Natural Selection Image Map

Sex Ratio:       

Lastly, a "sex ratio" (the ratio of males to females) helps us to characterize population. For instance, if your karate class has four boys and six girls, the sex ratio is 4 : 6.  What would be the sex ratio of an all-boy karate class?  Why would such ratios be important to ecosystems? 

Recently, scientists have recognized the significance of the sex ratio in the Columbia River.  In response to environmental pollutants, developing male salmon sometimes undergo a sex reversal.  They change sex!  This phenomenon is considered a major factor in the observed decline in salmon population size.  Why?


You’ve probably heard the word “extinction” before -- most likely in the context of a discussion of dinosaurs or dodo birds.  But did you know that without extinction, we wouldn’t be here?  Since life on earth began, 99.9% of all species have become extinct! 

This means that of all the species that ever existed, only a very small fraction remains around today.  How does this happen?  As the characteristics of a population, like density, age distribution, and sex ratio, change, so does the population’s ability to survive.  Unlike the salmon, most organisms aren’t able to adapt to an environment with only one sex and they become extinct. How do conditions necessary for extinction arise?

Each population occupies a specific place, or niche, in the ecosystem.  If something disturbs that ecological niche, the population’s size may change.  But don’t think that the only way to go is down!  A change may produce population growth as well as decline, which may or may not lead to extinction.  To see how this affects you, see the section "Why It Matters."

Populations on the Genetic Level

In the Cells are Us section on Information Coding and Transmission, you were introduced to genes.  But you may be wondering about their significance in the context of ecosystems?  Think about what members of a population share with each other?  Being of the same species, you would expect individuals to be very similar. 

To get a better perspective, look around at your classmates.  Unless you have a pair of identical twins in your class, each individual will possess a different appearance.  What accounts for this distinction between individuals of the same species?  A large part of this difference comes from having different combinations of dominant and recessive alleles. 


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