Isolating Mechanisms and Speciation

Hyla versicolor
Hyla chrysoscelis
Using the (older) typological definition of species, these
two groups of frogs were classified as one. Upon further
investigation, it was discovered that there are two groups
that differ only in their mating calls. They call for mates at
different frequencies.
A biological concept of a species is a population or group of
populations that are able to interbreed, under natural conditions to
produce fertile offspring.
The selective mechanisms that favour beneficial traits, natural
selection, are also responsible for speciation (the formation of an
entirely new species).
Evolutionary changes that occur at the species level are known as
Reproductive Isolating Mechanisms
Biologists rely on morphological features from fossil records to
distinguish thousands of different species.
 For living populations that are morphologically similar, behavioral or
other biological methods are needed to distinguish species.
 One method used is to identify the species’ reproductive isolating
mechanisms (any behavioral, structural or biochemical traits that
prevent individuals of different species from reproducing successfully
 There are two types of isolating mechanisms
 prezygotic isolating mechanisms
 postzygotic isolating mechanisms.
Prezygotic mechanisms prevent interspecies mating and
fertilization. There are four types of isolation that prevent mating
from occurring, thus maintaining species isolation.
ecological isolation
temporal isolation
behavioural isolation
mechanical isolation
gametic isolation
Prezygotic isolating mechanisms
Ecological isolation is when species occupy separate habitats or
niches and do not encounter one another to reproduce due to
some geographic or ecological barrier.
Example – ground squirrel species occupy different habitats.
Woodchucks live in fields at low elevation
Marmots live in the Rocky
Mountains at high elevations
Ecological isolation
Example – Atlantic blueheaded wrasse and the
Pacific Cortez rainbow
Temporal isolation is when two species are found in the same
area, but are incapable of mating due to different reproductive
cycles for flowering or mating.
Example – Red and black sea urchins live in the same
location, but release their gametes at different times of the
Behavioural isolation is when distinct mating
rituals by one species may prevent members
of another species from recognizing or
selecting a mate.
Example: male jumping spiders dance
(shake their legs and wave their palps).
Females of different species do not respond
to the dance.
Example: Different species of fireflies do not
recognize each others' mating signals, and
as a result do not generally interbreed.
Mechanical isolation is when structural
differences in reproductive organs
prevent successful fertilization.
Example - This is especially true in
flowering plants that have evolved
specific structures adapted to
certain pollinators. Nectar-feeding
bats searching for flowers are
guided by their echolocation system.
Therefore, plants which depend on
these bats as pollinators, have
evolved acoustically conspicuous
flowers that assist in detection.
Gametic isolation – may prevent reproduction at a molecular level.
Example – in coral reefs, many species with external fertilization
may release gametes simultaneously, so trillions of sperm and
eggs may be in the shallow water at one time. Sperm and eggs
of the same species recognize each other by molecular markers.
This can also be the reason pollen from one species will not be
able to form a pollen tube if it lands on the stigma of a different
Postzygotic mechanisms prevent the hybrid zygote from developing
into healthy and fertile adults. There are three likely cases that will
occur to ensure that the hybrid does not reproduce:
Zygote mortality is a result of chromosomal incompatibility.
Hybrid inviability is when the embryo does develop but the hybrid
experiences reduced fitness and often an early death.
Hybrid infertility occurs when a hybrid develops into a mature adult
but is unable to undergo successful meiotic division, and is unable
to produce offspring. (seen in donkey-horse hybrids: mules)
Postzygotic Isolating mechanisms
Modes of Speciation
Allopatric Speciation
 Populations begin to diverge when gene flow between them is
restricted. Geographic isolation is often the first step in allopatric
 Examples: Galapagos finches and tortoises, Asian elephants, and
the following experiment by Diane Dodd in 1989.
Sympatric Speciation
 Sympatric speciation happens when members of a population
develop some genetic difference that prevents them from
reproducing with the parent type.
 This mechanism has occurred several times in plants, where failure
to reduce chromosome number results in polyploid plants that
reproduce successfully only with other polyploids.
 This causes instant speciation due to non-disjunction.
 Sympatric speciation has also occurred to cichlid fish in Lake
Victoria in Africa.
Parapatric Speciation
 Two separate regions occur with a zone of hybridization where
the two species overlap
 Examples
◦ Bullocks Orioles and Baltimore Orioles
◦ Ensatina salamanders