C. Evolution · EvolutionPatterns of Selection: Stabilizing, Directional, Disruptive and Sexual Selection
C3.4 C2.1 A1.10 A1.12About 70 minutes
Learning goals- I can describe stabilizing, directional and disruptive selection and sketch the effect of each on a bell curve.
- I can identify the type of selection from a description or a graph.
- I can explain sexual selection, including competition between males and mate choice.
- I can explain why sexual selection can favour traits that reduce survival.
Words to know
normal distribution
A bell-shaped graph of a trait: most individuals near the average, fewer at the extremes.
stabilizing selection
Selection that favours the average form of a trait and works against both extremes.
directional selection
Selection that favours one extreme of a trait, shifting the average in that direction.
disruptive selection
Selection that favours both extremes and works against the average.
sexual selection
Selection for traits that improve an individual's chances of mating.
intrasexual selection
Competition between members of the same sex (usually males) for mates.
intersexual selection
Mate choice: members of one sex (usually females) choose mates based on traits.
sexual dimorphism
Differences in appearance between males and females of the same species.
Not all selection pushes the same way
Measure the beak depth of 100 finches, the height of 100 maple seedlings or the mass of 100 newborn babies, and graph how many fall in each range. You usually get a bell-shaped curve, a normal distribution: most individuals near the average, a few at each extreme.
Natural selection can change that curve in three different ways, depending on which individuals are most successful. And there is a special kind of selection, sexual selection, that can produce some of the most spectacular (and impractical) traits in nature.
Three patterns of natural selection
1. Stabilizing selection favours the average and works against both extremes. The curve gets narrower and taller, but the average stays the same.
- Example: human birth mass. Historically, babies of very low or very high birth mass had lower survival rates than babies of average mass, so average birth mass was favoured. (Modern medical care has weakened this selection.)
- Example: the number of eggs in a bird's clutch. Too few eggs means few offspring; too many means parents cannot feed them all.
2. Directional selection favours one extreme. The whole curve shifts toward that extreme.
- Examples: darker peppered moths in sooty forests; deeper beaks in medium ground finches after a drought; antibiotic resistance.
3. Disruptive selection favours both extremes and works against the average. The curve can split into two peaks.
- Example: in a West African finch called the black-bellied seedcracker, birds with small beaks crack soft seeds efficiently and birds with large beaks crack hard seeds, but medium beaks do neither well. Both small and large beaks are common; medium beaks are rare.
Disruptive selection matters because it can be a first step toward speciation, the formation of new species (see the lesson on speciation).
Worked exampleExample 1: Let's identify the type of selection
Name the pattern of selection in each case: (a) After years of heavy fishing that removes large fish, the average size of adult cod declines. (b) Spiders that build medium-sized webs catch the most insects, and very small or very large webs catch fewer. (c) On an island, some snails live on pale sand and some in dark seaweed; birds eat snails with medium-toned shells more than very light or very dark ones.
- (a) One extreme (small size) is favoured because large fish are caught before they breed. The average shifts toward smaller fish: directional selection (caused by humans).
- (b) The average is favoured and both extremes do worse: stabilizing selection.
- (c) Both extremes (light and dark) are camouflaged on one background each; the average stands out everywhere: disruptive selection.
(a) directional; (b) stabilizing; (c) disruptive.
Sexual selection
Darwin noticed that some traits are hard to explain by survival. A peacock's enormous tail makes it slower and easier for predators to spot. Why would natural selection keep it?
Darwin's answer was sexual selection: selection for traits that improve an individual's chance of mating. A trait can spread if it increases reproduction more than it decreases survival. Remember, fitness is about offspring, not long life.
Sexual selection takes two forms:
- Intrasexual selection ("within one sex"): individuals of the same sex, usually males, compete for access to mates. This favours size, strength and weapons such as antlers and horns. Male moose and white-tailed deer spar with their antlers during the breeding season (the rut).
- Intersexual selection ("between the sexes"): members of one sex, usually females, choose mates. This favours bright colours, long tails, songs and displays. Choosy females may benefit because showy traits can signal a healthy male with good genes.
Sexual selection often produces sexual dimorphism: males and females that look different, like the colourful male and brown female mallard duck.
Worked exampleExample 2: Let's analyse a sexual selection experiment
Male red-winged blackbirds, common in Canadian wetlands each spring, show bright red shoulder patches when they defend their territories. In a classic experiment, researchers dyed the red patches of some males black. Many of the dyed males lost their territories to rivals, while most undyed males kept theirs. What does this suggest?
- The independent variable is the red patch (present or hidden). The dependent variable is whether the male keeps his territory.
- Dyed males lost territories more often, so the red patch helps males compete with other males.
- Males with territories attract mates and raise young, so the patch increases reproductive success.
- This is evidence of intrasexual selection: the patch works as a signal in male-male competition.
The red shoulder patch helps males hold territories against rivals, so it is favoured by intrasexual selection.
Balancing survival and mating
Sexually selected traits come with a cost. Long tails, bright colours and loud songs can attract predators and use energy. Selection balances these costs and benefits: a trait can become more extreme until the extra mating success is outweighed by the extra risk. In some fish (guppies) studied in streams with many predators, males are less colourful than in streams with few predators. Predation pulls one way, mate choice the other.
Just for funA male bowerbird in Australia spends a great deal of time building and decorating a structure of sticks just to impress females, sometimes arranging objects of one favourite colour. It's the bird version of cleaning your room before company comes over.
