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The Cost of Sex

Although some species reproduce asexually, the majority
reproduce sexually

Therefore, it seems that sex has a selective advantage is
most species

But, sex has a cost
The cost of sex

Consider an asexual female. Her offspring will be clones; genetically
identical individuals

Now consider a sexual female. Each one of her offspring will have only
50% of her genes.

Reproductive rates differ as well. Let’s say each female produces two
offspring and then dies
• Asexual female: 100  200  400 
• Sexual female: 100  100 females + 100 males  100 f + 100 m 

To compensate, it would seem that sexually produced offspring should
be twice as fit as asexually produced offspring

Understanding the cost of sex has been described as “the
outstanding puzzle in evolutionary biology”


Some argue that sexual reproduction results in greater
variation for natural selection to act upon
The point is true but can’t explain the cost of sex
• The argument requires that individuals act for the good of
the species rather than for their own good
• Altruism is not an adaptive behavior
Possible solutions to the cost of sex problem

Muller’s Ratchet: Sexual reproduction allows females to reduce the
number of deleterious alleles in their offspring

The Lottery Model or the Tangled Bank Hypothesis: habitat quality varies
spatially and genotypes vary in their ability to grow in different habitats it’s better to have 100 different lottery tickets than 100 copies of the
same ticket

The Red Queen Hypothesis
• "The most curious part of the thing was, that the trees and the other
things round them never changed their places at all: however fast
they went, they never seemed to passanything. "I wonder if all the
things move along with us?" thought poor puzzled Alice. And the
Queen seem to guess her thoughts, for she cried, "Faster! don't try to
talk."
Through the Looking Glass. Lewis Carroll
• The coevolution of parasites and hosts may produce rapid
environmental change
The Legend of Johnny Appleseed
Johnny Appleseed

Born John Chapman in 1774

Spent 49 years planting apple trees inIllinois, Indiana,
Kentucky, Pennsylvania and Ohio

We know there is tremendous variation in the quality of
apples arising from sexual reproduction

Most of the apples that Johnny Appleseed planted were likely
none too tasty
Apple grafting