B3. Speciation and Genetic Drift Flashcards
What is speciation?
Speciation is the development of a ____ species from an ________species. It occurs when populations of the same species become _____________ ________-changes in _________ ___________cause changes in ___________, which mean they can no longer ______________to produce _________ _________.
Reproductive isolation can occur when a _________barrier, e.g. a flood or an earthquake, divides a population of a species, causing some individuals to become separated from the main population. This is known as _________________isolation. There is no _____ _____(transfer of genes) between the two populations, which can lead to _____________ ____________.
Alternatively, speciation can also occur when a population becomes ____________ ___________without any _________separation. This is known as ______________speciation
Speciation is the development of a new species from an existing species. It occurs when populations of the same species become reproductively isolated -changes in allele frequency cause changes in phenotype, which mean they can no longer interbreed to produce fertile offspring.
Reproductive isolation can occur when a physical barrier, e.g. a flood or an earthquake, divides a population of a species, causing some individuals to become separated from the main population. This is known as geographical isolation. There is no gene flow (transfer of genes) between the two populations, which can lead to allopatric speciation.
Alternatively, speciation can also occur when a population becomes reproductively isolated without any physical separation. This is known as sympatric speciation
Allopatric speciation
Populations that are _____________ ____________will experience slightly different conditions. For example, there might be a different ________on each side of the physical barrier. The populations will experience different selection pressures and so different changes in allele frequencies could occur:
- __________ __________will be more ________________in the different ___________, so ___________ __________occurs. For example, if ____________separation places one population in a colder climate than before, longer fur length will be _____________. ______________ ___________will then act on the alleles for fur length in this population, _____________the ____________of the ________for longer fur length.
- _________ _____________will also change as mutations will occur
______________in each population.
- _________ ______may also affect the _______ ____________in one or both ______________.
Over time, this can lead to ___________. The changes in ________ ____________will lead to differences accumulating in the _____ _____of the separated populations, causing changes in ___________frequencies. Eventually, individuals from the different populations will have changed so much that they won’t be able to breed with one another to produce fertile offspring -they’ll have become reproductively isolated. The two groups will have become ________ _________
Populations that are geographically separated will experience slightly different conditions. For example, there might be a different climate on each side of the physical barrier. The populations will experience different selection pressures and so different changes in allele frequencies could occur:
- Different alleles will be more advantageous in the different populations, so natural selection occurs. For example, if geographical separation places one population in a colder climate than before, longer fur length will be beneficial. Directional selection will then act on the alleles for fur length in this population, increasing the frequency of the allele for longer fur length.
- Allele frequencies will also change as mutations will occur
independently in each population.
- Genetic drift may also affect the allele frequencies in one or both populations.
Over time, this can lead 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 from the different populations will have changed so much that they won’t be able to breed with one another to produce fertile offspring -they’ll have become reproductively isolated. The two groups will have become separate species
Figure 1: Diagram showing allopatric speciation.
Sympatric speciation
Sympatric speciation can occur when _________ __________within a __________prevent ___________that carry the _________from ____________with other members of the population that don’t carry the _________. It doesn’t involve _______________ ____________.
It’s generally thought that sympatric speciation is pretty ____, as it’s difficult for a section of a population to become completely reproductively isolated from the rest of the population without being geographically isolated too (as is the case with allopatric speciation).
Sympatric speciation can occur when random mutations within a population prevent individuals that carry the mutation from breeding with other members of the population that don’t carry the mutation. It doesn’t involve geographical isolation.
It’s generally thought that sympatric speciation is pretty rare, as it’s difficult for a section of a population to become completely reproductively isolated from the rest of the population without being geographically isolated too (as is the case with allopatric speciation).
Sympatric speciation - Example
Most eukaryotic organisms are diploid - they have two sets of homologous
(matched) chromosomes in their cells. Sometimes, mutations can occur that increase the number of chromosomes. This is known as polyploidy. Individuals with different numbers of chromosomes can’t reproduce sexually to give fertile offspring-so if a polyploid organism emerges in a diploid population, the polyploid organism will be reproductively isolated from the diploid organisms. If the polyploid organism then reproduces asexually, a new species could develop. Polyploidy can only lead to speciation if it doesn’t prove fatal to the organism and more polyploid organisms can be produced. It’s more common in plants than animals.
Most eukaryotic organisms are diploid - they have two sets of homologous
(matched) chromosomes in their cells. Sometimes, mutations can occur that increase the number of chromosomes. This is known as polyploidy. Individuals with different numbers of chromosomes can’t reproduce sexually to give fertile offspring-so if a polyploid organism emerges in a diploid population, the polyploid organism will be reproductively isolated from the diploid organisms. If the polyploid organism then reproduces asexually, a new species could develop. Polyploidy can only lead to speciation if it doesn’t prove fatal to the organism and more polyploid organisms can be produced. It’s more common in plants than animals.
