Natural selection on a discrete trait

Most sexually reproducing populations contain individuals that display variable behaviors, structures, molecules, and other phenotypes, like the guppies you just studied. Doug Schemske and Paulette Bierzychudek used another variable population and investigated whether that variability was maintained by natural selection. Schemske and Bierzychudek worked with desert snow ( Linanthus parryae ), a small annual plant that has a flower color dimorphism; some plants have blue flowers and others have white flowers. It lives in the deserts of southwest North America. Flower color is determined by a single gene, with blue dominant to white. {Connections: Mendelian inheritance is discussed in Sections 3.1 and 3.2.} Populations of this plant can contain all blue flowered individuals, all white flowered individuals, or a mix of the two.

Schemske and Bierzychudek asked how selection acted on individuals that had different flower color. Individual populations of desert snow tend to be stable in flower color; that is, the frequencies of phenotypes found in one area remains constant over time. They found a shallow ravine where the plants on one side were predominantly blue flowered and the plants on the other side were predominantly white flowered. Over a period of 7 years, they sampled plots along two lines that crossed from one side of the ravine to the other, one at the northern end of the ravine and one at the southern end (Figures 19.5A and C). The flower color data were averaged over many annual censuses.

Figure 19.5 Spatial pattern of flower color along a north (A) and south (C) line that cut through a ravine and estimates of frequencies for four allozymes along the north (B) and south (D) lines. The vertical bar near the center of each panel marks the position of the ravine that divides the populations. From Schemske and Bierzychudek, Figure 4, Copyright 2007, John Wiley and Sons.

The researchers hypothesized that there was intense local selection for flower color, such that blue flowers were favored on one side of the ravine and white on the other. To support this hypothesis, the researchers tested four other genes, predicting that other genetic loci would show no pattern across the ravine if selection were only on flower color; the other genes were not selected for or against (Figures 19.5B and D). To determine the frequencies of alleles of the four genetic loci, the scientists collected individual desert snow plants across the ravine. They extracted enzymes and separated allozymes for the four genes using electrophoresis. The frequency of the most common allozyme produced by each gene across the ravine is plotted in Figures 19.5B and D.

The scientists also planted both white and blue flowered plants in plots on both sides of the ravine to determine their seed production success in the two habitats (Figure 19.6). Note that 1995 was wetter than average, and 1996 was drier than average.

Finally, Schemske and Bierzychudek collected data on environmental factors on the two sides of the ravine to determine what selective factors there might be in the two habitats. They looked at the other plants in the community, which are potential competitors, as well as soil properties (Figure 19.7).

Figure 19.6 Mean seed production (± 1 SE) in 1995 and 1996 for blue- and white-flowered desert snow plants from plots located on the primarily blue-flowered side of the ravine and the primarily white-flowered side of the ravine. NS, not significant; SE, standard error. From Schemske and Bierzychudek, Figure 6, Copyright 2007, John Wiley and Sons. Figure 19.7 Environmental variation of desert snow on two sides of a ravine. A, Area covered for ten plant species. Asterisks indicate that the cover for a species was statistically different on the two sides. B, Differences in soil composition along a transect that crossed the ravine. Ca, calcium; K, potassium; Mg, magnesium; Na, sodium; P, phosphorus; S, sulfur; EC, electroconductivity; %OM, percent organic matter; NH4-N + NO3-N, total nitrogen. * = 0.05, ** = 0.01, *** = 0.001, **** = 0.0001 probability of the observed differences if the null hypothesis of no difference is true. From Schemske and Bierzychudek, Figures 7 & 8, Copyright 2007, John Wiley and Sons.

The scope of Schemske and Bierzychudek’s study, in terms of the area studied and the number of desert snow populations studied, was small. Within the two small populations studied, however, the researchers established that there was a clear difference in flower color on each side of the ravine and that the observed differences persisted over time. That is suggestive of two populations adapted to two specific habitats, although limited dispersal could also explain the distribution of flower types.

Because flower color is determined by one gene, examination of flower color can lead to estimates of allele frequencies at that genetic locus. On the east side of the ravine, where white flowered plants predominate, almost 100% of the alleles in the population were for white flower color. You know this because that characteristic is recessive. On the west side, if all the blue-flowered individuals were heterozygous, the frequency of the white flowered allele would be 50%, and if all blue flowered individuals were homozygous dominant, that frequency would be 0%. {Connection: Recessive and dominant genes are explored and genetic loci are defined in Section 3.1.} We don’t know the exact percentage in the blue flowered populations, but you observed that the frequency of one allele changed across the ravine, something that alleles of the other genes tested do not do. Schemske and Bierzychudek found that four other genes from plants sampled across the ravine varied little. That suggests no selection for or against those enzymes and that only flower color is selected for.

