Monday, February 25, 2013

It's all in the genes

OK, today pretend you are an Irish farmer in the mid-1840s, and your primary food supply, your potato crop, has turned to toxic slime underground. Hold that thought.

There's diversity everywhere!
image: Gary Larson
In the previous post, I highlighted 3 main levels that scientists use to better understand the diversity of life around us. But, perhaps you are asking, so what?

Diversity is important! Variety at every spatial scale, from whole ecosystems, to individual species, to the genes we see only under the microscope, has been the norm on Earth for thousands of years.

More recently, though, humans have succeeded in modifying and simplifying it, very thoroughly, but possibly to our own detriment.

Huh? We're doing what?

A greater diversity of ecosystems in a region tends to support more of the species and the ecological processes – such as pollination, seed dispersal, water retention and flow, soil formation, and nutrient retention in soil and plants – that are useful to humans. Most of these are irreplaceable (more details).

Having a higher number of tree species in production forests has been shown to be associated with higher returns, in the form of more wood and improved functioning. This effect may occur in large part because trees of one species that live among trees of several other species: 1) are not close enough to one another to transmit disease easily, and 2) interact with the other species and with the microbes that live all around them, which can lead to higher resistance to disease, insects, and other pathogens that prevent tree growth.

Genetic diversity

Diversity at a much smaller scale – the genetic diversity within conspecific individuals – also contributes to resistance of the species to harm.

You might be asking: if individuals of a certain plant or animal species have a trait that works, why does the world need other individuals with different versions of that trait?

It's a good question that may most often be answered during tough times.

Farmers, as well as dog breeders, recognize the importance of crop or breed diversity and the cross-breeding of plants or animals to combine features not normally found together, such as large grains + high pest resistance or improved flavor + drought resistance. Are you getting that theme of resistance here?

Lower diversity = lower resistance to disease. Mass-producing a single species of plant, usually a high-yield variety with little or no genetic diversity among plants - a practice known as a monoculture- invites disease. When all of the plants have the same genetic makeup, all have the same (lack of) resistance to a pathogen.

This is what happened in Ireland. A single pest or disease can wipe out the crop of an entire country, as the Irish potato famine of 1845 demonstrated.

image: UC Berkeley Evolution 101
With very few potato varieties brought in from the New World, the potato blight destroyed crop after crop, across the country. Food became scarce. Over one million people died, and many more moved abroad to survive. This case was disastrous for potato plant fitness and human fitness.

The advantage to a plant species of having some individuals with slight genetic variations, or having different types of bees or earthworms around, may not be obvious until some uncertain future, when the oddball traits they carry suddenly come in handy. Those plants that carry certain traits may withstand a potato blight, or wetter/drier/hotter conditions, better than others in the population and thus survive or reproduce successfully, despite new environmental conditions.

Thus, variation in how certain genes are expressed, such as thicker cell walls or faster-growing leaves, affects the chances that some individuals will survive a disease or disastrous environmental catastrophe (think drought, fire, flood), enabling the species as a whole to carry on.

Disease outbreaks or environmental changes in a system where all the plants are clones of each other (such as a potato or corn crop or an apple orchard or oil palm plantation) means all are equally susceptible to pests, diseases, drought or flood conditions, or the loss of their pollinator.

Diverse Malaysian tropical rain forest (right) replaced by a
simplified oil palm plantation  photo: Lisa Kelley,
If certain individual plants have traits that help them survive in such a different environment, those traits will get passed on more frequently, but if none have such traits, all the plants might succumb to harsher conditions. In contrast, a natural forest, with its diversity of genes in many species, has a far better chance of containing those genetic traits needed to withstand a new disturbance.

Read more on importance of genetic diversity

Monoculture and the potato famine (Evolution 101, UC Berkeley)
FAO page on harvesting nature's diversity (Food and Agriculture Organisation)


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