Chapter 18 Speciation

Question 1

Define species.

Answer 1

an evolutionarily independent population or group of

populations.

Question 2

Explain, in general, how speciation occurs (i.e., what two steps are involved)?

Answer 2

Speciation: the process by which one species splits into two or more distinct species.

Speciation results from:
1. Genetic isolation
• Results from lack of gene flow.
2. Genetic divergence.
• Results from selection, genetic drift, and mutations, which occur independently in the isolated populations.
• Example: If a new mutation creates an allele that changes the phenotype of individuals in one population, without gene flow, there is no way for the allele to appear in the other population.
• As a result, allele frequencies and other characteristics in the populations diverge.
• After some time, this divergence leads to the population becoming a distinct species.

Question 3

Biological Species concept

Answer 3

Species: a group of populations whose members have the potential to interbreed in nature and produce viable,
fertile offspring, but who cannot produce viable, fertile
offspring with members of other such populations.
• A biological species are reproductively
isolated from other populations.

• Advantages:
• Strong theoretical foundation and is logical – can be confident populations are evolutionarily independent since no gene flow occurs between them.
• Limitations:
• Fossils – how to evaluate?
• Asexual reproduction – cannot apply.
• Gene flow can sometimes occur between species that are morphologically and ecologically distinct.
• Similar species whose ranges do not overlap – lack information about interbreeding.

Question 4

Define reproductive isolation

Answer 4

• Reproductive isolation is the existence of biological
barriers that prevent members of two species from
interbreeding and producing viable, fertile offspring.
• Typically, these barriers are intrinsic to the organisms.
• Reproductive isolation prevents populations belonging to different species from interbreeding, even when their ranges overlap.

Question 5

Distinguish between prezygotic and postzygotic isolating mechanisms.

Answer 5

Prezygotic- barrier functions before the formation of zygote

Postzygotic: Barrier functions after formation of zygote

Question 6

Describe 5 prezygotic isolating mechanisms and give an example of each.

Answer 6

1) Habitat isolation: occurs when species use 
different habitats (even in the same geographic area) and are thus unlikely to encounter each other to even attempt mating. example: snakes on the land and snakes in the water 

2)Temporal isolation: occurs when species breed during different times of day, different seasons, or different years. ex. american toad that mates early in summer and Fowlers toad which mates in late summer

3) Behavioral isolation: occurs when species
do not recognize each other as mates due to behavioral
differences. Observed in species that use unique and elaborate courtship behaviors to identify and/or attract
mates and to synchronize gonadal maturation example: different crickets have different mating songs

4) Mechanical isolation: occurs when matings between closely related species fail (though they may make an attempt to do so) because they are anatomically incompatible and transfer of sperm is not possible. Example: snails with shells going 2 diff ways
5) Gametic isolation: occurs when the gametes of two species do not form a zygote because of incompatibilities preventing fertilization. example: Gametic isolation: occurs when the gametes of two species do not form a zygote because of incompatibilities preventing fertilization.

Question 7

Describe 3 postzygotic isolating mechanisms.

Answer 7

1) Reduced hybrid viability: occurs when development of the hybrid is aborted at some embryonic stage or frail offspring are produced that do not survive.

2) Reduced hybrid fertility: occurs when hybrids are
infertile, and cannot backbreed with either parent species.
This infertility may be due to problems in meiosis
because of differences in chromosome number or
structure. (horses and donkeys)

3) hybrid breakdown : when first generation hybrids are viable and fertile but the next generation is feeble or sterile

Question 8

Distinguish between allopatric and sympatric speciation, providing an example for each.

Answer 8

• Allopatric speciation: speciation occurs when geographic separation of populations restricts gene flow.
• Sympatric speciation: speciation occurs in geographically overlapping populations when biological factors, such as chromosomal changes, habitat differentiation, and non-random mating, reduce gene flow.

Question 9

Morphological species concept

Answer 9

• Species defined by body shape, size, and other structural features.
• The oldest and still most widely applicable.
• Advantages:
• It can be applied to asexual and sexual species, and fossils.
• Useful even without information about the extent of gene flow.

• Limitations:
• Features used to distinguish species are subjective, and
researchers may disagree about which characters identify a species.

Question 10

Phylogenetic species concept

Answer 10

• Species defined as the smallest group of individuals that share a common ancestor and form one branch on the tree of life.
• Distinguish groups of individuals that are sufficiently different to be considered separate species.
• Advantages:
• It can be applied to any population (fossil, asexual, or sexual)
• It is logical because populations are distinct enough to be monophyletic only if they are evolutionarily independent.
• Limitations:
• Determining the degree of difference required to indicate separate species.

Question 11

Distinguish between how vicariance and dispersal lead to genetic isolation of populations.

Answer 11

1. Vicariance: involves geologic processes that fragment a
   population into two or more isolated populations.
   • Mountain ranges, glaciers, land bridges, or splintering of lakes may divide one population into isolated groups.
2. Dispersal: some individuals colonize a new, geographically remote area and become isolated from the parent population.

Question 12

Explain two ways in which a population can become genetically isolated without geographic separation.

Answer 12

Although much less common than allopatric speciation,
sympatric speciation can occur if gene flow is reduced by
such factors as polyploidy, habitat differentiation, and
sexual selection.

Question 13

Distinguish between an autopolyploid and an allopolyploid species.

Answer 13

1. An autopolyploid mutant is an individual that has more than two chromosome sets, all derived from a single species.
2. An allopolyploid mutant has more than two chromosome sets, derived from different species.

Question 14

Describe how cichlid fishes may have speciated in sympatry in Lake Victoria (one of Africa’s Great Lakes).

Answer 14

• Cichlid species of the Great Lakes in Eastern Africa.
• In these lakes almost 2000 species of cichlids have evolved in the last 10 million years from a single ancestor.
• One hypothesis – populations became adapted to different food resources:

1. Split into two groups.
   • Sandy benthic or rocky benthic habitat
2. Rock and sand dwellers began using
   diversity based on foraging strategies.
   • Natural selection favored physical + behavioral adaptations best suited to food choice.