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Evolutionary Constraints

By Levi Clancy for Student Reader on

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The three main constraints on evolution are:

  1. Disequilibrium - Natural selection only produces optimal adaptations at equilibrium

  2. Local vs global optima - Natural selection is blind, and doesn’t always reach optima

  3. Laws of physics and chemistry


At equilibrium, population will be stable. Change in any direction will be disadvantageous. Natural selection only produces optimal adaptations at equilibrium. Evolution will maintain status quo. But if environmental change was recent, population won’t be well-adapted. For example, on the Galapagos Islands, droughts follow the El Nino weather storms; birds need to "catch up" to their new conditions.

The growing hazards to our health
By Jeannine Stein, Times Staff Writer

More people are gaining weight and binge drinking than ever before, resulting in a host of preventable problems and deaths, according to two federal studies published last week in the Journal of the American Medical Assn. And the number of related illnesses isn't expected to decline in the year ahead.

The country's obesity rate rose from 19.8% in 2000 to 20.9% in 2001. Diabetes diagnoses climbed as well, from 7.3% in 2000 to 7.9% in 2001, according to the federal Centers for Disease Control and Prevention, which surveyed about 200,000 adults across the country. In addition to diabetes, obesity is also linked to high blood pressure and heart disease.

Other examples of disequilibrium in humans:

  • Carpal tunnel syndrome

  • Cavities

  • Nearsightedness

Local vs Global Optima

Natural selection is blind, and doesn’t always reach optima. Imagine a mountaineer sets off to reach mountain peak. It’s foggy. She follows rule, “Always go up” On Mt Kilimanjaro, rule works fine. On Mt Kenya, rule may not work. Selection is like a near-sighted mountaineer. Imagine fitness landscape is like topographic map. Population can be at local peak. Population can’t reach global peak without dropping into valley. Selection avoids valleys. Population will not always reach global peak.

Laws of Physics & Chemistry

Why can't male mammals lactate? Hormones that facilitate milk production inhibit testosterone production. Why can't elephants fly? Heavy animals need strong bones. Strong bones are heavy. Heavy animals can't fly.