Sexual Reproduction and Multicellularity

What is the evolutionary significance of sexual reproduction?

Sometime after eukaryotic cells arose, they began to reproduce sexually. The development of sexual reproduction sped up evolutionary change because sexual reproduction increases genetic variation.

Significance of Sexual Reproduction When prokaryotes reproduce asexually, they duplicate their genetic material and pass it on to daughter cells. This process is efficient, but it yields daughter cells whose genomes duplicate their parent's genome. Genetic variation is basically restricted to mutations in DNA.

In contrast, when eukaryotes reproduce sexually, offspring receive genetic material from two parents. Meiosis and fertilization shuffle and reshuffle genes, generating lots of genetic diversity. That's why the offspring of sexually reproducing organisms are never identical to either their parents or their siblings (except for identical twins). The more heritable variation, the more “raw material” natural selection has to work on. Genetic variation increases the likelihood of a population's adapting to new or changing environmental conditions.

Multicellularity Multicellular organisms evolved a few hundred million years after the evolution of sexual reproduction. Early multicellular organisms underwent a series of adaptive radiations, resulting in great diversity.

19.3 Assessment

Review Key Concepts

1. 1. Review What was Earth's early atmosphere like?

   2. Explain What does Miller and Urey's experiment tell us about the organic compounds needed for life?

   3. Predict You just read that life arose from nonlife billions of years ago. Could life arise from nonlife today? Why or why not?

2. 1. Review What does the endosymbiotic theory propose?

   2. Explain According to this theory, how did mitochondria evolve?

   3. Apply Concepts What evidence supports the theory?

3. 1. Review Why is the development of sexual reproduction so important in the history of life?

   2. Sequence Put the following events in the order in which they occurred: sexual reproduction, development of eukaryotic cells, free oxygen in the atmosphere, and development of photosynthesis.

WRITE ABOUT SCIENCE

Explanation

4. Write a paragraph explaining the “RNA world” hypothesis. What parts of the hypothesis have yet to be proved? Is it possible that we will never know the origins of RNA and DNA? Explain your answer.

Table of Contents