Seminar Two
Evolution of sexuality
Asexuality in nature
Do plants reproduce asexually?
Yes, in many different ways. Some clone themselves through vegetative reproduction, such as grasses that spread by means of underground roots and woody plants that sprout new saplings from the base of the parent tree. Some plants reproduce asexually by apomixis, in which genetically identical seeds are formed without prior formation of gametes (as in some dandelions). Often the same plant that reproduces asexually also reproduces sexually by forming seeds.
Do tiny eukaryotes reproduce asexually?
Algae and bdelloid rotifers do. The survival of asexual bdelloid rotifers for tens of millions of years has been called an “evolutionary scandal” by the evolutionary biologist John Maynard Smith! These tiny freshwater invertebrates may have “gotten away” with asexuality because they have extraordinary capabilities: they can dry up completely and blow away on the wind, thereby ridding themselves of a deadly fungal pathogen, which can’t survive the dessication and may not naturally occur in the new environment (Science 327:574-576, 29 Jan. 2010).
Do other multicellular animals reproduce asexually?
Yes—like bdelloid rotifers, by parthenogenesis, in which the egg develops without fertilization. Parthenogenesis is known in invertebrates, fish, amphibians, reptiles, and one bird, the turkey. The egg develops into a female adult. Some parthenogenetic vertebrates require the stimulus of sperm from a male of another species to initiate egg development, without fertilization.
To recap, there are three fundamental “divides” in reproductive biology:
- Sexual versus asexual reproduction
- Internal versus external fertilization
- Parental investment
We are now ready to make the difficult leap to human sexuality, which confronts us with the most challenging questions of all: how did male and female mating priorities evolve, and how do we unravel the tapestry of nature versus nurture? To prepare for this leap, let's take a quick look at behavioral ecology.
