Variation is how a species survives

Word	What it means
Biological diversity	The variety of living things: how many species live in an area, and how much the members of each species differ.
Species	Organisms that can breed together and produce fertile offspring. Horses and donkeys can produce mules, but mules are almost always infertile, so horses and donkeys are two species.
Variation	Differences among members of one species. Discrete variation falls into clear groups (blood type); continuous variation takes any value in a range (height).
Niche	A species' role in its ecosystem: what it eats, where and when it feeds, what eats it.
Extinct / extirpated	Extinct: gone everywhere, for good. Extirpated: gone from one area but still living elsewhere.
Asexual reproduction	One parent, and offspring genetically identical to it, called clones.
Sexual reproduction	Two parents each supply a gamete (sex cell). The gametes join at fertilization, and the offspring is a new mix of both.
DNA / gene	DNA is the molecule that carries an organism's instructions. A gene is a section of DNA that carries the instructions for one characteristic.
Chromosome	One long, coiled strand of DNA. Human body cells have 46, in 23 pairs.
Mitosis / meiosis	Mitosis makes two cells identical to the original, for growth, repair and asexual reproduction. Meiosis makes gametes, with half the chromosomes.
Dominant / recessive	A dominant form of a gene shows whenever it is present. A recessive form shows only when both copies are recessive.
Heritable	Passed on through genes, like blood type. A scar or a language you learned is not.
Natural / artificial selection	Natural: the environment decides which individuals survive and reproduce. Artificial, or selective breeding: people decide which individuals breed.
Genetic engineering	Changing an organism's DNA directly, often by adding a gene from another species.

Discrete or continuous. There is nothing between blood type A and blood type B, so blood type is discrete. Measure the height of a class and plot it, and the bars make a smooth hump instead of separate columns, so height is continuous.

Niches reduce competition, the struggle between organisms for the same food, light or space. Several species of warbler feed in the same spruce trees, each in a different part of the tree, so they are not all after the same insects. When two species need exactly the same resources in the same place, one usually pushes the other out.

On the test. Two populations of the same wildflower: one varies widely in height, colour and leaf shape; the other is nearly uniform. A new disease arrives. Which is more likely to survive? The varied one — because it is more likely to include a few individuals that happen to resist this particular disease.

• Binary fission: the cell splits in two. Bacteria, amoebas.
• Budding: an outgrowth forms on the parent and breaks off. Yeast, hydras.
• Spores: tiny cells released in huge numbers, each able to grow into a new organism. Bread mould, mushrooms, ferns.
• Vegetative reproduction: a new plant grows from part of an old one — strawberry runners, potato tubers, and the root sprouts that make a stand of trembling aspen one connected clone.

	Asexual	Sexual
Offspring	Identical copies, fast and many	Varied, slower and usually fewer
Needs a mate?	No	Yes
Best when	Conditions are stable	Conditions change

Counting chromosomes. A human body cell has 46. Mitosis makes two cells with 46 each. Meiosis makes gametes with 23 each, and at fertilization 23 + 23 = 46. If gametes carried 46, the number would double every generation; halving it is the whole point of meiosis.

Why copies are a risk. In the 1840s Ireland depended on potatoes, which are planted from pieces of potato rather than seed, and most of the crop was a very few varieties. When potato blight arrived, nearly every plant was vulnerable in the same way and the harvest failed across the country. A population of clones has no individuals that happen to be different.

nucleus → 46 chromosomes → each one long DNA molecule → genes are sections of that DNA

Worked. In Gregor Mendel's pea plants, studied in the 1860s, purple flowers are dominant over white. Write the dominant form P and the recessive form p: PP and Pp plants are purple, and only pp is white. Cross two purple plants that are both Pp. Each parent passes on P or p, so the offspring can be PP, Pp, pP or pp:

Pp × Pp → PP, Pp, pP, pp → 3 purple : 1 white

So two purple parents can have a white-flowered offspring. The white was not new; it was hidden in both parents and showed when one offspring received the recessive form from each.

Heritable	Not heritable	Both play a part
Blood type, natural eye colour	A scar, dyed hair, a language you learned	Height: genes set a range; food and health decide where in it you end up

Natural selection, worked: why antibiotics stop working. A population of bacteria varies, and a few happen to carry a gene that lets them survive an antibiotic. The antibiotic kills most of the others. The survivors reproduce by binary fission, all their offspring carry the gene, and soon most of the population is resistant. No bacterium became resistant because the antibiotic was there; the resistant ones already existed, and the antibiotic removed their competition.

Artificial selection and genetic engineering. Every dog breed descends from tamed wolves, shaped by people choosing which dogs to breed; canola was bred from rapeseed in Canada in the 1970s. Genetic engineering moves a gene directly: since the 1980s, human insulin for people with diabetes has been made by bacteria given the human insulin gene.

Losing diversity. Habitat loss is the biggest cause, then over-hunting, pollution and invasive species. The passenger pigeon, once among the most numerous birds in North America, is extinct; the last one died in a zoo in 1914. The swift fox was extirpated from the Canadian prairies by the late 1930s and has been reintroduced to southern Alberta and Saskatchewan since the 1980s.

Keeping it. A monoculture — one variety planted over a huge area — strips variation out of a field, the Irish lesson again. Seed banks, such as the Svalbard Global Seed Vault in Norway, store seeds of thousands of varieties so their genes survive even if farms stop growing them.

On the test: which kind of reason? A company proposes a genetically engineered crop. “Pollen could carry the new gene to wild relatives” is a scientific concern; “the seed will cost farmers more” is economic; “nobody should own a living thing” is ethical. All three can be true at once.

• Saying an individual adapted or became resistant. Populations change because survivors reproduce.
• Mixing up extinct and extirpated. Extirpated means gone from one place only.
• Thinking dominant means common or stronger. It only means it shows when present. A dominant form can be rare.
• Swapping mitosis and meiosis. Mitosis copies (46 → 46); meiosis halves (46 → 23).
• Calling an acquired characteristic heritable. A scar or a skill is not in the DNA.