Hybridism

“The sterility of first crosses and of their hybrid progeny has not been acquired through natural selection… When two species are crossed… we can understand, on the view that there is no essential distinction between species and varieties, how it is that the sterility is of such variable degree.” — Charles Darwin

If species evolved from common ancestors, why are hybrids between different species often sterile or inviable? The mule—sturdy but sterile—seemed to mark an unbridgeable gulf between species. Darwin argues that hybrid sterility is not a specially endowed barrier but an incidental result of other differences that accumulated during species divergence.

The Sterility Puzzle

Hybrid sterility was considered strong evidence for the fixity of species. The argument went: if species can interbreed and produce fertile offspring, they’re the same species; if not, they’re different species with an impassable barrier between them.

The Challenge

If species evolved gradually from varieties, and varieties can interbreed freely, how did the barrier of sterility arise? It seems to require a special mechanism to prevent species from merging back together.

Sterility Is Not Absolute

Darwin begins by showing that hybrid sterility is not the clear-cut barrier it appears:

Variability in Hybrid Fertility

• Some species crosses produce fully fertile hybrids
• Fertility varies greatly between different species pairs
• The same species may produce fertile hybrids with some relatives but not others
• Reciprocal crosses often differ—A × B may be more fertile than B × A
• Environmental conditions affect hybrid fertility

This variability suggests sterility is not a single, specially created barrier but a complex result of many factors.

Sterility Is Incidental, Not Designed

Darwin’s key argument: sterility was not selected for—it’s a byproduct of other divergences:

Why Sterility Wasn’t Selected

• No advantage: Sterile hybrids can’t pass on the genes that made them sterile
• Can’t be selected: Selection can only favor traits that increase reproduction
• Therefore: Sterility must arise as a side effect of other changes, not as a directly selected trait

Degrees of Sterility

Darwin catalogs the spectrum from full fertility to complete sterility:

The Continuum of Fertility

1. Full fertility: Varieties cross freely and produce fertile offspring
2. Slight reduction: Crosses produce fewer offspring than normal
3. Reduced hybrid fertility: First generation is fertile, but fertility decreases in later generations
4. Sterile hybrids: Offspring are healthy but cannot reproduce
5. Hybrid weakness: Offspring are weak or inviable
6. Failure to cross: Species cannot mate or fertilization fails

This gradation parallels the gradation from varieties to species. Just as species status is a matter of degree, so is reproductive isolation.

Causes of Sterility

Darwin considers various factors that might cause hybrid sterility:

Factors Contributing to Sterility

• Constitutional differences: Accumulated divergences affect reproductive compatibility
• Functional disruption: Hybrid reproductive systems may malfunction
• Incompatible development: Embryonic development may be disrupted
• Environmental sensitivity: Changed conditions may exacerbate incompatibilities

Modern genetics has confirmed Darwin’s intuition: hybrid sterility typically results from incompatibilities between genes that evolved independently in each lineage—exactly the kind of incidental side effect Darwin proposed.

Varieties and Hybrids Compared

Darwin compares crosses between varieties (within species) and between species:

The Parallel

Crosses between very different varieties sometimes show reduced fertility—not as severe as species crosses, but the same phenomenon at a smaller scale. If varieties are incipient species, this is exactly what we’d expect: the factors that cause sterility between species begin to appear in differences between varieties.

Summary of the Argument

Darwin concludes that hybrid sterility, far from being evidence against evolution, is exactly what evolution predicts:

The Evolutionary Explanation

• Species diverge gradually from common ancestors
• Each lineage accumulates changes suited to its conditions
• Some of these changes incidentally affect reproductive compatibility
• The more divergent two species, the more likely their crosses will fail
• Sterility is a byproduct of divergence, not a barrier specially created to prevent crossing

Key Takeaways

• Hybrid sterility is not absolute but varies greatly between species pairs
• Sterility was not selected for—it can’t be, because sterile hybrids don’t reproduce
• Sterility is an incidental byproduct of other differences accumulated during divergence
• There’s a continuum from full fertility (varieties) to complete sterility (distant species)
• Even varieties sometimes show slight fertility reduction when crossed
• Hybrid sterility supports, rather than refutes, common descent