2Speciation By the end of this lesson you should be able to: Explain what a species isKnow what is meant by a gene poolKnow what happens during speciationName the 6 stages leading to the development of anew speciesName 3 different isolating mechanismsKnow how the 3 sub-species of European wren haveevolved
3IntroductionSpeciation is the evolutionary process leading to the formation of a new species.Populations first need to be isolated from each other so that gene pools can diverge.The isolated populations develop different characteristics & eventually will no longer be able to breed with each other.
4Species What is a species? Read page 130 of Torrance if you can’t remember!
5What is meant by a Gene Pool? The total of all the different genes in a population is the gene pool.
6What is meant by a Gene Pool? As long as the population is large and mating is random then the gene pool is considered stable.Gene frequency is the frequency of occurrence of an allele of a gene in a population.
7How does a gene pool change? Any changes to gene frequency will change the gene pool.These include:MutationNatural selectionGene migrationNon-random matingGenetic driftChance
8How does a gene pool change? MutationNew alleles appear & are reshuffled during meiosis when chromosomes cross-over & are independently associated to each other.
9How does a gene pool change? Natural selectionThe frequency of an allele increases in a population if it provides a selective advantage.
10How does a gene pool change? Gene migrationThis is when alleles move between populations by individuals arriving from a different population & breeding.These individuals could have a different gene pool and so introduce new alleles into the population.
11How does a gene pool change? Gene migrationHowever, continual gene flow decreases the differences between the gene pools of populations.
12How does a gene pool change? Non-random matingThis increases the number of homozygous individuals (but does not change the frequency of the alleles)Inbreeding is a common form of this.In humans this can increase the chances of the offspring inheriting a disease caused by a homozygous recessive genotype (e.g. cystic fibrosis or sickle cell anaemia)
13How does a gene pool change? Genetic driftThis tends to occur in small populations- small populations isolated from each other can vary greatly from each other.
14How does a gene pool change? ChanceChanges to an allele frequency can occur due to random loss.e.g. an individual possessing a certain allele my die or fail to reproduce.
15How does a new species arise? Speciation is the formation of a new species.Evolution is the mechanism by which speciation occurs and involves changes to the genotype & phenotype.These changes enable the organism to better exploit the environment.
16Stages in Speciation There are 6 stages. Stage 1 A large interbreeding population of one species exists- sharing the same gene pool.
17Stages in Speciation Stage 2 A barrier separates the population into 2 isolated sub-populations.Barriers can be:geographicalecologicalreproductive
18Stages in SpeciationgeographicalThis canyon is isolated by mountains
19Stages in Speciation geographical Different terrains on neighbouring mountains
20Stages in Speciation geographical .geographicalWater creates a difficult barrier to cross
21Stages in Speciation.geographicalDeserts are hostile environments
22Stages in Speciation e.g. changes in temperature, humidity ecologicale.g. changes in temperature, humiditypH, salinity, altitude
23Stages in Speciation reproductive e.g. different populations are fertileat different times of the year.OR changes to sexual organsOR different courtship behaviourOR individuals no longer attracted to each otherOR pollination/fertilisation fails
24Stages in Speciation Stage 3 Random mutations in each sub-population give rise to new variation within each group (but not shared by both groups)
25Stages in Speciation Stage 4 Different selection pressures act on each sub-population depending on local conditions like:ClimatePredatorsDisease.
26Stages in Speciation Stage 4 Natural selection affects each sub-group in a different way, by favouring the alleles that allow the individuals to better exploit their environment.
27Stages in Speciation Stage 5 Over a very very long time…… Stages 3 & 4 cause the two gene pools to become so altered that the groups become genetically distinct & isolated.
28Stages in Speciation Stage 6 If the original barrier is removed, the two sub-populations are no longer able to interbreed (since their chromosomes are no longer able to form homologous pairs).There are now 2 separate distinct species.Speciation has occurred!
32Speciation Can you do it? Explain what a species is Know what is meant by a gene poolKnow what happens during speciationName the 6 stages leading to the development of anew speciesName 3 different isolating mechanismsKnow how the 3 sub-species of European wren haveevolved