124 Environment and Phenotype

Key Idea: An organism's phenotype is influenced by the environment in which it develops, even though the genotype remains unaffected.

` Environmental factors can modify the phenotype encoded by genes without changing the genotype. This can occur both during development and later in life. Environmental factors that affect the phenotype of plants and animals include nutrients or diet, temperature, altitude or latitude, and the presence of other organisms.

The effect of temperature

3D

` The sex of some animals is determined by the incubation temperature during their embryonic development. Examples include turtles, crocodiles, and the American alligator. In some species, high incubation temperatures produce males and low temperatures produce females. In other species, the opposite is true. Temperature regulated sex determination may provide an advantage by preventing inbreeding (since all siblings will tend to be of the same sex). ` Colour-pointing is a result of a temperature sensitive mutation to one of the melanin-producing enzymes (melanin is the pigment responsible for darker skin and hair). The mutated enzyme is thermally unstable and fails to work at normal body temperatures. However it is active at the lower temperatures found in the extremities of the body (paws, face, tail), so these areas are darker than the surrounding pale coloured hair (pointed). Colour-pointing is seen in some breeds of cats and rabbits (e.g. Siamese cats and Himalayan rabbits).

The effect of other organisms

` The presence of other individuals of the same species may control sex determination for some animals. Some fish species, including Sandager's wrasse (right), show this characteristic. The fish live in groups consisting of a single male with attendant females and juveniles. In the presence of a male, all juvenile fish of this species grow into females. When the male dies, the dominant female will undergo physiological changes to become a male.

The male and female look very different. ` Some organisms respond to the presence of other, potentially harmful, organisms by changing their body shape. Invertebrates, such as some Daphnia species, grow a helmet when invertebrate predators are present.

The helmet makes Daphnia more difficult to attack and handle. Such changes are usually in response to chemicals produced by the predator (or competitor) and are common in plants as well as animals.

Female

Non-helmeted Daphnia

Chemical signal Male

Helmeted Daphnia

1. (a) Give two examples of how temperature affects a phenotypic characteristic in an organism:

(b) Why are the darker patches of fur in colour-pointed cats and rabbits found only on the face, paws, and tail?

2. How is helmet development in Daphnia an adaptive response to environment?

Growth to genetic potential

Cline

Increasing altitude can stunt the phenotype of plants with the same genotype. In some conifers, e.g. Engelmann spruce, plants at low altitude grow to their full genetic potential, but growth becomes progressively more stunted as elevation increases. Growth is gnarled and bushy at the highest, most severe sites. Gradual change in phenotype over an environmental gradient is called a cline. The chemical environment can influence the phenotype in plants and animals. The colour of hydrangea flowers varies with soil pH. Blue flowers (due to the presence of aluminium compounds in the flowers) occur in more acidic soils (pH 5.0-5.5) in which aluminium is more readily available. In less acidic soils (pH 6.0-6.5) the flowers are pink.

3. (a) What is a cline?

(b) What physical factors associated with altitude could affect plant phenotype?

4. Describe an example of how the chemical environment of a plant can influence phenotype:

5. Vegetable growers can produce enormous vegetables for competition. How could you improve the chance that a vegetable would reach its maximum genetic potential?

6. Two different species of plant (A and B) were found growing together on a windswept portion of a coast, Both have a low growing (prostrate) phenotype. One of each plant type was transferred to a greenhouse where "ideal" conditions were provided to allow maximum growth. In this controlled environment, species B continued to grow in its original prostrate form, but species A changed its growing pattern and became erect in form. Identify the cause of the prostrate phenotype in each of the coastal grown plant species and explain your answer:

Plant species A:

Plant species B:

7. With the ability of scientists now to clone animals there is often talk of people cloning their much loved pet so that they can have the same animal again after the original pet has died. Explain why it is highly unlikely that the cloned animal with be exactly like the original:

8. Some cattle breeders on the east coast of New Zealand decided to produce a cattle breed suited to the dry east coast conditions by selecting the fastest growing cattle from each generation for breeding. A buyer from the west coast was impressed by the speed at which the cattle grew and the size they obtained and decided to try raising a herd on a west coast farm. However the buyer found subsequent generations of the cattle grew slowly and were quite small. Explain why there was such a difference in the growth and size of the cattle on the east and west coasts.

9. To examine the role of environment on phenotype, scientists try to use organisms that are genetically identical. These might include identical siblings (e.g. identical twins) or using plant cuttings.

(a) Why would scientists use genetically identical organisms in gene-environment studies?

(b) Various studies on IQs have been carried out to see if cognitive ability is related to environment or genetics (the heritability of intelligence). When comparing IQs of different groups the studies have found the following:

Group

Identical twins raised together Identical twins raised apart Fraternal twins Parent-child Half-siblings Cousins

% similarity in IQ

88 69 60 42 31 15

Use this information to discuss the effect of genes and environment on IQ: