GEOG20009 Lecture Notes - Lecture 13: Allele Frequency, Extinction Event, Genetic Drift
15 May 2018
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LECTURE 13: SPECIATION AND EXTINCTION
• Species: a basic unit of biological classification and a taxonomic rank
• The fundamental unit of biogeography is the geographic range of a species
• Speciation: branching event, leading to the accumulation of distinguishing characteristics (traits) over time
• Divergence of an ancestral species into two or more daughter species requires genetic change
• Types of characteristics:
o Apomorphy: derived character(s) that uniquely identify a monophyletic lineage
o Pleisomorphy: primitive character traits
SPECIATION
• Adaptive Radiation: organisms diversify rapidly from an ancestral species into a multitude of new forms
o Particularly when a change in environment makes new resources/niches available, then colonisation of
new area, appearance of new resources, evolution of a new trait or extinction of species previously
using resources
o E.g. Darwin’s finches – evolved in fragmented landscapes – speciation in 2 parts: allopatric speciation,
then adaptive radiation (specialisation)
Species Emergence
• Cladogenetic speciation: main form of speciation
• Anagenesis: phyletic speciation
• Reticulate speciation: hybridisation
Genetic Change
Mutation
• Random changes in DNA
• Source of new alleles
• If beneficial: allele frequency increase
Genetic Drift
• Change in allele frequencies that occur entirely from change
• Largest effects in small, isolated populations
Geographic Component
• Is gene flow (migration, dispersal) high enough to overwhelm
natural selection (adaption)?
• Natural selection:
o Facilitated by geographic isolation – gene flow back to parent population
o Facilitates ecologic separation – local adaptation to different environments
▪ Different traits are important
o Experiments show that planting seeds from different elevations and planting in
the same environment shows natural selection reduced gene flow
Speciation Modes
• Gene flow is blocked by physical barriers – results in allopatric speciation
• Vicariance: usually results in multiple species diverging at the same time
• Dispersal: only one species diverging at the same time
• Allopatric speciation: uses both natural dispersal and human-aided dispersal
EXTINCTION
• Important for opening of new resources and habitats for speciation to happen
• Background extinction: continuously operating low rate of extinction
• Mass extinction: high rate of extinction, short period, affect many lineages simultaneously
o Few possible causes of mass extinctions – often a combination of factors
o Causes: asteroid, volcanic activity, rapid global cooling (ice ages), falling sea levels
Natural Selection
• Non-random processes through which biological traits
become common in a population
• Traits alter interactions with environment to enhance
survival and reproduction
Gene Flow
• Movement of alleles within a population or between
populations caused by dispersal of gametes or offspring
• Acts counter to genetic drift and natural selection and
impede genetic divergence
Document Summary
Lecture 13: speciation and extinction: divergence of an ancestral species into two or more daughter species requires genetic change. Species: a basic unit of biological classification and a taxonomic rank. The fundamental unit of biogeography is the geographic range of a species. Speciation: branching event, leading to the accumulation of distinguishing characteristics (traits) over time. Types of characteristics: apomorphy: derived character(s) that uniquely identify a monophyletic lineage, pleisomorphy: primitive character traits. Darwin"s finches evolved in fragmented landscapes speciation in 2 parts: allopatric speciation, then adaptive radiation (specialisation) Species emergence: anagenesis: phyletic speciation, reticulate speciation: hybridisation. Change in allele frequencies that occur entirely from change. Natural selection: non-random processes through which biological traits become common in a population. Traits alter interactions with environment to enhance survival and reproduction. Gene flow: movement of alleles within a population or between populations caused by dispersal of gametes or offspring, acts counter to genetic drift and natural selection and impede genetic divergence, natural selection:
