University:
Cameron University
Course:
1004 | General Biology
Academic year:

2019
Views:

155

Pages:

1
Author:

SpaceCadet101

SPECIATIOn Reproductive Isolation Isolation Barriers Reproductive isolation occurs when barriers prevent two populations from interbreeding (gene pools kept separate) • Without gene flow, the gradual accumulation of genetic differences (mutations) will eventually lead to speciation Examples of pre-zygotic isolation barriers include: There are two categories of reproductive isolation barriers: • Pre-zygotic barriers (no offspring are produced) • Post-zygotic barriers (offspring are not viable or infertile) Prezygotic Barrier Postzygotic Barrier Temporal Isolation • Populations have distinct / separate reproductive cycles • E.g. Leopard and wood frogs mate at different times Behavioural Isolation • Populations exhibit or respond to specific courtships • E.g. Different cricket species have distinct mating calls Geographic Isolation • Populations occupy different habitats / niches in an area • E.g. Lions and tigers don’t often interact within a region Types of Speciation Allopatric Speciation • Occurs when geographic barriers isolate populations • The physically separated populations are exposed to different environmental conditions and begin to diverge Sympatric Speciation • Occurs when populations diverge within a shared location (i.e. the populations are not physically separated) • Reproductive isolation leads to genetic divergence Polyploidy Pace of Speciation Sympatric speciation may be caused by polyploidy • A failure to undergo cytokinesis during meiosis results in gametes with additional sets of chromosomes (not haploid) Speciation may occur by one of two alternative models: Polyploidy is more common in plant species (e.g. Allium) that can self-pollinate or reproduce asexually (vegetative propagation) Zygote Parent Meiotic error 2n = 6 Diploid Self-fertilise 4n = 12 Tetraploid Phyletic Gradualism • Speciation occurs at a constant pace over a period of time • Involves a continuous / gradual accumulation of mutations • Theory is supported by the presence of intermediate fossils (e.g. evolution of the modern horse hoof) Punctuated Equilibrium • Speciation occurs in rapid bursts with periods of stability • If conditions are stable, characteristics are maintained • Environmental change promotes rapid divergence • Gaps in the fossil record provide support for this theory