Natural Selection A cause of evolution

Forces of evolutionary change Natural selection – traits that improve survival or reproduction will accumulate in the population adaptive change Genetic drift – frequency of traits can change in a population due to chance events random change

Essence of Darwin’s ideas  Criteria for natural selection  variation exists in populations – important in a dynamic environment  overproduction of offspring  more offspring than the environment can support  competition for limited resourses  for food, mates, nesting sites, escape predators  differential survival  successful traits = advantage  differential reproduction  successful traits become more common in population

Natural Selection Survival and reproduction of the fittest Evolutionary fitness is measured by REPRODUCTIVE SUCCESS! Selection acts on any trait that affects survival or reproduction – predation selection, physiological selection, sexual selection

Predation selection – act on both predator & prey behaviors (ex. safety in numbers) camouflage & mimicry speed defenses (physical & chemical)

Physiological Selection Acting on body functions (physiological) – disease resistance – efficiency at using oxygen, food, etc. – biochemical versatility (ex. able to use multiple food sources) – protection from injury HOT STUFF! Some fish had the variation of producing anti-freeze protein 5.5 mya The Antarctic Ocean freezes over

Sexual Selection Acting on reproductive success – attractiveness to potential mate Example: bird coloration – fertility of gametes – successful rearing of offspring Survival doesn’t matter if you don’t reproduce!

Sexual selection It’s FEMALE CHOICE, baby!

The lion’s mane… Females are attracted to males with larger, dark manes Correlation with higher testosterone levels – better nutrition & health – more muscle & aggression – better sperm count / fertility – longer life But imposes a cost to male – HOT! Is it worth it??

Effects of Selection Deviations from the normal distribution of a population Evolution is NOT goal oriented – different variations can be selected for in different environments Environment affects which way evolving population will deviate and how fast If environment changes, it can change deviation 3 ways a population can deviate:

Distribution Histograms Y axis – number of organisms X axis – some measurable trait

Stabilizing Selection Stabilizing selection – natural selection favors the average individual – Ex. Plant height Too short – can’t compete for sunlight Too tall – sustains wind damage

Directional Selection Directional selection – natural selection favors one extreme or the other – Seen often in changing environments – Ex. Giraffe necks Short-necked cannot reach taller plants

Disruptive Selection Disruptive selection – natural selection favors both extremes (selects against average) – Occurs often when there is 2 microenvironments – Example: limpets growing on light and dark rocks

Peppered moth What was the selection factor? – early 1800s = pre-industrial England low pollution lichen growing on trees = light colored bark – late 1800s = industrial England factories = soot coated trees killed lichen = dark colored bark – mid 1900s = pollution controls clean air laws return of lichen = light colored bark Year% dark% light

Long Term Effects of Selection Some variations significant increase, decrease, or disappear Adaptation – variation that is favored by selection and provides an advantage in a particular environment

Artificial selection Selective breeding can use variations in populations to create vastly different “breeds” & “varieties” “descendants” of the wolf “descendants” of wild mustard

Why does evolution matter now?

Applications of Evolution 1.Antibiotic-resistant bacteria (ex. MRSA)

Bacteria reproduce and mutate at a very high rate. Some bacteria’s DNA has mutated to be resistant to many known antibiotics.

When the resistant bacteria reproduce, they pass this ability onto their offspring. Bacteria can also pass the resistance to other bacteria through conjugation.

To help prevent the development of antibiotic resistant bacteria, – don’t overuse antibiotics (they do not kill viruses!) – use a variety of antibiotics (not just the same one over and over again) – finish the whole dose of antibiotics

Applications of Evolution 2.Pesticide-resistant insects Same mechanism as above. To help prevent this, – don’t overuse pesticides (or don’t use them at all) – raise/buy organic food – use biocontrols (such as lady bugs)

Natural selection in action Insecticide & drug resistance – Insecticide/drug didn’t kill all individuals – resistant survivors reproduce – resistance is inherited – Insecticide/drug becomes less & less effective

Fig. 22-UN2