Evolving like a weed: mustard adapts quickly to climate fluctuations


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By Elsa Youngsteadt

Biologists studying the common weed Brassica rapa, a member of the cabbage family also known as field mustard, have kept a repository of seeds from each years crop of plants. From 1999 to 2004, an unusually long and severe drought struck that region subjecting the field mustard populations to hot, dry conditions for five years in a row. Such conditions usually favor plants that flower early in the spring, plants that grow and reproduce before the heat gets too withering. Scientists wondered whether the mustard populations might have evolved to have earlier flowering times over the course of the five-year drought. They noticed that the plants did indeed flower earlier in 2004 than they had in 1997, but that observation alone could not prove that evolution had occurred. Individual plants might simply adjust their flowering times in response to environmental conditions without underlying genetic changes.

Because the scientists still had seeds from the pre-drought mustard plants collected in 1997, they were in a perfect position to find out whether the change in flowering time was really due to evolution. They grew pre-drought seeds alongside seeds collected from the exact same sites in 2004, after the drought. If the difference in flowering time was a direct response to environmental conditions, the difference between pre-drought and post-drought plants should have gone away when they were grown together at the same time in the same greenhouse. Instead, the post-drought seeds gave rise to plants that flowered earlier than those of the pre-drought seeds, even in the greenhouse, confirming that the shift in flowering time really was due to an underlying change in the plants' genetic program.

Field mustard was probably able to respond quickly to a new natural selection pressure (hot dry weather), because of its fast reproductive rate of one generation per year. Other studies have also demonstrated microevolution - evolution that results in change within a species but not formation of new species - in organisms from fruit flies to squirrels. However, previous studies have done so by comparing information collected from past and present populations, or from members of the same species living in different regions. This study is unique in having stored dormant representatives of a plant population and compared them in real time to individuals collected from the same population at a later date.


University of California-Irvine news release

Yahoo news story

PNAS Article

Climate change information from the EPA

Science magazine coverage of other fast evolution cases

Questions for review and discussion

1. How long did the drought last, and what characteristic of the field mustard changed in response to that drought?

2. How did the researchers show that that change was due to genetic change rather than direct, flexible response to environmental conditions?

3. Define global warming and climate change. How are they related to the process of evolution in this study?

4. What characteristic of the field mustard enabled it to evolve quickly? What other organisms might also be able to adapt quickly to a rapidly changing climate? What organisms would be incapable of such fast evolution?