Questions: Peripatric Speciation and Founder Effects

Question 1

How does peripatric speciation differ from standard allopatric speciation, and what makes it capable of producing reproductive isolation more rapidly?

• A. Peripatric speciation involves selection operating on both populations simultaneously, accelerating divergence
• B. In peripatric speciation, a small founding population is subject to intense genetic drift, which rapidly alters allele frequencies beyond what gradual selection alone would produce
• C. Peripatric speciation differs only in geography — island populations always speciate faster because of different food resources
• D. Peripatric speciation requires a shorter geographic barrier because island populations are more reproductively isolated to begin with

Answer: (B)

The defining feature of peripatric speciation is the extreme smallness of the founding population, not just geographic isolation. In a large allopatric population, allele frequencies change slowly — the law of large numbers keeps rare alleles rare and common alleles common, requiring many generations for substantial divergence. In a tiny founding population of tens or hundreds of individuals, random genetic drift dominates: allele frequencies can swing dramatically in just a few generations, including the possible fixation of rare alleles or loss of common ones. This rapid reshuffling can affect alleles involved in reproductive compatibility, potentially generating reproductive isolation on timescales that large-population allopatric speciation cannot match.

Question 2

A mainland bird population of millions sends a small group of individuals to an isolated island. The mainland population has an allele at a mate-recognition locus at 3% frequency. After 50 generations on the island, this allele is at 70%. What is the most likely explanation?

• A. Strong positive selection favored this allele in the island's new environment
• B. The allele was at high frequency in the small founding population by sampling chance (founder effect), and drift maintained it
• C. Gene flow between the island and mainland populations drove the allele to high frequency
• D. Mutation pressure generated many new copies of this allele on the island

Answer: (B)

The founder effect is the sampling bias that occurs when a small group carries only a fraction of the ancestral population's genetic variation. By chance, the few founders may carry this rare allele at a higher-than-average frequency (or carry it at all, if they happen to be heterozygous). Once the founding population is small, drift can rapidly move this allele from the founding frequency to 70% in relatively few generations without requiring any selective advantage. While selection (option A) is possible, the scenario describes a mate-recognition locus with no stated environmental difference, making drift the parsimonious explanation. Gene flow (option C) would tend to keep island and mainland frequencies similar, not diverge them.

Question 3

In peripatric speciation, genetic drift in the small founding population can produce reproductive isolation even without natural selection acting on any loci directly involved in reproduction.

• T. True
• F. False

Answer: True

This is counterintuitive but important: reproductive isolation does not require selection explicitly favoring reproductive barriers. Drift randomly fixes or nearly-fixes alleles — including those involved in gamete recognition, developmental timing, or mating signals — that happen to be different from the mainland population. When island and mainland individuals later meet, these drifted alleles at compatibility loci can produce inviable or infertile hybrids, even though no selection was specifically targeting reproductive isolation. The isolation is a side effect of the rapid allele frequency changes driven by drift in the small population.

Question 4

The founder effect ensures that a founding population begins with the same allele frequencies as the ancestral population, just expressed in a smaller number of individuals.

• T. True
• F. False

Answer: False

This inverts the meaning of the founder effect. The founder effect describes a sampling error: when a small group is drawn from a large population, the sample's allele frequencies will deviate from the source population's by chance. Rare alleles may be absent entirely from the founding group; other alleles may be at higher frequency than in the source simply because the few founders happen to carry them. The smaller the founding group, the larger the expected deviation. This initial deviation, amplified by subsequent drift, is precisely what makes peripatric speciation potentially faster than standard allopatric speciation in large populations.

Question 5

Why does small founding population size accelerate speciation compared to a large geographically isolated population, even when both are equally isolated from the ancestral group?

Model answer: Population size determines how strongly genetic drift operates relative to selection. In a large isolated population, drift is weak — allele frequencies change slowly and predictably, and selection is the dominant force shaping divergence over many generations. In a small founding population, drift is the dominant force: allele frequencies can change dramatically within a few generations regardless of selective value, and rare alleles can reach fixation (or be lost) by chance. This includes alleles involved in mate recognition, gamete compatibility, and developmental processes — the very alleles whose divergence can produce reproductive isolation. Small populations also face novel environments with strong selective pressures. The combination of rapid genetic change by drift plus potentially strong local selection produces reproductive isolation on a compressed timescale.

The classic examples — Hawaiian honeycreepers, Galápagos finches — illustrate this: small founders on isolated islands diversified into dozens of morphologically and behaviorally distinct species. The same process in a large continental population would require orders of magnitude more time, because drift-driven frequency changes are averaged away by the large sample size.