The criteria used to define a species · What is a species? o "No one definition has satisfied all naturalists; yet every naturalist knows vaguely what he means when he speaks of a species" C Darwin, The Origin of Species o Difficult to define, because species change due to evolutionary processes and may give rise to new species. o Speciation is a process not an event o Features that might define a species: " Reproductive isolation from other species (inability to produce fertile offspring between member of 2 different species) " Species form morphological and/or genetic clusters that are distinct from other such clusters Members of a species share a common ancestor " Group that is adapted to a particular ecological niche · The Biological Species Concept (Ernst Mayr) o A species is a group of actually or potentially interbreeding natural populations that are reproductively isolated from other such groups. o Problems with this concept: · Difficult to test for reproductive isolation · Cannot apply to asexual organisms Cannot be tested in extinct groups · The Genotypic Cluster Species Concept (James Mallet) o A species is a [morphologically or genetically] distinguishable group of individuals that has few or no intermediates when in contact with other such clusters. " The figure below shows there is variation w/in each of the butterfly groups, but this variation doesn't overlap between species, and there are gaps between the ranges of phenotypes and genotypes of the different groups. This concept can be more easily measured · How do new species form? o Speciation requires the evolution of reproductive isolation " Pre-zygotic (before fertilisation): e.g. behavioural isolation (no mating), incompatibility of reproductive organs · Post-zygotic (after fertilisation): some type of genetic incompatibility e.g. mules · The evolution of reproductive isolation o The figure below shows a species over time. An event splits the population in two (could be a physical event/barrier, or an ecological process) o Over time, if these populations maintain this separation, mutations would accumulate in each population, and the populations will continue to be compatible, and potentially
interbreeding. However, eventually they become so different that they're no longer able to interbreed. At this point they would typically be referred to as different species (though the exact point is a matter of interpretation) A Mutations 1 B Time Genetic difference / Populations able to interbreed x Populations unable to interbreed Different types of speciation, and key processes leading to divergence · Species and speciation o New species form by splitting of existing species. · Cladogenesis (phylogeny): evolutionary splitting event where a parent species splits into two distinct species, forming a clade (ft speciation) · allopatric: geographically isolated, · parapatric: adjacent, · sympatric: without spatial separation (host, timetti) · In reality: a continuum between these geographic extremes o New species form by change over time · Anagenesis: cumulative change within lineage · Allopatric speciation o A parental species splits into 2 populations (A & B), and for whatever reason, there is a geographic barrier. These