Populations & Evolution (AQA A-Level Biology): Revision Notes
Speciation
Understanding allelic frequencies and selection
Natural selection changes how often particular alleles appear within a population's gene pool. The gene pool contains all the alleles from every gene in all individuals of a population at any given time. When we measure how frequently a specific allele occurs within this gene pool, we get the allelic frequency.
Selection pressures influence these frequencies by affecting which organisms survive and reproduce successfully. Environmental changes therefore alter the probability of certain alleles being passed to future generations, leading to evolution by natural selection - essentially a change in allelic frequencies within a population over time.
What is speciation?
Speciation refers to the evolutionary process through which new species develop from existing ones. This process has operated over millions of years, creating the vast diversity of life forms we observe today.
Species Definition
A species can be defined as a group of organisms sharing common ancestry and the same genes (though with different alleles). Most importantly, members of a species can interbreed with one another to produce fertile offspring, but are reproductively separated from other species.
How new species form
The formation of new species occurs primarily through reproductive separation combined with genetic changes due to natural selection. Although individuals within a species typically breed among themselves, they retain the ability to interbreed with members of other populations of the same species.
When populations become separated and undergo different mutations, they begin to diverge genetically from other populations. Each separated population experiences distinct selection pressures based on their local environment, leading to adaptive radiation. This process causes changes in allelic frequencies as each population adapts to its specific conditions.
Over time, these genetic differences may become so significant that even if populations are reunited, they cannot successfully interbreed. Each population now represents a distinct species with its own gene pool.
Genetic drift in small populations
Genetic drift becomes particularly important in small populations due to their limited genetic diversity. Small populations possess fewer alleles compared to large populations, restricting genetic variety to those alleles present in the founding individuals.
In small populations, each individual's contribution to the gene pool carries greater weight. When specific alleles are passed on, they quickly influence the entire population due to their high frequency. Mutations affecting these alleles can also rapidly spread throughout the population.
Why Genetic Drift Matters More in Small Populations
The effects of genetic drift are more pronounced in small populations, causing relatively rapid changes that increase the likelihood of developing into separate species. Conversely, large populations experience weaker genetic drift effects because mutations affecting single alleles have less impact on the much larger gene pool.
Allopatric speciation
Allopatric speciation (meaning "different countries") occurs when populations become geographically separated by physical barriers. These barriers might include oceans, rivers, mountain ranges, or deserts - obstacles that prevent interbreeding between populations.
The effectiveness of barriers varies between species. While oceans separate populations of land animals like hedgehogs, many birds can cross these same barriers easily. A small stream might isolate snail populations, whereas it poses no barrier to larger, more mobile organisms.
When environmental conditions differ on either side of a barrier, natural selection influences each population differently, leading to adaptations suited to their local conditions. These changes may take hundreds or thousands of generations, but can ultimately result in reproductive separation and new species formation.
Worked Example: Galapagos Finch Speciation
The Galapagos finch provides a classic example of allopatric speciation:
Step 1: Initial colonisation
A single ancestral species colonised different Galapagos islands
Step 2: Geographical isolation
Each island population became isolated from mainland populations
Step 3: Local adaptation
Each population evolved distinct adaptations to suit local food resources and environmental conditions, including different beak shapes and sizes for handling various seed types
Step 4: Reproductive isolation
The accumulated changes eventually prevented successful interbreeding, resulting in multiple separate finch species
Sympatric speciation
Sympatric speciation (meaning "same country") describes species formation within populations occupying the same geographical area, without physical separation. Instead, these populations become reproductively separated through other mechanisms.
Worked Example: Apple Maggot Fly
The apple maggot fly demonstrates sympatric speciation in progress:
Original situation: These insects only laid eggs in hawthorn fruit native to North America
Change introduced: When apple trees were introduced, some flies began laying eggs in apples instead
Behavioural isolation: Female flies tend to lay eggs on the same type of fruit where they developed, while males seek mates on their preferred fruit type
Result: Hawthorn-raised flies usually mate with other hawthorn flies, while apple-raised flies mate with apple flies. Although not yet separate species, continued genetic changes in each population could eventually lead to reproductive isolation.
Worked Example: Temporal Isolation in Frogs
Different frog varieties show how temporal isolation can lead to sympatric speciation:
Mechanism: Wood frogs, Pickerel frogs, Tree frogs, and Bullfrog varieties have different breeding seasons throughout the year (March through August)
Effect: This temporal separation prevents interbreeding even when populations share the same habitat
Outcome: Reproductive isolation occurs without geographical separation
Isolation mechanisms
Several types of isolation mechanisms can prevent successful interbreeding between populations:
| Isolation Type | Description |
|---|---|
| Geographical | Physical barriers like oceans, mountains, or rivers separate populations |
| Ecological | Populations inhabit different habitats within the same area, rarely encountering each other |
| Temporal | Populations have different breeding seasons, preventing mating opportunities |
| Behavioural | Differences in courtship patterns, mating calls, or visual signals prevent successful mating |
| Mechanical | Physical incompatibilities prevent successful mating (e.g., mismatched reproductive structures) |
| Gametic | Biochemical incompatibilities prevent fertilisation even when mating occurs |
| Hybrid sterility | Offspring from cross-species mating are sterile and cannot reproduce |
Worked Example: Hybrid Sterility
Cross-breeding: When horses (64 chromosomes) mate with donkeys (62 chromosomes)
Offspring produced: The resulting mule has 63 chromosomes
Problem during reproduction: During meiosis, these chromosomes cannot pair correctly, producing non-viable gametes
Result: Mules are sterile and cannot reproduce, maintaining separation between horse and donkey species
Key Points to Remember:
- Species are groups capable of interbreeding to produce fertile offspring, reproductively isolated from other groups
- Speciation occurs through reproductive separation followed by genetic divergence due to different selection pressures
- Allopatric speciation involves geographical separation, while sympatric speciation occurs within the same area
- Genetic drift has stronger effects in small populations, accelerating species formation
- Multiple isolation mechanisms can prevent interbreeding, including geographical, temporal, behavioural, and genetic barriers