Speciation in a morphological species concept entails: - Emphasis on phenotype (hence morphological ) and evolution of morphological novelty - Accumulation of differences over time between two populations, usually in allopatry - Can be a gradual or sudden process. Patterns of Speciation inferred from the fossil record: A B C B time time C A B phenotype A phenotype Gradual (Anagenesis) Punctuated (Cladogenesis) Process-dependent - species concepts - mid-20th century. These are derived from notions of what processes are assumed to be responsible for speciation - Biological species concept: reproductive isolation - this implies that selection leads to reproductive isolation - Mate-Recognition concept: mating specificity - this implies that selection favors proper mate selection (isolation is a side-effect) Speciation in a process-dependent species concepts entails: - Evolution of reproductive isolation - Mechanisms of reproductive isolation either pre- or postmating (pre- and postzygotic) - These can involve either rejection of unlike individuals or recognition of like individuals for mates. From our discussion of phylogeny, it is apparent that in most cases speciation is accompanied by phenotypic innovation (apomorphies), yet many discussions of speciation dwell not on the evolution of phenotypic innovation, but on the evolution of reproductive OR on the branching of lineages whether there is phenotypic change or not. This is part of the reason why the debate about speciation has been so controversial over the years! Page 90

4 Reproductive Isolation Speciation requires that gene flow, or exchange of genes through sexual reproduction, between ancestor-descendant lineages of organisms have ceased, or be extremely small, prior to the origin of a new species. The term reproductive isolation often is used in two different ways leading to confusion: 1) A history of no prior crossing or gene flow (little r reproductive isolation) this could be simply a consequence of geography or other physical isolation. This is necessary for speciation to occur, but not always sufficient by itself 2) An evolved mechanism to prevent crossing (Big R Reproductive isolation) this is usually attributed to selection acting to create this mechanism to prevent gene flow, but also could be a non-selective consequence of divergent evolution resulting from #1. This is often considered sufficient for speciation to occur, but not always necessary. Does this mean that the fact of two lineages being non-interbreeding is sufficient to say that they are species, even if no evident change has occurred? Sibling species - result when reproductive isolation occurs without any phenotypic innovation. These would only be considered species, IF you hold that reproductive isolation is the defining basis for speciation. Examples: 1) Behavioral isolation in Hawaiian Drosophila 2) autopolyploidy in plants (some plant species have 2n, 4n, 6n, 8n, 16n, etc.) Phylogenetic species concepts s-1990's - species are identified by derived traits shared by organisms - assumes that evidence of evolutionary novelty or phylogenetic divergence is sufficient to define speciation having occurred - evidence of a phylogenetic split (for example, DNA evidence only) is sufficient to denote speciation Speciation in a phylogenetic species concept entails: - Emphasis on history of divergence in phylogenetic context - In theory, this could mean historical divergence without phenotypic divergence, but in practice advocates having traits (synapomorphies) that diagnose species Page 91

5 Phenotypic Innovation When there is no fossil record, speciation denotes the branch points in a phylogeny: A B C D E Time Speciation 'events' Does this mean that the fact of two lineages having diverged is sufficient to say that they are species, even if no phenotypic change has occurred? Mechanisms of Speciation are usually conceived to be either: Gradual - accumulation of genetic differences - selection for reproductive isolation through reduced fertility of hybrids, or mechanisms that prevent crossing (for example, pollinator specificity) OR Sudden: - polyploidy - hybridization accompanied by chromosomal or ploidal change - "quantum" or "macromutational" change hopeful monster of Goldschmidt, perhaps including homeotic mutations transform one organ into another (e.g., loss of one pair of wings in insects) The evolution of a new species rarely occurs by splitting into two fully formed monophyletic species, but is a process that occurs over time as populations go extinct, migration and gene flow occur between populations, and coalescent processes work to make each species monophyletic. Species A gives rise to a new, isolated form called species B Page 92

6 Processes such as extinction and migration start to homogenize species A Eventually each species becomes monophyletic How does speciation occur? Two terms often used in discussions of speciation and reproductive isolation: Allopatric speciation reproductive isolation occurring in separate geographic distributions Sympatric speciation reproductive isolation arises while occupying the same or overlapping geographic distributions -- This is perceived to be rare and some say it does not occur. It requires some mechanism to prevent reproduction between individuals in close proximity. Conventional (Selectionist) view: A population in allopatry is subject to a different environment. Directional selection drives the evolution of morphological novelty as an adaptation to the new Page 93

7 environment. At the same time the new species evolves differences that make it less compatible with its progenitor and therefore is reproductively isolated. Reinforcement, through evolution of physical or genetic barriers to crossing Reproductive isolation, may occur if the two species come back into contact. Alternative (Non-selectionist) view: Release-from-selection. In this scenario, a population, most likely in allopatry, is subjected to a relaxation in selection by virtue of being in a new environment where some new resource is abundantly available, or some new niche space is available. In this setting, phenotypic variability is greatly expanded and new phenotypes at the extreme of the range of variability survive and reproduce that normally would not. If one of these is turns out to be better adapted to the new environment, then selection will rapidly fix the new phenotype. This often is accompanied by a phenomenon called the founder effect, which results in divergent phenotypes due to the chance loss of much genetic variability in the small founding population. - If the new environment has many available resources/niches, or if the evolutionary novelty allows exploitation of previously unused resources, then many new forms may arise in a short period of time - Phylogenetic Radiation - Examples: - oceanic islands, such as Hawaii - new adaptive zones, such as bats - Key innovation is a concept that matches a novel trait with a sudden rapid radiation of species (example: nectar spurs on flowers). Where does the debate stand today? Continuing debate centers around these two pairs of questions: 1) Are species things that exist in nature (resulting from the process of evolution)? OR Are species human constructs (arbitrary units to be defined based on consistent criteria by those who study them)? 2) Are species evolutionary lineages historically independent of other lineages? OR Are species cohesive groups of organisms held together by some process? These two philosophies derive from two very different ways of viewing the relationship between nature and the role of taxonomy. What do you think? Page 94