lecture 6 kinds of biological variation

• Polyphenism- a biological mechanism that causes

a trait to be polyphenic*polyphenic trait is a trait for which multiple,

discrete phenotypes can arise from a single genotype as a result of differing environmental conditions.

- the occurrence within a species of distinct morphological types or morphs in response to environmental cues.

*the morphs, which may overlap in space and/or time, are the result of particular signals in the animals' environment, such as nutrition, temperature, or daylength, that interact with the animals' genes and ‘switch’ development of the young from one ‘program’ to another.

*the resultant morphs are better adapted to their environmental conditions or to fulfil a certain role in a social group.

Environmental cues/signals:

1.) seasonal polyphenism- temp., moisture, daylength- in moths & butterflies

2.) nutrional – food, diet

3.) pheromonal - due to pheromones

4.) kairomonal – signals released from one species that can be recognized by another

• Seasonal polyphenism is not uncommon in many butterfly species

-more than one environmental cue is often able to induce the polyphenism

e.g.: in the cabbage white (Pontia) and the sulfur butterfly (Colias)

- where their ventral hindwings become darker when the temperature drops, allowing them to heat more quickly when they alight on leaves.

Figure 1   Polyphenic variation of Pontia (Pieridae) Top row: Summer morphs, P. protodice female (left) and male (center), P. occidentalis male (right); bottom row: Spring morphs, P. protodice "vernalis" female (left) and male (center), P. occidentalis "calyce" male (right).

Figure 2   Polyphenic variation of Colias eurytheme (Pieridae). Top row: Summer morph, male (left) and female (right); bottom row: Cold-season "ariadne" morph, male (left) and female (right).

• Social variation (insect castes)

- in social insects, bees & wasps, particularly in ants & termites; castes have developed

- these are definite groups of individuals w/in a colony in addition to the reproductive castes (queens & males or drones)----workers, soldiers

*structural types may result from different larval food or may due to hormonal or other controls

*taxonomic names should not be applied to these intracolonial variants; belong to the same colony but in different structural types

●Ecotype - a genetically distinct geographic

variety, population or race within species (or among closely related), which is adapted to specific environmental conditions.

- closely related to morphs

*typically, ecotypes exhibit phenotypic differences (such as in morphology or physiology) stemming from environmental heterogeneity and are capable of interbreeding with other geographically adjacent ecotypes without loss of fertility or vigor.

*have no main taxonomic rank in modern biological classification. However, in Environmental Encyclopedia they are said to be "taxonomically equivalent to subspecies".

● Allometric variation- allometric growth my result in the

disproportionate size of a structure in relation to the size of the rest of the body

*particularly marked among insects- heads of ants, mandibles of stag

beetles, & frontal horns & thoraxes of scarabs

Allometric growth– Is the proportioning that helps give a body its

specific form

Newborn 2 5 15 Adult

(a) Differential growth rates in a human. The arms and legs lengthen more

during growth than the head and trunk, as can be seen in this conceptualization of an individual at different ages all rescaled to the same height.

Age (years)

Different allometric patterns– Contribute to the contrasting shapes of human

and chimpanzee skulls

Chimpanzee fetus Chimpanzee adult

Human fetus Human adult

(b) Comparison of chimpanzee and human skull growth.

The fetal skulls of humans and chimpanzees are similar in shape. Allometric growth transforms the rounded skull and vertical face of a newborn chimpanzee into the elongated skull and sloping face characteristic of adult apes. The same allometric pattern of growth occurs in humans, but with a less accelerated elongation of the jaw relative to the rest of the skull.

● Parasite- induced variation

- parasites may produce structural modifications

*in bees, Andrena, parasitism by Stylops (Order Strepsiptera) frequently results in reduction in the size of the head, enlargement of abdomen, & changes in pubescence & wing venation

Stylops are twisted-winged parasites usually have active first stage larvae that attach to adult hosts, ride back to the host’s nest and burrow into the host’s larva where they develop as legless internal parasites, staying inside their hosts at least until adulthood.

This photo shows a paper wasp with strepsipteran pupae sticking out between the abdominal tergites.

● Post-mortem changes

-changes in preserved specimens

- from the chemical action of preservatives or killing agents

e.g.: certain yellow insects, wasps, are overexposed to cyanide which turn bright red

● Sexual dimorphism (genetic variation)

1. primary sex differences – involving the primary organs utilized in reproduction (gonads)

2. secondary sex differences- very distinct differences between males & females, as in birds of paradise, hummingbirds, & ducks

● Gynandromorphs (intersexes)

- individuals that show male characters in one part of the body & female in another part

- thus, the 2 halves of the body may be of opposite sexes, the division may be transverse, or may be scattered in a mosaic

- by unequal somatic distribution of chromosomes particularly in sex chromosomes

A gynandromorph of Papilio dardanus showing the difference in morphology of males and females

a mosaic gynandromorph of a male Morpho rhetenor cacica, left side of this butterfly is all male, while the right side shows a good deal of the female coloration

mosaic gynandromorph