several conditions - self-fertilisation, vegetative
propagation, higher fitness than the diploid, or niche
separation from the diploid - may enable new polyploidy
to increase & form a viable population
because polyploidy apparently confers new physiological &
ecological capabilities it may play important role in plant evolution
their increased number of genes
may enhance their adaptability
polyploidy doesn't confer major new
morphological characteristics eg
differences in structure of flowers or fruits,
so seems unlikely to cause evolution of
new genera or other higher taxa
recombination speciation - arises when hybrid
genotypes, that are fertile but reproductively
isolated from the parent species, increase in
frequency forming a distinct population (aka hybrid
speciation) - more common in plants than animals
hybridisation, by generating diverse gene combinations on
which selection can act, can be source of new species with
novel morphological & ecological features
speciation rates
vary greatly:
expected to be very
slow if it proceeds by
mutation & drift of
neutral alleles
expected to be faster
if driven by ecological
or sexual selection
expected to be
accelerated if
reinforcement plays
a role
allopatric speciation could be
slow or very rapid, depending on
strength of divergent selection &
on genetic variation in relevant
traits
polyploidy, recombination speciation & sympatric speciation
should be very rapid when they occur - but may occur rarely,
resulting in long intervals between speciation events
ecological speciation
can be very rapid
characteristics that
seem most likely to
increase speciation rate
include:
pollination
by animal
(rather than
wind) in
plants
features that
indicate
intense
sexual
selection in
animals
these observations suggest diversification in some groups of
animals owes more to simple evolution of reproductive isolation
(due to sexual selection) than to ecological diversification
MOST IMPORTANT
CONSEQUENCE OF
SPECIATION IS THAT
ITS THE SINE QUA
NON (ESSENTIAL
CONDITION OR
PREREQUISITE) OF
DIVERSITY
for sexually reproducing organisms,
every branch on phylogenetic tree
represents a speciation event
speciation stands at border between microevolution - genetic
changes within & among populations - & macroevolution - evolution
of higher taxa in all their diversity
evidence that speciation contributes to both rates
of molecular & morphological evolution, based on
expected differences between patterns of
divergence expected under models of phyletic
gradualism vs punctuated equilibrium
in gradual
model, rate
of evolution
of lineage
is
unaffected
by number
of
speciation
events
in contrast,
evolutionary
change
requires
speciation &
increases with
number of
speciation
events, in
pure
punctuated
equilibrium
model
agreed no
reason to think
speciation
triggers
morphological
evolution
nevertheless,
morphological
change might be
associated with
speciation
because
reproductive
isolation enables
morphological
differences
between
populations to
persist in the long
term