Download Life: The Science of Biology, 10e

Lecture Notes
by Donna Francis
Life: The Science of Biology, Tenth Edition
Sadava • Hillis • Heller • Berenbaum
Chapter 23: Speciation
23.4 What Happens when Newly Formed Species Come into Contact?
Genetic differences accumulate in isolated populations over time, reducing the
probability that individuals can mate successfully when they come back into contact.
If hybridization occurs, and hybrids are less fit, selection will result in reinforcement of
mechanisms that prevent hybridization.
Prezygotic isolating mechanisms prevent hybridization from occurring.
Postzygotic isolating mechanisms reduce the fitness of hybrid offspring.
Postzygotic mechanisms result in selection against hybridization, which in turn reinforces
prezygotic mechanisms.
Prezygotic isolating mechanisms (before fertilization occurs):
 Mechanical
 Temporal
 Behavioral
 Habitat
 Gametic
Mechanical isolation:
 Differences in size and shape of reproductive organs makes mating impossible.
 In plants such as orchids, mechanical isolation may involve pollinators.
FIGURE 23.10 Mechanical Isolation through Mimicry
Temporal isolation:
 Mating periods do not overlap.
 In sympatric populations of three closely related leopard frogs, each species
breeds at a different time of year.
FIGURE 23.11 Temporal Isolation of Breeding Seasons
Behavioral isolation:
 Individuals reject or fail to recognize potential mating partners.
 Breeding calls of male frogs quickly diverge between related species. Female
frogs ignore calls from other species.
© 2014 Sinauer Associates, Inc.
FIGURE 23.12 Behavioral Isolation in Mating Calls
MEDIA CLIP 23.1 Narrowmouth Toads Calling for Mates
Mate choice of one species may be mediated by behavior of other species.
Whether two plant species hybridize can depend on the food preferences of their
pollinators.
Example: Two species of columbines are reproductively isolated because their floral
shapes attract different pollinators.
FIGURE 23.13 Reproductive Isolating Mechanisms May Be Mediated by Species
Interactions
Habitat isolation:
 When two closely related species evolve preferences for living or mating in
different habitats, they may never come into contact during mating periods.
Examples: apple maggot flies, cichlid fishes in Lake Malawi.
Gametic isolation:
 Eggs of one species do not have appropriate chemical signals for sperm of another
species; or sperm is not able to attach to and penetrate the egg.
 In sea urchins the protein bindin attaches the sperm to eggs but will only bind to
eggs of the same species.
Postzygotic isolating mechanisms
 Low hybrid zygote viability—zygotes fail to mature or have severe abnormalities.
 Low hybrid adult viability—offspring have lower survival rates.
 Hybrid infertility—offspring are infertile (e.g., mules).
If hybrid offspring have low fitness, natural selection may favor prezygotic barriers.
Individuals that can avoid mating with members of another species would have a
selective advantage.
In Texas, Phlox drummondii has pink flowers over most of its range.
Where it is sympatric with another pink species, P. drummondii has red flowers.
An experiment indicated that reinforcement could explain the evolution of red flowers.
FIGURE 23.14 Flower Color Reinforces a Reproductive Barrier in Phlox
If reinforcement is occurring, sympatric species should evolve more effective prezygotic
barriers than allopatric species.
This is illustrated by temporal and behavioral isolation in frogs: Breeding seasons and
call frequencies overlap less in sympatric populations.
If populations are reunited before complete reproductive isolation has developed,
interbreeding can occur.
© 2014 Sinauer Associates, Inc.
If hybrid offspring are fit and interbreed with both parental populations, gene pools will
be mixed and no speciation occurs.
If hybrid offspring are less fit, reinforcement may result in more prezygotic barriers, and
complete reproductive isolation may evolve.
A hybrid zone may develop in the absence of reinforcement or before reinforcement is
complete.
Hybrid zones contain recombinant individuals resulting from many generations of
hybridization.
Genetic studies in hybrid zones show why they may be stable for long periods of time.
Example: Two species of European toads have a long narrow hybrid zone.
FIGURE 23.15 A Hybrid Zone
The toad hybrids have many defects, some of which are lethal.
On average, a hybrid toad is only half as fit as a purebred individual.
The hybrid zone is narrow because there is strong selection against hybrids. But it
persists because individuals of both species continue to move into it and mate.
© 2014 Sinauer Associates, Inc.