Exam Tip
Make sure you’re clear on the difference between sympatric and allopatric speciation, and can remember which is which. If it helps, you could think of Sympatric speciation happening in the _____place, and Allopatric speciation occurring in populations that are ______ from each other.
Exam Tip
Make sure you’re clear on the difference between sympatric and allopatric speciation, and can remember which is which. If it helps, you could think of Sympatric speciation happening in the Same place, and Allopatric speciation occurring in populations that are Away from each other.
Tip: Polyploidy can be a mechanism of _______________isolation.
Tip: Polyploidy can be a mechanism of reproductive isolation.
Tip: ________________ ____________is necessary for sympatric or allopatric speciation to take place.
Tip: Reproductive isolation is necessary for sympatric or allopatric speciation to take place.
Mechanisms of reproductive isolation
Reproductive isolation occurs because changes in _______, __________, and ____________prevent individuals with these changes from successfully ______________with individuals without them. These changes include:
Reproductive isolation occurs because changes in alleles, genotypes, and phenotypes prevent individuals with these changes from successfully breeding with individuals without them. These changes include:
- Seasonal changes-individuals develop different flowering or mating seasons, or become sexually active at different times of the year. This means that they can’t breed together, as they aren’t reproductively active at the same time.
- Mechanical changes-changes in the size, shape or function of genitalia can prevent successful mating, preventing individuals from breeding
- Behavioural changes- a group of individuals may, for example, develop courtship rituals that aren’t attractive to the rest of the species, such as a change in song for birds. This prevents individuals from breeding with each other, even if they could do so successfully.
Tip: A species is a group of individual organisms that can _______ _________to _________ ____________ ____________
Tip: A species is a group of individual organisms that can breed together to produce fertile offspring
Tip: Don’t confuse geographical isolation with reproductive isolation. Populations that are geographically isolated are ____________separated, but may still be able to reproduce if brought back together. ________________isolation can lead to _____________isolation if _________ __________significantly changes the _________ _____________in the two separated populations.
Tip: Don’t confuse geographical isolation with reproductive isolation. Populations that are geographically isolated are physically separated, but may still be able to reproduce if brought back together. Geographical isolation can lead to reproductive isolation if natural selection significantly changes the allele frequencies in the two separated populations.
Tip: All sorts of behavioural changes can cause ____________ __________-eg, different groups may develop different preferences about where they breed, which could prevent them from breeding with each other.
Tip: All sorts of behavioural changes can cause reproductive isolation-eg, different groups may develop different preferences about where they breed, which could prevent them from breeding with each other.
Evolution via genetic drift
___________ ___________ can change the ___________ ___________ of a population over time. This is evolution by ________ ____________
Evolution also occurs due to genetic drift-this just means that
instead of environmental factors affecting which individuals _______, ______and _______on ______ ________, chance dictates which _________are passed on. For this reason, genetic drift is sometimes called _________drift.
Here’s how it works: (3 steps)
Genetic drift can lead to differences in ________ ___________ between two isolated populations. If enough differences in _______ __________build up over time, this could eventually lead to _____________ ___________ and ___________.
Selection pressures can change the allele frequencies of a population over time. This is evolution by natural selection
Evolution also occurs due to genetic drift-this just means that
instead of environmental factors affecting which individuals survive, breed and pass on their alleles, chance dictates which alleles are passed on. For this reason, genetic drift is sometimes called random drift.
- Individuals within a population show variation in their genotypes
(e.g. A and B, see Figure 3). - By chance, the allele for one genotype (B) is passed on to more offspring than the others. So 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.
Genetic drift can lead to differences in allele frequency between two isolated populations. If enough differences in allele frequency build up over time, this could eventually lead to reproductive isolation and speciation.
Figure 3: Diagram to show genetic drift in a population.
Evolution via genetic drift - Genetic drift and population size
Natural selection and genetic drift work alongside each other to drive evolution, but one process can drive evolution more than the other depending on the population size. Evolution by genetic drift usually has a greater effect in __________populations where chance has a __________influence. In __________populations any chance factors tend to even out across the whole population.
Natural selection and genetic drift work alongside each other to drive evolution, but one process can drive evolution more than the other depending on the population size. Evolution by genetic drift usually has a greater effect in smaller populations where chance has a greater influence. In larger populations any chance factors tend to even out across the whole population.
