10.4.3 Sympatric Speciation

Question 1

Sympatric speciation

Answer 1

• Sympatric speciation is the formation of a new species from a subpopulation that is physically located within the parent population.
• Sympatric speciation is a major driving force in the evolution of plants. A mechanism of sympatric speciation is polyploidy.
• Polyploidy is a chromosomal modification in which the organism has more sets of chromosomes than the typical diploid number (two sets) of chromosomes.
• Nondisjunction of chromosomes causes polyploidy during mitosis or meiosis.
• The two major types of polyploidy are:
1. autopolyploidy—all chromosomes come from the same species.
2. allopolyploidy—the chromosomes come from two different species.
• There are very few documented cases of sympatric speciation in animals.

Question 2

Review:

Answer 2

• Allopatric speciation is a type of speciation that
  can occur when a geographical barrier separates a splinter population from the original population.
• A typical cell has two sets of chromosomes (2n).
  An egg or sperm cell receives half (the haploid, or n) number of chromosomes. During sexual reproduction, gametes unite to form a diploid zygote (2n).

Question 3

Question: Does geographic isolation always have to occur in order to make a new species?

Answer 3

• The formation of the Grand Canyon led to allopatric

speciation of antelope squirrels, which now comprise two species.

Question 4

sympatric speciation

Answer 4

• During sympatric speciation, a parent population gives rise to a new species, and the subpopulation is physically located within the parent population.
• Sympatric speciation has played a major role in the
  evolutionary history of plants. One mechanism by which it occurs is polyploidy.

Question 5

note

Answer 5

• The cells of a polyploid individual have more than the normal 2 sets of chromosomes. The gametes of a polyploid may have more than the haploid number of chromosomes.

Question 6

autoploidy

Answer 6

• One type of polyploidy is autopolyploidy, a phenomenon that results when a self-fertilizing individual produces diploid gametes and gives rise to offspring that are tetraploid (having a 4n number of chromosomes).
• Many plants can self-fertilize, including Mimulus guttatus.
• Autopolyploidy begins during gamete formation in the parent plant. Within a cell, all of the chromosomes fail to separate during meiosis, resulting in a diploid (2n) gamete (this is known as nondisjunction). After self-fertilization, the zygote is tetraploid. The tetraploid can self-fertilize or mate with other tetraploids.

Question 7

Allopolyploidy

Answer 7

• occurs when two different species interbreed,
  producing an individual with more than two sets of
  chromosomes. Hybrids are typically sterile, but two different mechanisms can produce successful allopolyploid species from the hybrids.
• The illustration shows one way allopolyploidy can lead to the production of fertile hybrids. The gametes of species A and B unite, forming a sterile allopolyploid with five chromosomes. The hybrid is sterile because not all of the chromosomes have a homologue to pair with. The hybrid produces an unreduced gamete with five chromosomes that combines with a normal gamete of species B. The resulting hybrid is fertile, having a diploid number of 10.

Question 8

Sympatric speciation occurs by __________ in plants.

Answer 8

• polyploidy

Question 9

Referring to the illustration, which one of the following statements is true?

Answer 9

• In the cell labeled “B”, 2n = 10.

Question 10

Which of the following is an example of a species that developed through polyploidization?

Answer 10

• bread wheat

Question 11

Which one of the following statements about sympatric speciation is true?

Answer 11

• Sympatric speciation does not require geographical isolation

Question 12

In the meiotic error in which a gamete has doubled chromosomes, what are the genetic consequences when pairing with another normal gamete?

Answer 12

• No homologs exist

Question 13

What is the polyploid chromosome complement of a gamete with 4n uniting with a gamete of 6n?

Answer 13

• 10n