Topic 7—B: Populations and Evolution- 3. Speciation and genetic drift Flashcards
What is speciation?
The development of a new species from an existing species
When does speciation occur?
- when populations of the same species become reproductively isolated
Definition of reproductive isolation
- changes in allele frequency causes change in phenotype which mean they can no longer interbreed to produce fertile offspring
When can reproductive isolation occur?
- when a physical barrier e.g. a flood or an earthquake, divides a population of species, causing some individuals to become separated from the main population
(Geographical isolation) - there is no gene flow (transfer of genes) between the two genes
Which can lead to allopatric speciation
Sympatric speciation
- speciation can also occur when a population becomes reproductively isolated without any physical separation
Allopatric speciation
- populations that are geographically separated will experience slightly different conditions
- e.g. 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
- e.g. 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 from fur length in this population, increasing the frequency of the allele for longer fur length
- allele frequencies will also change as mutations
Genetic drift may also affect the allele frequencies in one or both populations
Allopatric speciation
- over time this can lead to speciation
- 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 2 groups will have become separated species
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
- doesn’t involve geographical location
- it’s generally thought that its 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
Example of Sympatric speciation
- most eukaryotic organisms are diploid (they have 2 sets of homologous 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 equally to give fertile offspring so if a polyploid organisms 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
- If more common in plants than animals
Why does reproductive isolation occur?
- because changes in alleles, genotypes, and phenotypes prevent individuals with these changes from successfully breeding individuals without them
What do the 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 successsfully.
What can selection pressures change?
The allele frequencies of a population over time
- this is evolution by natural selection
What can evolution also occur due to?
Genetic drift
What does genetic drift mean?
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
How does genetic drift work?
- individuals within a population show variation in their genotypes
- by chance the allele for one genotype 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
What does genetic drift lead to?
Differences in allele frequencies between 2 isolated populations
If enough differences in allele frequencies build up over time, this could eventually lead to reproductive isolation and speciation
What works alongside eachother to drive evolution ?
Natural selction and genetic drift
Which process has a greater effect in smaller populations where chance has a greater influence?
Evolution by genetic drift
What do chance factors tend to do in larger populations
They tend to even out across the whole population
What is the diversity of life on earth today as a result of?
Speciation and evolutionary change over millions of years
- to start with there was one population of organisms
- the population was divided and the new populations evolved into separate species
- the new species were then divided again and the new populations evolved into more separate species
- this process has been repeated over a long period of time to create millions of new species
