Expert: Florence M Rollwagen - 7/5/2008

Question
What is the relative success of internal and external fertilisation in relation to the
colonisation of terrestrial environments?

Answer
Hi Stephanie:  Thanks for your question.

Although I think this may be a homework problem, I'll give you some hints to answer it  yourself.

You didn't mention whether you were talking about colonization by plants or animals, but the same principles apply.  Think about plants and animals that live in the water.  They have sperms and eggs that survive on their own, and after fertilization, the embryos are capable of independent life while they grow up.  Little newly hatched fish babies (fry) live on their own while they grow.  

Colonization of the land presents some major problems for both plants and animals, not the least of which is desiccation.  Things dry out when taken out of the water.  Some land animals (amphibians) have not completely overcome this problem, and have to go back to the water to reproduce.  Similarly, some plants (mosses and ferns) have a step in their life cycle where they need water (rain) to reproduce.  This is called alternation of generation.  

Higher plants and animals (trees, birds, reptiles and mammals) have largely overcome the problems of water fertilization.  How?  By adapting to dry conditions with internal fertilization.  Sperms and eggs are entirely internal, as is fertilization and the growth of the next generation.  Birds and reptiles lay eggs, mammals hold their young internally (placenta) until old enough to survive on their own.

I hope this answer has helped you.  Please write back if you have more questions.

FM Rollwagen, PhD