How can I create a dynamic Select on an IEnumerable<T> at runtime?

Given that I have an IEnumerable<T>, where T is any object, how can I select a specific property from it, given that I know the name of the one of the property names at run time as a string?

For example:

var externalIEnumerable = DataPassedFromConsumingCode(); // `IEnumerable<T>`

string knownPropertyName = "Foo";
var fooSelect = externalIEnumerable.Select(...);

In essence, I'm obviously just doing externalIEnumerable.Select(x=> x.Foo);, but I need to perform this Select at runtime, when I don't have control over when it's initially created.

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ANSWER: Based on AlanT's answer, here's what I actually did:

public Expression<Func<TItem, object>> SelectExpression<TItem>(string fieldName)
{
    var param = Expression.Parameter(typeof(TItem), "item");
    var field = Expression.Property(param, fieldName);
    return Expression.Lambda<Func<TItem, object>>(field, 
        new ParameterExpression[] { param });
}

I kept it as an Expression, because calling Compile caused the IQueryable to be Enumerated, which meant the database was hit unnecessarily. So, to use it, I just do the following:

string primaryKey = _map.GetPrimaryKeys(typeof(TOriginator)).Single();
var primaryKeyExpression = SelectExpression<TOriginator>(primaryKey);
var primaryKeyResults = query.Select(primaryKeyExpression).ToList();