Hands-on activityMeasure a trait and spot the bell curve
You need: a bag of peanuts in the shell, or pea pods, or about 30 tree leaves of one species (maple leaves work well), a ruler, paper and a pencil. If anyone in your home has a peanut allergy, use leaves or pea pods.
- Measure the length of each item to the nearest millimetre and record it.
- Make a frequency table with ranges (for example 30 to 34 mm, 35 to 39 mm, ...).
- Draw a histogram (bar graph of the frequencies). Does it look roughly bell-shaped? Find the average.
- Imagine a predator that only eats the longest items. Sketch the new curve and name the type of selection.
- Now imagine conditions where only very short and very long items survive. Sketch and name the type of selection.
What to look for: a correctly labelled histogram with a roughly normal shape (small samples can be bumpy); sketch 4 shifted toward shorter lengths (directional); sketch 5 with two peaks (disruptive).
Your turn
1Which type of selection favours the average and narrows the bell curve?
2Which type of selection shifts the average toward one extreme?
3Which type of selection favours both extremes and can produce two peaks?
4At Ram Mountain in Alberta, trophy hunters for many years shot bighorn rams with the largest horns. Over time, average horn size in that population decreased. What type of selection is this?
Directional selection (toward smaller horns), caused by human hunting.
Selection for traits that increase an individual's chances of mating.
6Male moose fighting with antlers is an example of which form of sexual selection?
Intrasexual selection (competition within one sex).
7Why can a trait that increases the risk of being eaten still spread?
If it increases mating success more than it decreases survival, carriers leave more offspring overall, so the trait's alleles become more common.
More practice: warm-up
1What shape is the graph of a trait in most populations?
A bell-shaped curve (normal distribution).
2What is sexual dimorphism?
Differences in appearance between males and females of the same species.
3Name the two forms of sexual selection.
Intrasexual (competition) and intersexual (mate choice).
4Is the peppered moth's change to dark colour stabilizing, directional or disruptive?
5Give one example of stabilizing selection.
Human birth mass (historically) or bird clutch size.
More practice: on your own
1In a population of plants, very short plants are shaded out and very tall plants are snapped by wind. Predict and explain the effect on the height distribution.
Stabilizing selection: medium-height plants survive best, so the curve becomes narrower around the average height. The average stays about the same.
2In a lake, some sticklebacks feed on the bottom (best with thick bodies) and some in open water (best with slim bodies). Fish with in-between bodies feed poorly in both places. What pattern of selection is this and why might it matter?
Disruptive selection. Over time it could split the population into two forms, a possible first step toward two species, especially if each form prefers to mate with its own type.
3Explain why female choice might favour males with bright feathers.
Bright feathers can be costly to produce and keep, so they can signal good health or good genes. Females that choose bright males may have healthier offspring, so their preference and the bright trait both spread.
4Guppies in streams with many predators are less colourful than guppies in streams with few predators. Explain using two kinds of selection.
Sexual selection (female choice) favours colourful males. Natural selection by predators works against colourful males because they are easier to see. Where predators are many, predation wins and males are duller; where predators are few, mate choice wins and males are brighter.
5A graph shows a population's average beak depth increasing from 9.0 mm to 9.5 mm after a drought. Which type of selection is shown? What would you expect if the drought ended and small soft seeds became common again?
Directional selection toward deeper beaks. If soft seeds become common again, selection may reverse (favouring smaller beaks) or become stabilizing, so the average could drift back.
Challenge
1Explain how heavy fishing that removes the largest cod could have long-term economic consequences.
Fishing acts as directional selection for fish that grow slowly and mature smaller and earlier. Over generations, the population's average size falls. Smaller fish produce fewer eggs and give lower catches per fish, so the fishery becomes less productive and slower to recover, even if fishing is reduced.
2In elephant seals, a few large dominant males do most of the mating. Predict how this affects (a) body size in males and (b) differences between males and females.
(a) Strong intrasexual selection favours very large, strong males, so male size increases over generations. (b) Females are not under the same pressure, so sexual dimorphism is extreme: northern elephant seal males are much larger than females.
3Sketch-in-words: describe how a bell curve of a trait would look after many generations of disruptive selection that is so strong that in-between individuals almost never survive. What might happen next?
The curve would have two separate peaks with very few individuals in the middle. If the two groups also begin to mate mainly with their own type (for example because they live in different microhabitats), gene flow between them falls and they may eventually become separate species (sympatric speciation).
Quiz
1. A curve gets narrower but its average stays the same. This is:
2. Antibiotic resistance spreading in bacteria is an example of:
3. Which pattern could split one population into two forms?
4. A female bird choosing a mate with the longest tail is an example of:
5. Why might a showy trait that attracts predators remain common?
6. Male and female mallards look very different. This is called:
Big idea (C3.4): selection can stabilize, shift or split a trait's distribution, and sexual selection can favour traits that help mating even at a cost to survival. Reading and sketching curves practises data skills (A1.10, A1.12).
How to help: Do the leaf or pea pod measuring activity together, and use the histogram to talk about how a predator would change it. Nature shows about animal courtship displays give many examples of sexual selection.
Common difficulties:
- Mixing up stabilizing and disruptive. Stabilizing = middle wins; disruptive = middle loses.
- Thinking sexual selection is separate from fitness. It is part of fitness: reproduction.
- Sketching curves. In directional selection, the whole curve slides; it does not just get taller.
Sources: stabilizing selection on human birth mass, the black-bellied seedcracker (disruptive selection), red-winged blackbird epaulette experiments, guppy colour and predation, and selection on bighorn sheep horns and fish size are documented in ecological research and textbooks.