Evolution of Sex and the Tangled Bank Hypothesis

QUESTION: Evolution of Sex and the Tangled Bank Hypothesis

ANSWER:

The Tangled Bank Hypothesis proposes that sex evolved in order to prepare offspring for the world around them. The term comes from Darwin’s book The Origin, in which he refers to a wide assortment of creatures all competing for light and food on a ‘tangled bank’. The concept states that in any given environment where there exists intense competition for space, food, and resources, a premium is placed on diversification.

Evolution of Sex and the tangled bank hypothesis - The difficulties
Although once popular, the theory turns out to face several difficulties. The theory would predict a greater interest in sex among animals that produce lots of small offspring that compete with one another. Sex is invariably associated with organisms that produce a few large offspring, whereas organisms producing small offspring frequently engage in parthenogenesis. In addition, the evidence from the fossil record suggests that species go for vast periods of geologic time without changing much. Some argue that bacteria ‘evolved’ in such a fashion as to ultimately be responsible for sexual reproduction. But in light of this model it becomes inexplicable why bacteria themselves remain virtually unchanged for billions of years of Earth history.

It should also be noted that we still see organisms today reproducing asexually as well as organisms that reproduce sexually. This raises another difficulty. Why do some organisms persist with asexual reproduction while others have evolved the ability to reproduce sexually?