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Selection Pressure
& Speciation
Pick Me! Pick Me!
We understand that direction of evolution is a
complex interaction of genetics, mutation and
environmental pressures.
Genetics acts as the source of variation but is
the environment that determines if the allele will
become more prevalent in society.
Selection is the pressure that exists against
those who are not favored by the environmental
conditions present.
Stabilizing Selection
Stabilizing Selection – A stable environment
where selection exists against individuals
exhibiting variations in a trait that deviate from
the current population average. It is the most
common type of selection.
Hummingbird beak and tongue lengths are a
good example of stabilizing selection. Too big
requires too much energy while too small can’t
access food very well.
Stabilizing Selection
Directional Selection
Directional Selection – The environment
selects for traits that represents an
increase or decrease in the value of a trait
from the current population average.
A change in the length of the flowers will
see a change in the length of the beak of
the hummingbird.
The salmon populations shrunk with the
introduction of gill nets.
Directional Selection
Disruptive Selection
Disruptive Selection – The environment
selects for variations of the trait that
represent the extreme ends of the trait
(they both differ from the current
population average).
Long or short flowers replace the medium
length flowers so the population shifts
towards longer or shorter beaks.
Disruptive selection is one of the
mechanisms that may lead to speciation.
Disruptive Selection
Sexual Selection
Sexual Selection – Traits that lead to mating
success are favored.
Sexual dimorphism may be apparent in which
there are striking physical differences between
the male and female forms of the species.
Behaviors and abilities (fighting, singing) are
also a source of sexual selection.
Sometimes the sexually selected trait comes into
direct conflict with another selection pressure. A
brightly coloured male lay look great to the
female form of his species but it also makes him
much more noticeable to predators as well.
Sexual Selection
The formation of a new species due to selective
pressures is called speciation.
Disruptive selection can result in two extreme
forms of a trait. These forms can be so different
that the two populations no longer associate with
each other or recognize each other as members
of the same species.
Changes in gene frequency and phenotypic
traits within a population and/or species are
termed microevolution – it can lead to
What is a Species?
A species is a group or population of
interbreeding individuals that are reproductively
isolated from other groups and evolve
Species can be differentiated using their
reproductive isolating mechanisms – any
behavioural, structural or biochemical traits that
prevent individuals of different species from
reproducing successfully together.
Reproductive Isolating Mechanisms
There are a variety of mechanisms at work that
prevent the successful production of viable
offspring between two separate species.
These mechanisms can be grouped into two
major categories – prezygotic and postzygotic.
Basically, those that stops a zygote from forming
and those that stop the being from developing
and/or mating after the zygote has been formed.
Pre-zygotic Mechanisms
Prezygotic mechanisms are those things that do not the
actual mating and/or fertilization to ever take place between
the two organisms.
 Ecological Isolation – Different habitats.
 Temporal Isolation – Different reproductive timing.
 Behavioural Isolation – Different rituals or actions used
to recognize a suitable mate.
 Mechanical Isolation – Reproductive parts of body that
can only function or be used in presence of the same
 Gametic Isolation – Prevention of fertilization at the
molecular level. Often seen in marine organisms.
Post-zygotic Mechanisms
Post-zygotic mechanisms are isolating
mechanisms that take effect after a zygote
or offspring has been produced.
 Zygotic Mortality – No fertilized zygotes
or embryos develop to maturity.
 Hybrid Inviability – Hybrid offspring do
not live long after birth.
 Hybrid Infertility – Hybrid offspring are
strong but sterile.
Modes of Speciation
Anytime you have a series of events that leads to
reproductive isolation, you may also have speciation
taking place soon after. Things like environmental change
and mutation are no longer shared between the two
groups. Geographical isolation is the most common source
of speciation.
 Allopatric speciation – The evolution of populations into
separate species as a result of geographic isolation. The
geography of the environment acts as a physical barrier that
cuts off the two populations.
 Sympatric speciation – The evolution of populations within
the same geographic area into separate species. Perhaps the
area is quite large and the two populations occupy their own
areas within the larger habitat or a mutation may have
occurred that instantaneously isolates the two populations.
Allopatric Speciation
Sympatric Speciation
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