Speciation

Macroevolution results in speciation - the origin of a new species. It occurs when populations become genetically distinct to the point where they can no longer interbreed and produce fertile offspring.

While microevolution focuses on changes in allele frequencies within a population, macroevolution looks at the broader patterns that emerge when populations diverge enough to become separate species.

In biology, a species is typically defined as a group of organisms that can successfully reproduce with one another and produce viable, fertile offspring. This is known as the biological species concept. If two organisms cannot do this - either because they do not mate, or because their offspring are not fertile - they are considered separate species. There are obviously issues with this definition, as it does not apply to asexual or extinct species, which is why there are multiple definitions for the concept of a species. The biological species is just the most commonly used.

The formation of new species usually requires reproductive isolation, meaning there is no gene flow between populations.

Allopatric Speciation

Allopatric speciation occurs when a population is divided by a physical barrier, such as a mountain range, river, or canyon. The term “allopatric” comes from Greek roots meaning “different country.” Once separated, the populations can no longer interbreed, and they may experience different mutations, selective pressures, and genetic drift. Over time, these differences can lead to the formation of new species.

Sympatric Speciation

Sympatric speciation happens within a single geographic area. The term “sympatric” means “same country.” In this case, reproductive isolation arises even though the populations are not physically separated. This can occur due to differences in behavior, habitat use, or genetics.

One example of something that can cause this is polyploidy, where a mistake during cell division results in an organism with extra sets of chromosomes. These individuals cannot reproduce with the original population, creating instant reproductive isolation. In animals, sympatric speciation can happen when certain individuals begin to prefer different mating behaviors, leading to reduced gene flow over time.

Reproductive Isolating Mechanisms

In order for speciation to occur, populations must be reproductively isolated. These mechanisms prevent individuals from producing fertile offspring, even if they live in the same area or come into contact. They are grouped into two categories: prezygotic (before fertilization) and postzygotic (after fertilization).

Prezygotic Isolating Mechanisms

These prevent mating or fertilization from happening.

Habitat isolation – Species live in the same area but occupy different habitats or niches.
Temporal isolation – Species breed at different times of day or year.
Behavioral isolation – Differences in courtship behavior or mating rituals prevent attraction.
Mechanical isolation – Physical differences in reproductive organs prevent successful mating.
Gametic isolation – Gametes are incompatible; fertilization cannot occur even if mating does.

Postzygotic Isolating Mechanisms

These occur after fertilization and reduce the viability or fertility of the offspring.

Reduced hybrid viability – The hybrid embryo fails to develop properly or dies early.
Reduced hybrid fertility – The hybrid survives but is sterile (e.g., a mule).
Hybrid breakdown – The first-generation hybrid is fertile, but its offspring are weak or sterile in later generations.

The Speed of Speciation

Not all speciation happens at the same speed. Scientists recognize two broad patterns for how evolution can occur, both of which can be found in nature:

Gradualism

Gradualism is the idea that species evolve slowly and steadily through the accumulation of small changes over long periods of time. In this model, you would expect to see many intermediate forms in the fossil record.

Punctuated Equilibrium

Punctuated equilibrium proposes that species remain relatively unchanged for long periods, but occasionally experience rapid bursts of change.

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