natural selection

Three main ideas underlie this pivotal aspect of Darwin's theory of evolution:

• all species produce more offspring than the environment can support, and only a fraction of offspring survive to adulthood;
• individuals of a species vary in their traits or characteristics; and
• offspring can inherit many of these variable traits from their parents.

From these three ideas, Darwin came to the theory of natural selection. He saw that when only a portion of the population will survive due to environmental limits, those individuals who have inherited traits that best adapt them to their environment are more likely to survive and reproduce. These individuals are more likely to have more offspring in the next generation, and those offspring that inherit the beneficial traits will also have better survivorship and reproduction. Natural selection is simply the unequal or differential reproduction of individuals in a population.

In nature, the environment determines which variations allow for higher reproduction in those individuals that have them. Over time, the population will change as more individuals have the traits natural selection favors. Eventually, this could lead to new species. Darwin used artificial selection, or selective breeding of plants and animals, as evidence for his theory of natural selection. In artificial selection, humans choose which individuals breed. For example, a farmer may selects a plant with high yields over ones that have long lives but lower yields. Or dog owner may choose to breed individuals with long noses and short ears. Over time, breeding can create new varieties of plants and animals that have very different traits than their wild ancestors. For example, broccoli, cauliflower, cabbage, brussel sprouts, and kale are all varieties of the same species derived from a wild mustard.

 

ocellus - (o-SEL-us), plural, ocelli

simple eyes of some insects. Monarch larvae have 12 ocelli.

 

ommatidium - (oh-mah-TI-dee-um) plural, ommatidia

the units which make up compound eyes in insects. Each ommatidium has two parts. One part gathers light through a lens, while the other senses it through nerves.

 

order

(Biol) the usual major subdivision of a class or subclass in the classification of organisms, consisting of several families

 

overwintering colonies

The two populations of monarch butterflies (west of the Rocky Mountains and east of them) overwinter in California and Mexico respectively. In California, monarchs overwinter in sheltered bays and inland areas along the coast, roosting in eucalyptus trees, Monterey pines, and Monterey cypresses. The eastern population, which constituted 95% of the North American monarch population, migrates to central Mexico. They roost in trees in the mountain ranges. The sites they choose must have trees and underbrush for resting on; cool temperatures but protection from snow and wind; water from clouds and fog; and nearby streams for additional water sources. These roosts were discovered by the scientific community in 1975 (although local villagers had known about them before that) after several years of following tagged butterflies.

 

palp - plural, palpi

paired appendages on an insect head used to sense, and test the quality of, food.

 

panmictic

Panmictic populations are ones that have complete mixing of individuals. This means that every female has an equal chance of mating with any male in the population and vice versa. In a panmictic population, gene flow is unobstructed within the group.

 

parasites

organisms that live in or on a host's body and depend on the host for nutrients and resources necessary to complete their life cycle. Parasites are usually smaller than their hosts (e.g., tapeworms that live in animal intestines) and usually do not kill the host directly, although they may weaken it and make it more susceptible to disease or predation.

 

parasitoids

insects that lay their eggs on or inside another insect species (called their host). The eggs hatch and feed on the host from the inside, eventually killing the host.

 

pathogens

disease-causing organisms. Bacteria, viruses, fungi, and parasites may all be pathogens.

 

pheromones - (FAIR-uh-mohns)

special chemicals released by some animals to communicate with other members of their species. They may be sensed over long distances and help mates find each other. They may also encourage mating at closer distances and help ensure that mating only occurs with other members of the same species.

 

proboscis - (pro-BAHS-kiss)

the adult Monarch's feeding tube, for sucking nectar, which is coiled under the head when not in use.

 

prolegs

the "false" legs on the abdominal segments of the Monarch larva. Only the three pairs on the thorax will become legs in the adult Monarch.

 

protozoan

Protozoans (from the Greek protos, first, and zoion, animal) belong to the kingdom Protista. Protozoans are eukaryotic (see bacteria for a definition of eukaryotic), single-celled organisms that eat food like animals rather than make their own food like plants. Protozoans can eat bacteria, other protists, or organic material suspended in fluid. Some are parasites on animals, including those that cause harmful human diseases, such as sleeping sickness and malaria.

