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Mechanisms of Speciation,
Maintaining a Species, and Types of
Evolution (Chap. 20)
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
interbreed
• often results in endemic species (species that are
found nowhere else)
• Two main ways populations become geo. isolated:
– I. Individual(s) end up in new hospitable place
and survive resulting in new sp.
– II. The slow process of geological change that
divides a group such as continental drift
• 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 sp. infects hawthorn trees
and the other apple trees
Gene flow has been reduced between flies that feed on
different food varieties, even though they both live in
the same geographic area.
– hybridization between two species
– polyploidy
• disorders in meiosis can result with 2N
gametes rather than N
– union of these would give 4N zygote;
instantly another species!
– Plants: very common
– Animals: very rare
Geographic models of speciation emphasize the development of reproductive isolation between geographic population components of a
species. (A) Allopatric: reproductive isolation develops between populations in physical isolation. (B) Founder Effect: reproductive
isolation develops in a small population separated from the main body of the species, in consequence of the biology of small populations. (C)
Parapatric: reproductive isolation develops among contiguous components of a population, in consequence of their limited vagility. (D)
Sympatric: reproductive isolation develops within the "cruising range" of an existing species, in consequence of special biological
properties (e.g., host-parasite specificity).
Reproductive Isolating Mechanisms /
Barriers Between Gene Pools
• means by which species maintains their
integrity and thereby continues 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
Four types (look in text)
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
– meadowlarks’ diff. songs
– trouts’ diff. mating time (temporal isolation)
3. Mechanical barriers – genitalia don’t fit
or certain insects only pollinate specific
plants because they only fit/visit those
plants
4. Gametic barriers – even if two different
species did mate, fertilization of egg
won’t take place because incompatibility
between egg and sperm
Postzygotic barriers - mechanisms that
act after a zygote is formed to prevent
the survival of the hybrid species
Three types
1. Hybrid inviability – zygote dies because of
genetic differences
2. Hybrid sterility – F1 generation survives but
cannot reproduce.
– ex. female horse + male donkey = mule
– ex. lion + tiger = ligers or tigons
3. Hybrid breakdown – F1 generation survives and
reproduces, but F2 cannot produce offspring
Patterns of Evolution
• Divergent evolution – process where sp.
become increasingly different over time.
– adaptive radiation is a form of divergent
evolution where a number of diverse species
form from a single ancestral one.
– ex. Galapagos finches observed by Darwin
• Convergent evolution – different species
“grow” more alike due to similar
environments. Occurs on all levels –
biochemical to morphological
– ex. Marsupials in Australia have a striking
resemblance to placentals on other continents.
(Fig. 20.18 in text)
• Coevolution – the reciprocal influence of
two species on each other’s evolutionary
direction
– ex. predator-prey relationships
– ex. yucca plant is dependent on the yucca
moth for pollination and the moth only lays
its eggs on the plant where larva eat the
seeds (Fig. 20.19)