Rapid micro-evolution and loss of chromosomal diversity in Drosophila in response to

climate warming

A study by Francisco Rodriguez-Telles and Miguel A.

Rodriguez

The Problem:

• Global warming is introducing ecological conditions that organisms have never before encountered

• Empirical evidence: bacteria and other simple organisms can evolve quickly in response to ecological pressure– Short generation time– Large, varied populations– Frequent mutation

• Can more complex organisms with smaller population sizes and longer generation times evolve rapidly enough to meet pressures due to higher temperatures?

Study population:Drosophila subobscura

–Drosophila is isothermal – habitat determines

body temperature

The O chromosome

• The O chromosome is linked to thermo-tolerance traits– Hsp70 gene

• Inversions of at least one gene on the O chromosome cause measurable phenotypic differences among individuals– 15 arrangements exist in the population

• Inversion arrangements affect physiology and behavior

Common arrangements

• OST, O3+4, O3+4+7, O3+4+8

• Climate affects geographic distribution of arrangements

• Frequency of arrangements fluctuates with the seasonal cycles

• This fluctuation parallels changes in temperature

Methods

• Drosophila were collected in spring, early summer, late summer, and autumn from 1976-80 and 1988-91 in a southern Palearctic habitat

• Frequencies of common O chromosomal inversion arrangements were recorded in each year

• Chromosomal diversity was calculated:

• IFR = Index of free recombination• Number of inversions• Frequency of inversions• Length of inversions• Amount of euchromatin not involved in the

inversion loops

• Low IFR value means population is becoming more heterogeneous -- more arrangements are present in the population

Climate

• Statistical tests used to determine:– change in climate

over the study period– effect of each variable

and combinations of climactic variables on distribution of arrangements

Results

• Decrease in:– Relative humidity– Annual precipitation

• Temperature increased linearly

• Temperature had strongest correlation to shifts in frequency of arrangements

From top to bottom: summer, spring, autumn, winter

Arrows indicate years in which sampling occurred

Overall, 18.3% loss of diversity in chromosome arrangements

OST arrangement

• Decreased in frequency 40.7%– Negative correlation with year

• Frequency depends on temperature

• Decreased most in late summer and early fall when temperatures were highest

• Strong correlation with latitude– Present more in latitudes with colder

temperatures

O3+4+8 arrangement

• Decreased in frequency in the population– Negatively correlated with year

O3+4 arrangement

• Frequency increased over 16-year-period– Positively correlated with year

• Shift in frequency due to change in temperature and humidity

O3+4+7 arrangement

• No significant shift in frequency– No correlation with year

• Inferred advantage:

– Increased temp led to increased frequency of arrangement O3+4

– O3+4 is present in highest frequencies in warmer parts of the habitat

Therefore, O3+4 is advantageous in warmer climates

– Hypothesis: Selection for this arrangement in warmer temps

• Inferred disadvantage:– Increased temperature led to decrease in

frequency of OST and O3+4+8

– OST and O3+4+8 arrangements more prevalent in colder parts of habitat

– Individuals with OST arrangement are least active during the hottest parts of the day

– OST and O3+4+8 are advantageous in colder climates

– Hypothesis: Selection against these arrangements in warmer temperatures

Supporting Studies

• Gilchrist and Huey, 1999– Temperature sensitivity is heritable– Correlation between increase in preferred

temperature and decreased genetic variation (graph)

• Levitan and Etges– Identified “southern” chromosome arrangements that

have increased in frequency in northern populations in recent years

– Frequency of “northern” arrangements has decreased “almost to the point of extinction”

Implications

• Effect of increased global temperature on Drosophila models possible impact on humans and other complex organisms

• Warmer temperatures lead to a decrease of genetic variety in populations

• If trend continues, directional selection could move some arrangements to fixation and result in the loss of others

• Currently, Drosophila is not experiencing any negative effects from the loss of certain chromosomal inversion arrangements

• In fact the population is responding well to selection pressure

• However, the cost of rapid evolution may be a decrease in variation of arrangements

• Decreased genetic variation decreases species’ capacity to respond to future selection pressure, retain fitness, and avoid extinction

• Natural selection depends on:– Variation in traits– Differential reproductive success– Homogeneity inhibits response to selection

What if we didn’t believe in evolution?

• Could not attribute large-scale loss of genetic diversity to many small changes in each generation that have accumulated

• Could not explain how organisms adapt and evolve in response to conditions they have never before encountered, since ID believes they originated as they are now and have not changed over time

References

• Gilchrist, George W. and Huey, Raymond B. 1999. The direct response of Drosophila melanogaster to selection on knockdown temperature. Nature, 15-29.

• Levitan, Max and Etges, William J. 2005. Climate change and recent genetic flux in populations of Drosophila robusta. BMC Evolutionary Biology, 5:4.

• Rodriguez-Trelles, Francisco and Rodriguez, Miguel A. 1998. Rapid micro-evolution and loss of chromosomal diversity in Drosophila in response to climate warming. Evolutionary Ecology, 829-838.