 

pupa - (PU-puh) plural, pupae (PU-pee)

the third stage in metamorphosis, after the larval stage. In Monarchs this stage lasts 8-13 days.

 

pupate

to change from a larva (caterpillar) to a pupa (chrysalis).

 

pyrrolizidine alkaloids

Alkaloids are a class of organic chemicals that contain nitrogen. They are physiologically active in vertebrates, and examples of alkaloids include nicotine and caffeine. Many plants contain specific alkaloids in their tissues, which may ward off animals or help make the plant taste bad to potential herbivores. Pyrrolizidine alkaloids are a specific type of alkaloids that have two 5-member rings linked along the side that contains the nitrogen.

 

restriction enzymes

Restriction enzymes are bacterial enzymes. They cut up foreign DNA by recognizing a sequence of base pairs, attaching the DNA strands, and breaking them at a specific point in the sequence. This protects bacteria against intruding DNA from viruses, phages, and other organisms. Biologists use restriction enzymes to study genetic differences in many organisms, since the restriction enzymes will cut all DNA. In these studies, scientists put restriction enzymes in with the DNA from two different individuals. The enzymes cut the two sets of DNA into pieces called restriction fragments. Different DNA in the two individuals means that the pieces will be different sizes, since the enzymes will cut in different places. By separating the fragments by electrophoresis, scientists can quantify the difference between the two individuals' DNA.

 

scales

overlapping pieces of chitin (the same material of which exoskeletons are made) that insulate butterflies' bodies and wings, improve their aerodynamics, and give them color and markings. Many people think the scales look like fine dust on butterfly wings.

 

sequester

to separate or segregate something from the general whole without changing it. When Monarchs sequester cardenolides, for example, they do not metabolize, or break down, the chemicals but rather shunt them into specific locations in their bodies.

 

spinneret

the organ on the bottom of the larva head from which silk is spun. This is the only silk-producing organ in the larvae.

 

spiracles - (SPEER-uh-kulls)

openings on the thorax and abdomen of insects through which gases are exchanged with the outside air. These lead to long air tubes, or tracheae, that run throughout the body.

 

spore

a resistant stage of an organism's life cycle that can survive unfavorable environmental conditions (e.g., low moisture, high or low temperature, no host present, etc.).

 

stridulation

Over 150 species of butterflies and moths stridulate as pupae. A tiny rasp and file exist on the margins of abdominal segments, and the pupae rub these segments together to creating chirping, creaking, clicking, and humming noises. Some people think this noise is primarily for defense against predators, while other think it has more to do with announcing its emergence to potential mates or attracting ants that will protect them from predation in exchange for honeydew secretions. It is still unclear exactly what purpose stridulation serves for each species.

 

synthesize

Synthesis involves combining two or more parts to make a whole. Biochemically, synthesis occurs when a new product is formed either by joining chemical elements, groups, or simpler compounds, or by breaking apart a complex compound into simpler pieces. Every organism synthesizes biochemicals as part of life. For example, plants use the energy provided by the sun to make sugars (C₂H₆O₂) out of oxygen (O₂), carbon dioxide (CO₂), and water (H₂O).

The four main categories of compounds organisms often synthesize include proteins, nucleic acids, carbohydrates, and fats. Each of these molecules is made up of many smaller parts, called monomers (Greek, mono, one, and meros, part). Organisms can store energy in the bonds that hold together monomers in many of these molecules. When it needs more energy, it breaks the big molecule into its parts and uses the stored energy for other functions.

Some of the chemicals plants and animals synthesize are toxins (in plants, these toxins are often called secondary compounds). These chemicals are expensive, in terms of the energy required to make them. Many ecologists believe organisms invest energy in toxins because the toxins serve as a defense against predation, including herbivory. There are still many unanswered questions, however, about the trade-offs involved in investing in toxin production vs. investing in growth and reproduction.