Speciation

A species is defined as a group of organisms that can interbreed to produce fertile offspring. The key word is fertile — offspring that can themselves reproduce.

Horses and donkeys can physically mate, and they do produce offspring (mules). However, mules are sterile — they cannot reproduce. This means the criterion for being the same species is not met.

The reason horses and donkeys are different species is that their offspring (mules) cannot produce fertile offspring. Physical appearance and chromosome numbers may differ, but the defining criterion is reproductive compatibility leading to fertile offspring.

The question asks why two organisms that can mate are still classified as different species. This tests understanding of the Biological Species Concept.

Physical appearance alone does not determine species status. Many organisms look very similar but are different species; others look different but are the same species. Morphology is not the defining criterion here.

Geographic location is relevant to allopatric speciation but is not the definition of a species. Organisms from different locations can still be the same species if they can produce fertile offspring together.

While chromosome differences can contribute to sterility, this is not the stated reason given in the question for classifying them as different species. The question is asking about the species concept, not the mechanism.

The correct answer is that their offspring cannot produce fertile offspring of their own. This directly reflects the Biological Species Concept: the inability to produce fertile offspring means they are separate species.

The lizard population was physically separated from the mainland population by an ocean — a geographic barrier. This is the defining feature of one specific type of speciation.

Allopatric speciation occurs when populations are separated by a geographic barrier, preventing gene flow. The ocean between the island and mainland is exactly this kind of barrier.

Over 500,000 years, isolated populations evolve independently under different selection pressures until reproductive isolation develops. This is the classic process of allopatric speciation.

The question asks which type of speciation occurred, given that a population was separated by an ocean (geographic barrier) before diverging.

Sympatric speciation occurs without geographic separation — populations live in the same area. Here, the island and mainland are 200 km apart, clearly a geographic separation, so this is not sympatric.

Polyploidy involves errors in chromosome duplication during cell division and is most common in plants. There is no mention of chromosome doubling in this scenario.

'Temporal speciation' is not a standard classification. Temporal isolation is a type of reproductive isolation (breeding at different times), not a category of speciation itself.

Allopatric speciation is correct. The ocean is a geographic barrier that separated the populations, stopped gene flow, and allowed independent evolution until reproductive isolation developed.