Speciation + Genetic Drift

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Front (Term)

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Define – speciation?

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The development of a new species from and an existing species

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When does speciation occur?

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When populations of the same species become reproductively isolated

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At what point are two populations of the same species no longer the same species?

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When they can no longer breed to produce fertile offspring

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Reproductive isolation can occur when a physical barrier divides a population of a species or without physical separation

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Define - Allopatric speciation?

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Speciation that occurs when:

• Geographical isolation - pop. divided by physical barrier (e.g. flood, earthquake)

• Causes Reproductive isolation

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Explain how Allopatric speciation works?

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• Geographically separated populations experience slightly different conditions (e.g. different climate)

• The populations experience different selection pressures so different changes in allele frequencies could occur: different alleles selected by natural selection, different random mutations, genetic drift in one or both pops.

• Over time, this leads to speciation. The changes in allele frequency will lead to differences accumulating in the gene pools of the separated populations, causing changes in phenotype frequencies. Eventually individuals will have changed so much that they can’t breed with each other.

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Define - sympatric speciation?

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Speciation that occurs when:

• no geographical isolation (in the same place)

• mutations prevent some individuals breeding

• these individuals are reproductively isolated from the rest of the population without being geographically isolated

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Give some mechanisms of reproductive isolation?

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• morphological - mutation changes the penis shape - doesn’t fit the vagina

• individuals become sexually active at different times of the year - different flowering/mating seasons

• Behavioural changes - A group of individuals may develop courtship rituals that aren’t attractive to the rest of the species. Even though they can breed, they won’t.

• random mutations can occur which increase the number of chromosomes (polyploidy) - a polyploidy individual can’t reproduce sexually with diploid individuals - reproductively isolated. For speciation to occur more polyploidy organisms have to be produced - a polyploidy individual would have to reproduce asexually or find a mate of the opposite species with polyploidy

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Why does sympatric speciation occur more often in plants than animals?

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• Often caused by polyploidy (a mutation increases the number of chromosomes)

• For the emergence of polyploidy organisms to lead to speciation - polyploidy individuals must reproduce

• Plants can just reproduce asexually, whereas animals would have to find another polyploidy individual of the opposite sex (which is unlikely)

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Why does sympatric speciation occur more often in plants than animals?

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• Often caused by polyploidy (a mutation increases the number of chromosomes)

• For the emergence of polyploidy organisms to lead to speciation - polyploidy individuals must reproduce

• Plants can just reproduce asexually, whereas animals would have to find another polyploidy individual of the opposite sex (which is unlikely)

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Why is sympatric speciation pretty rare?

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It’s difficult for a section of the population to become completely reproductively isolated from the rest of the population without being geographically isolated too

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Evolution can occur via 2 pathways. What are they?

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• Evolution via natural selection - environmental factors change the allele frequencies

• Evolution via genetic drift - chance dictates how the allele frequencies change (which alleles passed on)

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Describe how genetic drift works?

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• Individuals within a population show variation in their genotypes

• By chance, one allele for a gene is passed on to more offspring than the other alleles

• the number of individuals with the allele increases

• If by chance the same allele is passed on more often again and again, it can lead to evolution, as the allele becomes more common in the population

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Natural selection - phenotypes become more common because they increase an organism’s chance of survival

Genetic drift - phenotypes become more common by chance

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When does evolution by genetic drift have a huge effect? Why?

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• Greater affect on smaller populations where chance has a greater influence

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The effect of genetic drift on evolution increases in smaller populations. Why does the effect of genetic drift decrease in large populations?

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Any chance factors tend to even out across the whole population

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Natural selection and genetic drift work alongside each other to drive evolution.

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Genetic drift can be dangerous for small populations. Explain why?

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• Genetic drift tends to cause the genetic diversity of a population to decrease

• Lack of genetic diversity may make species less able to adapt to future changes in their environment