Spea multiplicata is one of several species of American spadefoot toad.
(a) Young spadefoot toads are called tadpoles and live in water in ponds.
S. multiplicata tadpoles show three different phenotypes due to genetic variation. The three phenotypes are: detritus feeder, intermediate and carnivore.
Detritus feeders are small, and carnivores are large. Intermediates vary in size between the two extremes.
A detritus feeder and a carnivore are shown in Fig. 7.1.
Detritus feeders:
- eat detritus (small pieces of dead organic matter) and algae (photosynthetic protoctists)
- have smooth mouthparts, small jaw muscles and long intestines.
Intermediates:
- can eat all available food (detritus, algae and fairy shrimps)
- have teeth-like mouthparts, medium-sized jaw muscles and medium-sized intestines.
Carnivores:
- eat fairy shrimps and other small animals
- have teeth-like mouthparts, large jaw muscles and short intestines.
Scientists counted the number of each type of tadpole in two different ponds: pond 1 and pond 2.
(i) In pond 1, the scientists observed:
- a high density of tadpoles
- a low abundance of food
- that most of the tadpoles they counted were either detritus feeders or carnivores, with very few intermediates present.
Describe and suggest explanations for the type of natural selection that appears to be acting in pond 1.
(ii) In pond 2, the scientists observed:
- a low density of tadpoles
- sufficient food availability for all tadpoles
- that most of the tadpoles they counted were intermediates, with fewer detritus feeders or carnivores.
Describe and suggest explanations for the type of natural selection that appears to be acting in pond 2.
(iii) The intestine length of S. multiplicata tadpoles shows continuous variation.
Sketch a curve on Fig. 7.2 to show how intestine length varies in the tadpole population in pond 2.
(iv) A student suggested that the variation in S. multiplicata tadpoles could lead to sympatric speciation in some populations.
Outline the features of sympatric speciation.
(b) Fig. 7.3 shows the evolutionary relationships between three species of American spadefoot toad.
Explain how analysis of DNA allowed the evolutionary relationships shown in Fig. 7.3 to be determined.
▶️ Answer/Explanation
(a)(i)
The type of natural selection acting in pond 1 is disruptive or diversifying selection.
Explanation: In pond 1, we see both extreme phenotypes (detritus feeders and carnivores) being favored while the intermediate phenotype is selected against. This occurs because:
- The high density of tadpoles creates intense competition for limited food resources
- Detritus feeders specialize in eating detritus and algae, while carnivores specialize in eating fairy shrimp and other small animals
- Intermediates, which try to eat both types of food, are outcompeted by the specialists and don’t get enough of either food source
- The lack of sufficient food acts as the selection pressure driving this pattern
(a)(ii)
The type of natural selection acting in pond 2 is stabilizing selection.
Explanation: In pond 2, the intermediate phenotype is favored while both extremes are selected against. This occurs because:
- With low tadpole density and sufficient food, there’s less competition
- Intermediates have an advantage as they can eat all available food types (detritus, algae, and fairy shrimp)
- The greater dietary flexibility allows intermediates to grow and develop better
- Extreme phenotypes (pure detritus feeders or pure carnivores) are less successful because they can’t take advantage of all available food sources
(a)(iii)
Explanation: Since pond 2 shows stabilizing selection favoring intermediates, the variation in intestine length would follow a normal distribution where most individuals have medium-length intestines (the intermediate phenotype), with fewer individuals having either very short (carnivore) or very long (detritus feeder) intestines.
(a)(iv)
Features of sympatric speciation:
- New species form while living in the same geographic area (without physical separation)
- Reproductive isolation develops due to ecological or behavioral differences
- In this case, different feeding specializations (detritus vs. carnivorous) could lead to reproductive isolation
- Over time, genetic differences accumulate between the specialized groups until they can no longer interbreed
(b)
DNA analysis allowed determination of evolutionary relationships through:
- Comparing DNA sequences of all three species to identify similarities and differences
- Counting nucleotide/base differences between species – fewer differences indicate closer relationship
- Using bioinformatics tools/databases to analyze the genetic data
- The species with the fewest genetic differences (S. hammondii and S. bombifrons) are shown as most closely related
- The more distant species (S. multiplicata) shares fewer DNA similarities with the other two