SBI3U Chpt 8 - The Origin of Species

Selective pressure

• Selective pressures are anything in the environment that affects which individuals survive and reproduce.

• They can be abiotic (non-living) or biotic (living).

Directional Selection

• One extreme trait is favoured.

e.g. If most flowers become very deep, only birds with long beaks can reach the nectar.

→ Long-beaked birds survive more → population shifts toward longer beaks.

Stabilizing Selection

• The average trait is favoured; extremes are not.

e.g. If most flowers are medium depth, birds with medium-length beaks can feed best.

Beaks too short → can’t reach

Beaks too long → awkward, wastes energy

Medium beaks survive → population becomes more average.

Disruptive Selection

• Both extremes are favoured; the average is not.

e.g. Imagine a habitat with only very shallow OR very deep flowers — no medium flowers.

Short beaks → great at shallow flowers

Long beaks → great at deep flowers

Medium beaks → can’t feed well anywhere

Short and long beaks survive, medium beaks die out → population splits into two groups.

Sexual Selection

• Sexual selection is when certain traits increase an organism’s chances of attracting a mate, not surviving predators.

• It’s about who gets chosen to reproduce.

• Sexual selection can produce traits that look “unnecessary” or even “dangerous,” but they’re kept because they help attract mates.

Intersexual Selection

• Females choose a mate based on a trait.

e.g. Female birds choose males with bright feathers, a long tail, nice dance, etc.

Those males reproduce more → that trait becomes more common.

Intrasexual Selection

• Males compete against each other to mate with females.

e.g. Male deer fight with antlers. Bigger/stronger males win the right to mate with the females.

Disadvantages of Sexual Selection

• Avoiding predators is difficult.

e.g. Fringe-lipped bats locate male tungara frogs by listening for their mating calls. Male frogs that calls frequently are more likely to be eaten. Male frogs that never call emain safe but are unable to attract a mate.

Genetic Drift

• Genetic drift is a random change in allele frequencies in a population.

• It happens by chance, not because a trait is better.

• Such changes are more obvious in small populations.

Genetic Bottleneck

• A bottleneck happens when a large population suddenly becomes very small due to a disaster.

• Cause: Natural disasters, Diseases, Habitat loss

• Only a few survivors remain, and their alleles become the new gene pool — even if they’re not representative of the original population.

• Result: Less genetic diversity, lots of random allele changes, and more genetic drift.

Founder Effect

• The founder effect happens when a small group leaves the main population and “founds” a new population elsewhere.

• Cause:

  • Migration to a new area

  • Geographic isolation (islands, valleys)

• Effect: The new group only carries a small sample of alleles from the original population.

• Result:

  • Different allele frequencies from the original population

  • Reduced diversity

  • Strong genetic drift

Hardy–Weinberg Principle

• Allele frequencies in a population stay the same unless something disrupts them.

• It describes an ideal, non-evolving population — used as a baseline to see if evolution is happening.

Hardy–Weinberg Conditions

• Natural selection: favours the passing on of some alleles over others.

• Small population size: increases the likelihood of genetic drift.

• Mutation: introduces new alleles to a population.

• Immigration/Emigration: introduces or removes alleles in a population.

• Horizontal gene transfer: the gaining of new alleles from a different species.

Microevolution

Small changes in allele frequencies within a population over a short time.

Speciation

The formation of a new species when populations become genetically isolated.

Reproductive Isolating Mechanisms

Traits or behaviours that prevent different species from mating or producing fertile offspring.

Prezygotic Mechanisms

• Barriers that prevent mating or fertilization.

• Before fertilization, no zygote forms.

Examples: different habitats, mating seasons, courtship behaviours, incompatible sex organs.

Behavioral Isolation

• Prezygotic Isolation

• Different courtship behaviours prevent mating.

• Example: Different mating dances, calls, or pheromones.

e.g. Male frogs of different species have unique calls that attract only females of their species.

Temporal Isolation

• Species mate at different times (day, season, or year).

• Example: One species mates in spring, another in fall.

e.g. Pussy willows produce flowers in the early spring. They are reproductively isolated from plant species that produce flowers at a different time of year.

Ecological (Habitat) Isolation

• Species live or mate in different habitats within the same area.

• Example: One insect lives on tree tops, another on roots.

e.g. The mountain bluebird (Sialia currucoides) lives at high elevations, while the eastern bluebird (Sialia sialis) prefers lower elevations and does not encounter the mountain species.

Mechanical Isolation

• Physical differences prevent mating or pollen transfer.

• Example: Incompatible reproductive organs or flower shapes.

e.g. Male damselflies transfer sperm during an unusual mating flight. The male and female genitalia of each species are uniquely shaped and are physically incompatible with other species.

Gametic Isolation

• Sperm and egg cannot fuse even if mating occurs.

• Example: Sea urchin sperm can’t fertilize eggs of another species.

e.g. Many marine animals including corals, clams, and sea cucumbers release their sperm and eggs into open water. The sperm recognize eggs of their own species through chemical markers on the surface of the eggs.

Postzygotic Mechanisms

• Barriers that occur after fertilization.

• After fertilization, the zygote forms but fails.

Examples: hybrid offspring die early, are weak, or are infertile (like mules).

Zygotic Mortality

• The zygote forms but dies early during development.

e.g. Some species of sheep and goat can mate, but the zygote is not viable.

Hybrid Inviability

• The hybrid is born but is weak or cannot survive to adulthood.

e.g. When tigers and leopards are crossed, the zygote begins to develop, but the pregnancy ends in a miscarriage or stillborn offspring.

Hybrid Infertility

• The hybrid survives but cannot reproduce.

• Example: Mule (horse × donkey).

e.g. Mules are the sterile hybrid of a horse-donkey cross.

Allopatric Speciation

• New species form when populations are physically separated by a geographic barrier (mountains, rivers, islands).

• No gene flow → populations evolve differently → new species.

e.g. About two million years ago, the Isthmus of Panama formed and created land between the Caribbean Sea and the Pacific Ocean. Marine species that once lived in a single large population were split into two distinct groups. Because the groups could no longer interbreed, they evolved separately over time. Eventually, the populations became different species that cannot successfully reproduce with each other. In snapping shrimp, seven original species became fourteen distinct species, with one species on each side of the isthmus.

Sympatric Speciation

• New species form in the same geographic area, without physical separation.

• Reproductive isolation happens due to behaviour, niche use, or genetics (common in plants).

e.g. In North America, hawthorn flies originally laid their eggs on hawthorn trees, but after apple trees were introduced, some flies began laying eggs on apples instead. This caused the population to split into two groups that fed and mated on different plants. Disruptive selection favoured traits that helped each group survive on its food source, which reduced interbreeding. Over time, the two populations became reproductively isolated. In plants, sympatric speciation can happen quickly when a mutation prevents normal sexual reproduction and the plant reproduces asexually, creating a new species.

Sympatric Speciation + Polyploidy

• Sympatric speciation can also occur through polyploidy, which is a sudden increase in the number of chromosome sets.

• Because the change happens suddenly, polyploidy can create a new species in one generation, without geographic separation.

e.g. the Cope’s gray treefrog (Hyla chrysoscelis) and the eastern gray treefrog (Hyla versicolor). These two species live in the same geographic area and look almost identical, but the eastern gray treefrog has twice as many chromosomes as Cope’s gray treefrog. This chromosome difference prevents them from successfully interbreeding, even though they share the same habitat.