Explain how variation of traits, natural selection and extinction produce the diversity of species over many generations

Learning goal: Explain how variation of traits within a species, combined with changing environmental conditions, leads over many generations to evolution, the origin of new species and extinction; work a counting model of how common a trait becomes from one generation to the next; and read the fossil record as evidence that the number and types of species on Earth have changed.

Variation comes first. Individuals of one species are not identical. Beetles of one species differ in shell color, seeds from one parent plant differ in size, and puppies in one litter differ in coat and temperament. Part of that variation is inherited, passed from parents to offspring through genetic information. New variation arises from the mixing of genetic information in sexual reproduction and from mutation, rare changes in genetic information that are not aimed at any goal. The crucial point for this lesson is that the environment does not create the trait a population needs. Selection can only sort among the versions of a trait that are present at the time; new versions arise by mutation at random, not because they are needed, whether before or after conditions change. When the environment changes, it changes which of the traits present help their owners survive and reproduce. In the beetle model below, the variation is present before the change and no new variation appears.

A trait is beneficial only in relation to conditions. Thick fur is beneficial in a cold place and a burden in a hot one. A dark shell hides a beetle on dark soil and exposes it on pale sand. Because of this, the question "is this a good trait?" has no answer until someone names the environment. When environmental conditions change, the value of a trait for the survival and reproductive success of an organism, or of a whole species, changes with them. Notice that both words matter. A trait that helps an individual survive but leaves it unable to reproduce does not become more common, because inherited traits spread only through offspring.

Natural selection, stated with its conditions. In a population where (1) individuals vary in a trait, (2) the variation is at least partly inherited, and (3) individuals with some versions of the trait survive and reproduce more successfully than others under the current conditions, the versions linked with greater reproductive success become more common in later generations. All three conditions are needed. If the variation is not inherited, the offspring do not carry it. If every version reproduces equally, selection produces no shift, although in a small population chance alone can still change the proportions. Over many generations, repeated shifts of this kind change the population. That change across generations is evolution.

A counting model, worked in the prose. Suppose a hypothetical beetle population lives on pale soil until a flood leaves the ground covered with dark mud that stays for many years. Before the change, out of every 100 beetles, 80 are light and 20 are dark. Assume that in the new dark environment, birds find light beetles easily, so only 40 percent of light beetles survive to reproduce, while 80 percent of dark beetles survive to reproduce. Assume each survivor leaves the same number of offspring and each offspring has its parent's color. Generation 0: 80 light, 20 dark. Survivors: 80 × 0.40 = 32 light and 20 × 0.80 = 16 dark, 48 in all. If each survivor leaves two offspring, generation 1 has 64 light and 32 dark, 96 beetles. The dark share is 32 ÷ 96 = one third, about 33 percent, up from 20 percent. Repeating the same rule on that generation gives a dark share of one half, then two thirds, then four fifths, then eight ninths: rounded to whole beetles per 100, the dark share runs 20, 33, 50, 67, 80, 89 across generations 0 to 5. The diagram shows these six numbers. No beetle changed color. The population's mixture changed because dark beetles left proportionally more offspring, generation after generation.

What the model leaves out. It is a counting model of a visible trait, not a model of genes, so it cannot predict how genetic information is arranged in the population; that is a topic for a later biology course. Its arithmetic is deliberately nothing more than counting survivors; the algebra of gene frequencies belongs to high-school biology. It assumes survival rates stay fixed, that offspring always match the parent, that no beetles move in or out and that no new variation appears. Real populations break every one of these assumptions to some degree. Chance also matters: a storm can kill well-camouflaged beetles as easily as exposed ones. The model shows the direction of change that differences in reproductive success produce; it does not give exact real-world numbers.

Speciation: where diversity of species comes from. When a population is split, for example by a river, a mountain range, or simply distance, the two parts experience different conditions and different chance events. Each accumulates its own changes across many generations. If the differences grow until members of the two groups no longer interbreed successfully when they meet, they have become separate species. This process, repeated across the history of life, is the source of the diversity of species. It usually takes far more generations than a person can watch, so most of the evidence for it comes from fossils and from comparing living populations, though a few cases, in plants and in fast-breeding animals, have been observed in progress.

Extinction. Throughout Earth's history, a species has become extinct when the environment changed and the individual organisms of that species did not have the traits necessary to survive and reproduce in the changed environment. Extinction is not a rare accident; scientists estimate that more than 99 percent of the species that have ever lived on Earth are now extinct. A species that is well matched to its environment today has no guarantee for tomorrow, because conditions keep changing. Variation within a species is what gives it any chance of containing individuals suited to the new conditions.

The fossil record as evidence. A fossil is a preserved remain or trace of an organism: a shell, a bone, an impression of a leaf, a footprint. Fossils form in layers of sediment, and in an undisturbed sequence the lower layers formed earlier than the layers above them. Reading upward through layers is reading forward through time. Doing so shows that species appear in the record, persist for a time and disappear, and that the number and types of species differ from one age to another. Some fossils are transitional forms: organisms with features intermediate between an ancestral group and its later descendants, such as fossil relatives of whales that still had small hind limbs. Some transitional forms have been found; others are missing, because fossilization is rare. Trilobites, a group of hard-shelled sea animals, are abundant in Ohio's oldest exposed rock layers around Cincinnati and are found nowhere alive today; a trilobite, Isotelus, is Ohio's designated state fossil. The same limestone layers full of marine shells show that shallow seas once covered the area, so fossils are evidence of past environmental conditions as well as of past organisms.

