SPECIES CONCEPT
powerpoint version
How does natural selection and geographic isolation lead to the formation of new species?DEFINITION OF SPECIES
Groups of actually or potentially interbreeding populations that are reproductively isolated from other such groups (Ernst Mayr)
HOW DO SPECIES ARISE??
Natural populations are made up of genetically different individuals.
Environments of natural populations differ.
Natural selection acts on different populations.
Results in different gene frequencies in different populations.
Geographic barriers to stop gene migration increase genetic differentiation.
Populations eventually cannot interbreed.
New species have been formed (or one diverged from central population)
NOTE-- This is a continuous process. We should expect to find all stages of the process within the real world (e.g.,Salix).
HOW ARE NEW SPECIES MAINTAINED ONCE GEOGRAPHIC BARRIERS DISSAPPEAR?
1. Ecological
separation-- same area, but on different types of sites sufficiently separated
that gene flow is very low.
2. Temporal separation-- same area, similar sites, but flower at different times (P. sabiniana and P. coulteri)
3. Mechanical separation-- same area, sites and flowering time, but pollen is not transferred from anther to stigma of different species (usually for insect pollinated plants)
4. Pollination incompatibility-- pollen can be transferred to from anther to stigma of different species, but prevented from germinating because reaction of stigma (or other parts of pistil) prevents pollen tube from reaching ovule.
5. Hybrid inviability-- pollen tube can grow and pollination occurs-- but gene combinations from different parents are not well enough coordinated to make a viable (functional) individual and the embryo dies.
6. Hybrid sterility-- hybrid (cross between different species) is viable, but formation of sex cells is impossible (chromosomes not similar to line up in pairs during meiosis)