Speciation

Question 1

Speciation and reproductive isolation

Answer 1

Speciation occurs when a group within a species separates from other members of its species and develops its own unique characteristics

Two changes during speciation
–Divergence - species adapt to different environments or selection pressures
–Reproductive isolation - populations cannot interbreed

Species - a group of populations whose members have the potential to interbreed in nature and produce viable fertile offspring but do not produce this with members of other such groups

Reproductive isolation - the formation of a new species hinges on reproductive isolation, the existence of biological factors that impede members of two species from interbreeding and producing viable, fertile offspring

Question 2

Gene flow and disruptive selection

Answer 2

Speciation expected to be associated with disruptive selection

Disruptive selection can be countered by gene flow

Gene flow - exchange of genes between populations as a result of movement and interbreeding of individuals

Directional/stabilising selection + gene flow = holds a species together

Disruptive selection + low gene flow = speciation

Things that can stop gene flow
-Geographic barriers
-Ecological barriers
-Behavioural barriers
-Genetic incompatibility
-Human activities

Two main theories of speciation, allopatric and sympatric

Question 3

Allopatric speciation

Answer 3

Geographic and reproductive isolation first, then divergence

Physical barriers lead to geographic isolation of populations

Lack of gene flow leads to reproductive isolation

Long term isolation can result in divergence due to climate or ecological differences or random genetic drift (change in genotype frequency caused by random variation in individual reproduction)

Question 4

Sympatric speciation

Answer 4

Divergence first, then reproductive isolation

Divergence causes reduction in gene flow leading to reproductive isolation

Question 5

How is sympatric speciation supposed to occur?

Answer 5

Disruptive selection

Postzygotic isolation
-Reduced survival or viability of hybrids

Prezygotic isolation
-Blocking fertilisation or mating of different species

Question 6

What evidence is required to make a compelling case for sympatric speciation?

Answer 6

Species must be largely sympatric (occurring in the same area)

The sympatric species must be reproductively isolated

The sympatric taxa must be sister species

The biogeographic and evolutionary history of the species must make it unlikely that they were ever allopatric (occurring in separate non-overlapping geographical areas)

Question 7

Are there any really convincing cases of sympatric speciation?

Answer 7

North American apple maggot fly (Rhagoletis pomonella) which has diverged into several distinct host races that specialise on different types of fruit. This divergence occurred within the same geographic region, and is thought to have resulted from disruptive selection and reproductive isolation

Question 8

How important is sympatric speciation?

Answer 8

Sympatric speciation can occur in the absence of geographic barriers, which can allow species to rapidly diversify in response to ecological or selective pressures. It can also result in the formation of new species that are closely related and may share many traits, which can provide insights into the mechanisms of evolutionary change