Mechanisms of Speciation (Chap. 20)

Mechanisms of Speciation and
Maintaining a Species
Two methods of speciation (formation of a
new species) based on geography
1. Populations are geographically isolated
• allopatric speciation – a geographic process splits a
population so they evolve separately and no longer
• often results in endemic species (species that are
found nowhere else)
• two main ways populations become
geographically isolated:
– individuals end up in new hospitable
place and survive resulting in new
– the slow process of geological change
that divides a group such as continental
• Grand Canyon
squirrels (Fig.
20.11 in text)
• Snapping shrimp in Central America
2. Populations are overlapping (not
geographically isolated)
• sympatric speciation – occurs in populations
occupying the same habitat
– Maggot flies – one infects hawthorn trees and
the other apple trees, resulting in two diff. sp.
Gene flow has been reduced between flies that feed on
different food varieties, even though they both live in
the same geographic area.
Reproductive Isolating Mechanisms /
Barriers Between Gene Pools
• means by which species maintain their
integrity and thereby continue as a
separate species even though they live in
the same habitat
• normally do this by preventing hybrids
from forming (prezygotic) or ensure that
they fail to be successful (postzygotic)
Prezygotic barriers – mechanisms that
prevent formation of a zygote
1. Ecological barriers – two sp. share same territory
but occupy different parts of it
– Maggot flies – one sp. infects hawthorn trees
and the other apple trees
– lions and tigers (also a postzygotic barrier)
2. Behavioral barriers – behavior of two sp. is very
different and therefore don’t breed
– lions/tigers social tendencies
– different mating time
– Homo sapiens behavior may have exterminated
other Homo species
3. Mechanical barriers – genitalia don’t fit
or certain insects only pollinate specific
plants because they only fit/visit those
4. Gametic barriers – even if two different
species did mate, fertilization of egg
won’t take place because incompatibility
between egg and sperm
– may have prevented recent Homo sapiens (46
chromosomes) from successfully breeding with older
hominids (48 chromosomes)
Postzygotic barriers - mechanisms that act
after a zygote is formed to prevent the
survival of the hybrid species
1. Hybrid inviability – zygote dies because of genetic
may have prevented recent Homo sapiens (46
chromosomes) from successfully breeding with older
hominids (48 chromosomes)
2. Hybrid sterility – first generation survives but
cannot reproduce.
– ex. female horse + male donkey = mule
– ex. lion + tiger = ligers or tigons
3. Hybrid breakdown – first generation survives and
reproduces, but second can’t produce offspring
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