Lecture 12: Isolated Populations Come into Contact

Question 1

If divergence has occurred and prezygotic isolation exists,
then mating between the populations is rare, gene flow is
minimal, and the populations continue to diverge
* When prezygotic isolation does not exist, populations may
successfully interbreed
* Gene flow then occurs and may erase distinctions between
the two populations
* Other possible outcomes are r_______, development
of h______ zones, and speciation by h_______

Answer 1

Reinforcement, hybrid zones, hybridization

Question 2

If two populations have diverged extensively and are
genetically distinct, the fitness of hybrid offspring will
be LOWER than the parents’ fitness
– If populations are well adapted to different habitats,
then hybrid offspring will not be well adapted to either habitat

T or F

Answer 2

True

Question 3

When p______ isolation occurs, there is strong
natural selection against interbreeding

Answer 3

Postzygotic

Question 4

Natural selection for traits that prevent interbreeding
among populations is called r_____

Answer 4

Reinforcement

Question 5

Reinforcement:
* Some of the best data comes from laboratory
studies of Drosophila
* Closely related sympatric species will seldom
mate with one another in the lab
* Allopatric species are often willing to mate with
one another in the lab
* This is the pattern expected by reinforcement
– Natural selection can act to reduce mating between
species only when their ranges overlap

T or F

Answer 5

True

Question 6

Sometimes the hybrid offspring of diverged populations can mate and produce viable fertile offspring
* A h____ zone is a geographic area where interbreeding
between two populations occurs and hybrid offspring are
common

Answer 6

Hybrid zone

Question 7

Depending on the hybrid offspring fitness and extent of breeding between parental species, hybrid zones can be (3 points):

Answer 7

– Narrow or wide
– Long or short-lived
– Stable in one place or move over time

Question 8

Example: Townsend’s warblers and hermit warblers
hybridize extensively where their ranges overlap
– Hybrid offspring have intermediate characteristics
relative to the two parental species
* Data from mitochondrial (mt) DNA show
– Most hybrids form when Townsend’s warbler males
mate with hermit warbler females
– Townsend’s males invade hermit territories, drive off hermit males, and mate with hermit females
– In some regions, many Townsend’s warblers have
hermit mtDNA

This result suggests that hermit warblers were
once as far north as Alaska
– Townsend warblers have taken over their range,
causing the hybrid zone to move south
– The hybrid zone may continue moving south,
eventually leading to the extinction of the hermit
warbler

T or F

Answer 8

True

Question 9

Two species may interbreed and produce hybrid
offspring that survive, reproduce, and possess a
unique combination of traits adaptive to their particular
environment
– A new species might be the result
* Example: Hybridization between two species of
sunflowers, which had diverged previously, produced
a new species
– This new species had unique combinations of alleles from
each parental species and therefore different characteristics
– The new species was able to grow in drier habitats

T or F

Answer 9

True