SCNC1112 Chap.12 Evolution, Ecology and the Anthropocene
Evolution, Ecology and the Anthropocene
Natural selection is best handled as an argument rather than a slogan. More offspring are produced than can survive; resources are limited; individuals vary; much of that variation is heritable; individuals better suited to the current environment leave more offspring; and therefore the composition of the population shifts across generations.
Each statement can be checked separately, and the conclusion fails if any one of them does. Only the fifth does real work, and it is about reproduction rather than about strength. Two stages are routinely collapsed into one. Mutation creates heritable variation at random with respect to what would be useful, and selection is not random at all because the environment does the filtering.
Keeping them separate removes the persistent error that organisms mutate in the direction they need, and it explains why a trait favourable in one environment is a liability in another. Artificial selection, where a human chooses which individuals breed, and sexual selection, where mates do the choosing, share the same mechanism with a different filter.
Four independent lines of evidence support the framework, and their independence is the strength. The same bone arrangement appears in a human arm, a bat wing and a whale flipper doing quite different jobs. Reduced remnants with little current function persist, such as hip bones in whales and snakes. Fossils occur in a consistent order in undisturbed layers, and the order has never been found inverted.
Molecular differences between species match the branching pattern anatomy had already suggested, and neutral changes accumulating at a steady rate give a molecular clock that can be checked against the fossil record. The ecology material brings Module 0 back. An ecosystem can be read as an energy budget: producers capture light, consumers eat producers, and other consumers eat them.
At each transfer most of the energy is lost as heat and as the cost of staying alive, and only around a tenth is built into the next level. Food chains are therefore short and top predators are necessarily rare, which is a statement about energy rather than about ferocity, and the same arithmetic explains why a kilogram of meat costs far more land and energy than a kilogram of grain.
Matter behaves differently: carbon, nitrogen and phosphorus cycle rather than passing through once. The final teaching lecture asks what science can do about the human era, and biodiversity gives the argument a functional form. More diverse ecosystems capture resources more completely, recycle nutrients faster and resist disturbance better, because a declining species can be covered by another.
The rivet analogy makes the risk concrete: removing one or two rivets may change nothing visible, each removal raises the chance of sudden collapse, and some have no redundancy at all. Extinction is the one pressure on the list that cannot be reversed.
What this chapter covers
- 01
Selection Set Out as Six Statements
- 02
Random Mutation Against Non-Random Selection
- 03
Four Independent Lines of Evidence
- 04
Speciation and the Molecular Clock
- 05
Trophic Levels and Ten Per Cent Transfer
- 06
Biodiversity, Rivets and Irreversibility
Explaining antibiotic resistance without the phrase survival of the fittest
- 2Establish where the variation came from. Mutation produced variation in resistance before exposure, at random with respect to whether an antibiotic would ever appear.
- 2Name the filter. The antibiotic kills susceptible individuals, so resistant ones leave far more descendants and their share of the population rises quickly because generations are short.
- 2Say what did not happen. The bacteria did not develop resistance in response to the threat. Nothing about the exposure directed the mutations that were already there.
- 2Propose the discriminating evidence. Culture a population that has never met the antibiotic, isolate separate lines, then test whether resistance appears in lines that were never exposed. Finding it there shows the variation preceded the selection.
Key terms
- Natural Selection
- The non-random survival and reproduction of individuals whose heritable traits suit the current environment, acting on variation that arose at random.
- Homologous Structure
- A feature with the same underlying arrangement in different species used for different purposes, taken as evidence of common ancestry.
- Vestigial Structure
- A reduced remnant with little or no current function, retained from an ancestor in which it was functional.
- Speciation
- The process by which one population becomes two that can no longer interbreed, most often beginning with geographical separation.
- Molecular Clock
- The use of neutral sequence differences, which accumulate at a steady average rate, to estimate when two lineages separated.
- Trophic Level
- An organism's position in a food chain, with roughly a tenth of the energy at one level passing into the next.
- Rivet Hypothesis
- The view that species can be lost from an ecosystem with little visible effect until the accumulated loss brings sudden collapse.
Evolution, Ecology and the Anthropocene FAQ
Did humans evolve from chimpanzees?
No. Humans and chimpanzees share a common ancestor that lived several million years ago and is long extinct, so neither is descended from the other. The chromosome counts differ because sequence corresponding to one human chromosome is carried as two in the chimpanzee, which is a difference in packaging rather than in content; the genomes remain very similar.
Why can a species be lost with no measurable effect and still matter?
Because the observation is consistent with redundancy rather than with unimportance. Other species absorbed the role, which is precisely what diversity buys. The rivet argument is that the cost of each successive loss rises rather than staying flat, and that some roles have no second occupant at all.
What does the ten per cent figure actually describe?
The approximate share of the energy at one trophic level that is built into the bodies of the next. The rest is lost as heat and spent on staying alive. Two consequences follow directly: food chains are short because there is too little left after four or five steps, and predators at the top are necessarily rare relative to the levels below them.
Exam move
Rehearse selection as six numbered statements and practise saying which one a given objection is attacking. Then use the ten per cent rule to answer two questions about diets or land use, and finish by writing the difference between variation arising before and after a selection pressure in one sentence.
