Types of Speciation (HL) (HL IB Biology)

Types of speciation

• Evolution causes speciation: the formation of new species from pre-existing species over time
• There are two different situations in which speciation can take place:
  • Two populations of a species are geographically isolated; speciation that occurs as a result of this is known as allopatric speciation
  • Two populations of species are living in the same area; this type of speciation is known as sympatric speciation

Allopatric speciation

• Allopatric speciation occurs as a result of geographical isolation
  • It is the most common type of speciation
• Allopatric speciation occurs when populations of a species become separated from each other by geographical barriers
  • The barrier could be natural, e.g. a body of water or a mountain range
  • It can also be man-made, e.g. a motorway
• This creates two populations of the same species between which no gene flow is taking place
• Allele frequencies in the gene pools of the two populations may change in different ways due to 
  • Different selection pressures acting on them
  • The accumulation of random changes in allele frequencies, known as genetic drift
• Changing allele frequencies will lead to changes in the phenotypes of the two populations
• If enough allele frequency differences arise between the two populations then they will eventually be reproductively isolated, and can be said to be separate species

E.g. Allopatric speciation in trees

• A population of trees exists in a mountainous habitat
• A new mountain range forms that divides the species into two geographically isolated populations
• The geographical barrier prevents the two populations from interbreeding so there is no gene flow between them
• The two populations experience different environments, so differential selection occurs
• Different alleles are therefore more likely to be passed on in each population
• Different alleles become more frequent in each population
• Over thousands of years the divided populations form two distinct species that are reproductively isolated

Allopatric speciation diagram

The natural geographical barrier of a mountain range can lead to allopatric speciation in trees

Sympatric speciation

• Sympatric speciation takes place with no geographical barrier
• Isolation instead occurs when random changes in the alleles, and therefore phenotypes, of some individuals in a population prevent them from successfully breeding with other individuals in the population
• Examples of phenotype changes that can lead to reproductive isolation include
  • Seasonal changes
    • Some individuals in a population may develop different mating or flowering seasons to the rest of the population, i.e. their reproductive timings no longer match up
    • This is known as temporal isolation
  • Behavioural changes
    • Some individuals in a population may develop changes in their courtship behaviours meaning they can no longer attract individuals of the opposite sex for mating, i.e. their methods of attracting a mate are no longer effective
    • This is known as behavioural isolation
• The populations may still live in the same habitat but they do not interbreed
• The lack of gene flow between the two populations means that allele frequencies in the gene pools of the two populations may change in different ways
• Changing allele frequencies will lead to changes in the phenotypes of the two populations
• If enough allele frequency differences arise between the two populations then they will become reproductively isolated and will be two separate species

E.g. sympatric speciation in fruit flies

• A population of fruit flies exists in a laboratory
• A random allele change resulting from random mutation divides the species into two populations
  • The allele changes leads to a change in phenotype, e.g. food preference
• The difference in phenotype prevents the two populations from interbreeding so there is no gene flow between them
• Different alleles are therefore passed on in each population
  • This could be due to difference in selection pressure, e.g. certain enzymes are advantageous for the digestion of different foods or due to random passing on of different alleles
• Different alleles become more frequent in each population
• Over time the divided populations form two distinct species that can no longer interbreed to produce fertile offspring

Phenotypic changes resulting from random mutations prevent gene flow between two populations of fruit flies which may lead to sympatric speciation

• The definition of a species states that

A species is a group of organisms with similar characteristics that can interbreed to produce fertile offspring

• There are several reasons why individuals of different species cannot breed together to produce fertile offspring, e.g.
  
  • Incompatible chromosome numbers
  • Incompatible courtship behaviours
• The term 'hybrid' refers to the offspring of individuals of two different species
  • Hybridisation is the mechanism by which hybrids are produced, i.e. the mating, fertilisation, and development processes
• Hybrids are rare, and are usually infertile

Barriers to hybridisation: incompatible chromosomes

• The fusion of gametes from different species often leads to non-viable zygotes; this can occur if the chromosomes of the different species do not match
  • The gene at a particular locus on a particular chromosome needs to be the same in both chromosomes in a homologous pair
• Viable zygotes can sometimes occur, but such zygotes usually develop into infertile hybrids
  • Different species often have different chromosome numbers, resulting in gametes with different numbers of chromosomes
  • The new diploid cells formed during fertilisation contain an uneven number of chromosomes which are unable to pair up in homologous pairs 
  • These individuals will be unable to carry out meiosis and so will be infertile
• A well-known example of this is the mating of a horse and donkey to produce a mule:
  • Mule chromosomes cannot pair up during meiosis, so mules cannot produce gametes of their own

Hybrid sterility diagram

Mules have an odd number of chromosomes so cannot carry our meiosis and are sterile

Barriers to hybridisation: incompatible courtship behaviours

• In some species the process of successful breeding can be preceded by some form of courtship behaviour
• Courtship behaviour in animals is a ritual that eventually results in mating and reproduction
  • It can be a very simple process that involves a small number of visual, chemical or auditory stimuli
  • It can also be a highly complex sequence of behaviours involving two or more individuals, using several modes of communication
    • Many birds of paradise have intricate and impressive courtship rituals
• If the courtship rituals of two individuals do not match, then no mating will occur and hybridisation will be prevented