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7.10 Speciation

Keywords

English Term 中文翻译 Definition & Explanation
Speciation 物种形成 An evolutionary process in which one species splits into two or more species, resulting in the diversity of life forms.
Biological Species Concept 生物学物种概念 Defining a species as a group of populations whose members have the potential to interbreed and produce viable, fertile offspring.
Reproductive Isolation 生殖隔离 Biological barriers that prevent members of two different species from mating and producing viable, fertile offspring, blocking gene flow.
Allopatric Speciation 异域物种形成 Speciation that occurs when biological populations become geographically isolated from each other.
Sympatric Speciation 同域物种形成 Speciation that occurs within populations that live in the same geographic area, often due to polyploidy or behavioral shifts.
Adaptive Radiation 适应性辐射 A period of rapid evolutionary divergence from a single ancestor into many new species as they adapt to newly available ecological niches.

1. What is a Species?

As populations continue to evolve (as seen in 7.8) and gene flow between them is restricted, they become increasingly different. Eventually, this process leads to speciation—the creation of new species, which is the ultimate source of Earth's vast diversity of life forms.

But how do we define a "species"? The most commonly used definition for sexually reproducing organisms is the Biological Species Concept.

  • It states that a species is a group of populations whose members are capable of interbreeding in nature to produce viable, fertile offspring.
  • If they cannot produce fertile offspring together (like a horse and a donkey making a sterile mule), they are considered separate species.

Reproductive Isolation

Speciation officially occurs when two populations become completely reproductively isolated from each other. Various mechanisms maintain this isolation and prevent gene flow:

  • Prezygotic Barriers: Block fertilization from occurring. (e.g., they breed at different times of the year, have mismatched mating dances, or their gametes chemically reject each other).
  • Postzygotic Barriers: Occur after fertilization. (e.g., the hybrid embryo dies early in development, or the resulting hybrid is healthy but completely sterile).

2. Geography and Speciation

How do populations become isolated in the first place?

  1. Allopatric Speciation: Occurs via geographic separation. A physical barrier (like a river changing course or a mountain rising) divides a population into two. Over time, different mutations and different selective pressures in the two new environments cause them to diverge genetically until they can no longer interbreed.
  2. Sympatric Speciation: Occurs without geographic separation. The populations live in the exact same area, but gene flow is reduced by factors such as polyploidy (common in plants), habitat differentiation, or sexual selection.

3. Pace and Patterns of Speciation

Evolution does not always happen at a constant speed. Scientists recognize two different tempos of speciation in the fossil record:

  • Gradualism: Evolution occurs slowly and steadily over hundreds of thousands or millions of years.
  • Punctuated Equilibrium: Evolution occurs very rapidly (in a geological sense) in short bursts, followed by incredibly long periods of stasis (no change).

Divergent Evolution and Adaptive Radiation

Speciation is a form of divergent evolution, where adaptation to new habitats results in phenotypic diversification from a common ancestor.

Speciation rates can be especially rapid during times of adaptive radiation, which occurs when new, vacant habitats suddenly become available (e.g., after a mass extinction, or when a volcanic island forms). A single ancestor can rapidly speciate to fill all the empty niches.

Illustrative Examples of Adaptive Radiation

  • Hawaiian Drosophila: A few fruit flies blew onto the newly formed Hawaiian islands. Because the islands had diverse, empty habitats, they rapidly radiated into hundreds of unique species.
  • Caribbean Anolis Lizards: A similar radiation occurred on Caribbean islands, where lizards rapidly adapted to live in different parts of the vegetation (tree canopy, trunks, or grass).
  • Apple maggot Rhagoletis: A classic example of sympatric speciation happening right now. Some flies started mating and laying eggs on apples (introduced by humans) instead of native hawthorn fruits, creating reproductive isolation without a geographic barrier.

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