E.1.4 Explain how animal responses can be affected by natural selection, using two examples.

Natural selection is that individuals with favorable heritable variations are more likely to pass on their genes than individuals with unfavorable heritable variations. Good genes will increase reproductive success by prolonging survival by avoiding predators, avoiding disease, obtaining food or attracting mates.

Natural selection requires: a variation in genes, inheritance, over production of offspring and differences in survival and reproduction.

Example 1

Three-Spined Sticklebacks (Gasterosteus aculeatus) – a small fish that perform an elaborate mating ritual.

1.    Male stakes out an area of sand on the bottom of the pond which becomes his territory, defended against all other males. The male sticklebacks then turn red once they have staked a territory.

2.    The male stickleback then digs a little hole 2 inches deep and wide by shoveling sand with his snout. He gathers algae and puts it into the hole, forming a little mound. He then wiggles through the mound creating a tunnel.

3.    When the male turns red he will then begin to court females with a ‘zigzag’ dance by darting towards the female then veering towards the nest. The female, once attracted by the dance, will follow the male to the nest. She will only enter the nest if the male pokes his head in the sand.

4.    Once the female has entered a male’s nest, the male will prod the female near the base of her tail with his snout which will release an entire sequence of muscular actions involved in liberating her eggs causing eggs to be laid in the nest. The male fertilizes the eggs, and then chases the female off.

5.    Whilst the male is waiting for the eggs to hatch, he keeps an eye on predators and also makes sure the water around the eggs has sufficient oxygen by 'fanning' fresher water towards the eggs.

Behavior 1: The stickleback turning red is a stimulus that triggers a aggressive behaviour which makes attack any other male in its territory.

Experiment: Present models of fish painted in different colors. Red will only attack a red coloured model. Advantage of attacking red fish only is it prevents a male from attacking non reproductive males.

Behavior 2: The male poking his head of the sand is also a stimulus which causes females to enter the nest.

Experiment: Poke red model into the sand of an aquarium. This will cause the females to wriggle through the non-existent nests.

Behavior 3: Males do the zigzag dance with females which are full of eggs. The female’s swollen belly is a stimulus that triggers the male’s attraction. Fat belly=more eggs=increased off spring

Example 2

Wasps

1.    Wasps will climb a sprig of grass and place a paralyzed green caterpillar in the crook of the grass blade.

2.    They will then caress it with their antennae and memorize its location.

3.    The wasp will them move a few inches and dig a hole in the ground.

4.    The wasp will then move the caterpillar to the hole where she will pack it in with a single egg and camouflage the entrance with dirt. The egg will hatch a tiny grub which will eat the worm until it is consumed.

5.    Then it will weave an underground cocoon and transmute into a new wasp that will dig its way out and go hunt for a male.

Experiment: When the wasp goes to dig a hole; move the caterpillar to another nearby blade. When the wasp is done with the hole she will return to the original sprig of grass looking for her prey. When she doesn’t find it she will hurriedly return to the ground and retrace her path to the hole and back again. Then she will try some nearby blades, climbing them until finds where the caterpillar is. She will get acquainted with the new position and go dig another hole.

Her heritable pattern of activity is not flexible enough to accommodate the new position of the caterpillar with respect to the position of the first hole.