Download Selection and Speciation

Section 9.2 (Part 1)
SBI 3U1
pp. 360 - 365
SPECIATION: HOW SPECIES
FORM
MICROEVOLUTION

Changes in the gene pool of a population over
time which result in relatively small changes to
the organisms in the population
MACROEVOLUTION
Changes in organisms which are significant enough
that, over time, the newer organisms would be
considered an entirely new species.
 The new organisms would be unable to mate with
their ancestors, assuming we were able to bring them
together.

SPECIES

Species - A group of actually or potentially
interbreeding natural populations that ordinarily
do not interbreed with other such groups even
when there is opportunity to do so.
SPECIATION
 Speciation

The formation of new species from existing species.

The formation of new species is also sometimes called
macroevolution
TYPES OF SPECIATION
 Allopatric
Speciation
The formation of two or more species often requires
geographical isolation of subpopulations of the species.
 Only then can natural selection or perhaps genetic drift
produce distinctive gene pools.

 Sympatric

Speciation
Species diverge while inhabiting the same space and become
reproductively isolated
ALLOPATRIC SPECIATION
Allopatric speciation by migration
 Removal of the geographic barrier
 Lets try this again

SYMPATRIC
SPECIATION
What might keep two subpopulations from
interbreeding when reunited
geographically? There are several isolating
mechanisms.
REPRODUCTIVE ISOLATION
ISOLATING MECHANISMS

Pre-zygotic – Before the zygote is formed


mating is prevented
Post-zygotic – After the zygote is formed

mating is not prevented, but the hybrids have
reduced fitness (reduced chance of survival)
PRE-ZYGOTIC ISOLATING MECHANISMS
 Pre-zygotic
- Factors which cause species to mate
with their own kind

Temporal isolation - Individuals of different species do not
mate because they are active at different times of day or in
different seasons. E.g. orchid & blooming time

Ecological isolation - Individuals mate in their preferred
habitat, and therefore do not meet individuals of other species
with different ecological preferences. E.g. Garter snake
(meadow vs. near water)

Behavioural isolation - Potential mates meet, but choose
members of their own species. E.g. Songs of birds

Mechanical isolation - Copulation is attempted, but transfer
of sperm does not take place. E.g. Insects

Gametic Isolation - Sperm transfer takes place, but egg is not
fertilized.
POST ZYGOTIC ISOLATING MECHANISMS
 Post-zygotic.
Genomic incompatibility,
hybrid inviability or sterility.

Zygotic mortality/Hybrid Breakdown – Hybrid is
viable and fertile, but hybrid’s offspring is sterile or
weak. E.g. Cotton plant hybrid has offspring that die
as seeds or early in development.

Hybrid Inviability - Hybrid embryo forms, dies in
early development before birth. E.g. Hybrid embryo
between sheep and goat.

Hybrid sterility - Hybrid is viable, but resulting
adult is sterile. E.g. Mule from female horse and
male donkey
p. 360, Figure 9.12
TO DO LIST

Section 9.2 p. 363 #13 – 18, p. 365 #19 – 24

Unit Test for Evolution

Monday Nov 23, 2015