In the experiment where seeds from plants of each color were planted on both sides of the ravine, Schemske and Bierzychudek found that there were differences in seed production, although they were not consistent from year to year. Seed production, and thus reproductive success, varies for the two flower types in the two sides of the ravine. In 1995, a year with greater precipitation, white flowered plants on the white flowered side of the ravine produced more seeds per plant than blue flowered plants. On the blue flowered plant side, blue and white flowered plants produced equal numbers of seeds per plant. In 1996, a drier than average year, all plants produced far fewer seeds, but blue flowered plants produced more seeds per plant than white flowered plants on the blue flowered side of the ravine. White flowered plants are typically more successful in wet years and the blue in dry years. Plant success is also tied to other environmental conditions, evidenced by the different species of plants present and the different soil conditions that plants experience on either side of the ravine.

How the soil differences came to be so great over such a small spatial scale is unknown. The soil environment or some other unknown, unmeasured factor could have then given rise to variation in plant community composition. Either of these factors, soil or the other species of plants present on either side of the ravine, could be the source of selection for flower color on the two sides of the ravine, although Schemske and Bierzychudek did not test individual factors in the soil or in the plant community. However, Schemske and Bierzychudek showed that ecological factors can and do vary, and this variation leads to natural selection on a local scale over short periods of time. Natural selection can eliminate certain characteristics from a population, thereby reducing variation. But natural selection can also maintain variable characteristics by favoring certain types in different local habitats. We now turn our attention to how variation among individuals in a population leads to descent with modification and speciation over much longer periods of time.



19.2 How will communities respond to climate change?

· Context: Predicted future climate change may alter communities and species’ interactions.

· Major theme: Species evolve in the context of their environment.

· Bottom line: As global climate changes, species will evolve, leading to changes in entire communities and the interactions therein.

Biology Learning Objectives

· Describe how global climate is changing.

· Evaluate how organisms are responding evolutionarily to global climate change.

Figure 19.8 Observed continental and global-scale changes in surface temperature with results simulated by climate models using natural and human-caused factors. Ten-year averages are shown in the black line. Lines are dashed where measurements were taken in less than 50% of the area. Pink and blue shaded bands show the range in which 90% of the predictions from computer simulations fell. Figure SPM 4, IPCC, 2007: Summary for Policymakers. In: Climate Change 2007

The Intergovernmental Panel on Climate Change (IPCC), sanctioned by the United Nations and comprised of hundreds of climatologists and policymakers from many countries, comes out with a report on climate change every 5 to 6 years. The most recent report, published in 2007, stated that the average global temperature is rising on every continent and in the oceans (Figure 19.8) and that it is very likely that humans are contributing to the change.

The IPCC concluded this using observed data, theory, and computer models. In Figure 19.8 you see that the observed data are very close to or within the range of results obtained from computer models using both natural and human-caused factors, whereas models using only natural factors, such as solar activity and volcanoes, did not fit the observed data as well, especially during the last half of the 20th century. Human-caused factors associated with temperature increases include burning of fossil fuels, deforestation, and altered land uses, all of which affect the production of greenhouse gases. The IPCC projects that temperatures will continue to rise and the rate of increase will be dependent upon our actions to curb production of greenhouse gases. Global average temperature at the end of the 21st century is predicted to be 2o to 6o C warmer than the current global average temperature. For the next couple of decades, warming of about 0.2° C per decade is predicted. Other aspects of climate also change when average temperatures increase. {Connections: The impact of climate change on ecological system homeostasis is discussed in Section 30.3.} For instance, some regions of the planet are predicted to receive more precipitation, others less. Some areas will be subject to stronger and more frequent storms, including hurricanes.

Although climate has changed often during the history of the planet, the changes occurring now are rapid and global, and they are outside the range experienced by humans since before the development of agriculture. Ecological systems have begun to change during the past few decades, and studies have documented changes in species interactions, seasonal activity patterns, and expansion of geographic ranges. In this section, you will see some data on evolutionary responses of species to these climate changes.

Video on how climate change can increase agricultural problems

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