FIGURE 17–18 Molecular Clock By comparing the DNA sequences of two or more species, biologists estimate how long the species have been separated. Analyze Data What evidence indicates that species C is more closely related to species B than to species A?
dCalibrating the Clock The use of molecular clocks is not simple, because there is not just one molecular clock in a genome. There are many different clocks, each of which “ticks” at a different rate. This is because some genes accumulate mutations faster than others. These different clocks allow researchers to time different evolutionary events. Think of a conventional clock. If you want to time a brief event, you use the second hand. To time an event that lasts longer, you use the minute hand or the hour hand. In the same way, researchers choose a different molecular clock to compare great apes than to estimate when mammals and fishes shared a common ancestor.
Researchers check the accuracy of molecular clocks by trying to estimate how often mutations occur. In other words, they estimate how often the clock they have chosen “ticks.” To do this, they compare the number of mutations in a particular gene in species whose age has been determined by other methods.
Gene Duplication
Where do new genes come from?
Where did the roughly 25,000 working genes in the human genome come from? Modern genes probably descended from a much smaller number of genes in the earliest life forms. But how could that have happened? One way in which new genes evolve is through the duplication, and then modification, of existing genes.
Copying Genes Most organisms carry several copies of various genes. Sometimes organisms carry two copies of the same gene. Other times there may be thousands of copies. Where do those extra copies come from, and what happens to them?
Remember that homologous chromosomes exchange DNA during meiosis in a process called crossing-over. Sometimes crossing-over involves an unequal swapping of DNA. In other words, one chromosome in the pair gets extra DNA. That extra DNA can carry part of a gene, a full gene, or a longer length of chromosome. Sometimes, in different ways, an entire genome can be duplicated.
Table of Contents
- Formulas and Equations
- Applying Formulas and Equations
- Mean, Median, and Mode
- Estimation
- Using Measurements in Calculations
- Effects of Measurement Errors
- Accuracy
- Precision
- Comparing Accuracy and Precision
- Significant Figures
- Calculating With Significant Figures
- Scientific Notation
- Calculating With Scientific Notation
- Dimensional Analysis
- Applying Dimensional Analysis