Speciation
Speciation

The formation of new species – populations reproductively isolated = no
gene flow. May occur:
-
Gradually by slow accumulation of small changes
-
Instantly by changes in chromosome numbers through polyploidy

A reproductive isolating mechanism (RIM) is any factor stopping members
of populations from breeding. May be pre/post zygotic.
Prezygotic isolating mechanisms

Geographical isolation:
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Physical barriers between populations.
-
If they come back together, they are sufficiently different and unable to
breed.
-
EG – Kea & Kaka. Sea level rising and cutting off North and South Islands –
two genetically isolated populations from one ancestral population. Kaka
in the north and kea in the south.
Prezygotic isolating mechanisms

Structural (Morphological) isolation:
-
Differences in external reproductive organs that don’t allow transfer of
sperm.
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Common in insects which have genitals that are shaped in a speciesspecific way.
-
In flowering plants (angiosperms) the flowers often have adaptations for
pollination by a specific type or species of animal pollinator = co-evolution.
Flowers attracting particular pollinator and pollinators only going to
particular flower.
Prezygotic isolating mechanisms

Ecological isolation:
-
Differences in habitat within same geo area, so populations rarely come
into contact.
-
EG – Two species of herbivorous insects feed and mate on different
species of plants.
Prezygotic isolating mechanisms

Temporal isolation
-
Results from breeding behaviour occurring at different, non-overlapping
times.
-
Can be seasonal or daily differences in timing of mating; there can be
differences in the season when pollination occurs.
-
EG – Two species of petrel nest on sub-Antarctic islands. One lays eggs 6
weeks before the other.
Prezygotic isolating mechanisms

Behavioural isolation
-
Differences in behaviour, typically mating behaviour – courtship.
-
Females only recognise and respond to the courtship behaviour of males
of their own species.
-
EG – Moths release species-specific pheromones to attract a mate.
-
Although 2 species are sympatric, one could be diurnal and the other
nocturnal in their feeding habits.
-
Many flowers secrete nectar at a particular time of day.
Prezygotic isolating mechanisms

Gametic isolation
-
Results from incompatibility of gametes. Sperm may not fertilise an egg or
another species through:
•
Egg’s surface not having the right chemical receptors
•
Sperm can’t penetrate the surface of the egg
•
Sperm can’t survive the chemical environment of the female repro system
-
EG. Kina can’t be fertilised by any other species’ sperm present in the
water.
-
EG. Pollen landing on the stigma can only grow a pollen tube if there is
chemical compatibility.