Limits of the record. The fossil record is incomplete. Organisms without hard parts rarely fossilize, many layers have been eroded away, and layers of some ages are missing entirely from a region. Ohio's surviving bedrock, for instance, largely skips the age of the dinosaurs, so their absence in Ohio rock is a gap in the rocks, not evidence that none lived here. A careful reader says what the record shows for the layers examined and does not claim more.

Misconceptions. "The beetles turned dark because they needed to" treats an individual as if it could change its inherited traits on purpose; individuals do not, and the shift happens in the population's mixture. "Evolution makes species better" is wrong as stated; a change is only better or worse relative to particular conditions, and the same trait can go from beneficial to harmful when the environment changes. "If a species survived this long it must be safe" ignores that more than 99 percent of species did not.

Investigation you can do. Cut 80 small paper dots, 40 dark and 40 light; scatter 20 of each on a large sheet of dark paper and keep the rest as spares. A partner, acting as a bird, has ten seconds to pick up dots one at a time while looking away between picks. Count survivors of each color, then bring the total back to 40 with spare dots, adding each color in the same proportion as the survivors (round to whole dots), and repeat for four rounds, recording counts and percentages each round. A result in which the dark percentage rises across rounds supports the claim that differences in survival change the population's mixture. Record the starting counts, the time limit, the background and who acted as predator, and note what the model cannot show: real generations take time, offspring are not exact copies, and no new variation was allowed to appear.

Worked model, generation 1 with counts: Start with 100 beetles: 80 light, 20 dark. Survival to reproduction on dark soil: light 40 percent, dark 80 percent. Survivors: 80 × 0.40 = 32 light, 20 × 0.80 = 16 dark. Each survivor leaves two offspring of its own color: 64 light, 32 dark, total 96. Dark share: 32 ÷ 96 = 1/3 ≈ 33 percent. The count rose from 20 percent to 33 percent without any beetle changing color, because dark beetles left proportionally more offspring.

Worked model, generations 2 and 3 with counts: Only the shares matter, so pick a convenient population size each time. Generation 1 is two thirds light and one third dark: take 300 beetles, 200 light and 100 dark. Survivors: 200 × 0.40 = 80 light and 100 × 0.80 = 80 dark, equal numbers, so generation 2 is one half dark. Generation 2: take 100 light and 100 dark. Survivors: 100 × 0.40 = 40 light and 100 × 0.80 = 80 dark, so generation 3 is 80 ÷ 120 = two thirds dark, about 67 percent. Because dark beetles survive twice as often, the ratio of dark to light doubles each generation (1:4, 1:2, 1:1, 2:1, 4:1, 8:1), which is why the diagram climbs 20, 33, 50, 67, 80, 89. Nothing here is more than counting survivors.

Worked reversal, the environment changes back: Suppose that after generation 5 (eight ninths dark, one ninth light) the mud is washed away and the soil is pale again, so survival rates swap: dark 40 percent, light 80 percent. Take 90 beetles: 80 dark and 10 light. Survivors: 80 × 0.40 = 32 dark and 10 × 0.80 = 8 light, so the next generation is 32 ÷ 40 = 80 percent dark: the share falls from 89 percent to 80 percent and will keep falling if pale soil persists. The same trait went from beneficial to harmful when the environment changed; nothing about the beetles themselves changed.

Worked fossil reading: A hypothetical undisturbed cliff has four layers, and the fossils have been identified to species: brachiopod species A, trilobite species B, coral species C and coral species D. Bottom: A and B (2 species). Second: A, B and C (3 species). Third: A, C and D, no trilobites (3 species). Top: C and D only (2 species). Reading upward: C appears in the second layer and D in the third, B is not found above the second, A is not found above the third. This supports the claim that the number and types of species in this place changed over the time the layers formed. (Counting broad groups such as "corals" would not do; each group contains many species.) It does not by itself prove that species B went extinct everywhere, or even that it vanished from this place, because a layer can lack a species that lived there but was not preserved, a single cliff records one place, and nothing here says why any species disappeared.

Supported record: Build a table for the beetle model with one row for each generation, 0 to 5, and these columns: the dark share at the start of that generation; a convenient count of light and dark beetles with that share; the survivors of each color; and the dark share of the survivors, which becomes the starting share in the next row. Draw the starting shares for generations 0 to 5 as a bar chart and check them against the diagram's numbers (20, 33, 50, 67, 80, 89 per 100). Write one sentence stating which assumption of the model you think is least realistic and why.

Independent performance: Run the paper-dot predator simulation for four rounds on a dark background, then repeat the whole simulation on a light background. Success criteria: (a) starting counts, the time limit, the background and the predator are recorded for both runs; (b) counts and percentages of each color are recorded after every round and the arithmetic is correct; (c) a written explanation uses the words variation, survive, reproduce and generations correctly and states the claim the results support; (d) the explanation names at least two things the simulation cannot show about real populations; (e) it states what would have to be true of the environment for the trend in one run to reverse, and connects that to why a trait cannot be called beneficial without naming the conditions.