public Builder<G, C> offspringFraction(final double fraction) {
			_evolutionParams.offspringFraction(fraction);
			return this;
		}
The offspring fraction. <i>Default values is set to {@code 0.6}.</i>
This method call is equivalent to
{@code survivorsFraction(1 - offspringFraction)} and will override
any previously set survivors-fraction.

@see #survivorsFraction(double)

@param fraction the offspring fraction
@return {@code this} builder, for command chaining
@throws java.lang.IllegalArgumentException if the fraction is not
        within the range [0, 1].
jenetics/src/main/java/io/jenetics/engine/Engine.java