Conservation genetics and conserving biodiversity

by Dr. Joel Cracraft

Evolutionary biology plays a major role in understanding and conserving biodiversity. For example, people who manage endangered and threatened species need to know what genes allow populations to adapt to local environments. As ranges of natural populations shrink due to human activities, those populations tend to shrink too. Smaller populations mean more inbreeding and less genetic variation. This “inbreeding depression” can decrease the fitness of the populations, making it harder for them to adapt to new or changing environments. In such cases, conservation geneticists attempt to design breeding programs that increase genetic variation.

A well-known example is the African cheetah, which underwent a population bottleneck around 12,000 years ago, and then in more modern times was extirpated over much of its range. This has resulted in inbreeding depression, and conservation geneticists have documented a decrease in heterozygosity in wild populations.

Conservation geneticists also gain insight about the extinction process by comparing the genomes of endangered species with their larger historical populations to understand their history of population decline and inbreeding. Improved genetic methods have made it easier to extract DNA from museum specimens (ancient DNA) and thus compare genetic variability over time. This allows scientists to design breeding programs that increase genetic variability in surviving populations, both in the wild and in captivity.

Invasive exotic species are a major global problem. Each year the United States spends billions of dollars addressing the environmental impacts of these species. It's important to identify invasive species and discover their origins quickly. A highly invasive marine green alga, Caulerpa taxifolia, escaped from an Italian aquarium around 1984 and rapidly blanketed much of the coastal Mediterranean. The native strain, found in the Caribbean, the Indo-Pacific, and the Red Sea, grows in small patches and is genetically distinct from the aquarium strain. In 2000, C. taxifolia was discovered in several bays along the southern California coast. Was it the aggressive Mediterranean strain, or the less aggressive native one? Phylogenetic analysis of DNA sequences identified it as the former, catalyzing an immediate effort to eradicate the alga, which was successful by 2006.

By domesticating wild plants and shaping their traits through breeding, humans have made use of the genetic diversity of wild organisms for thousands of years. This is one of the major ways people have used other species for their benefit. Domesticating wild plants and animals allowed human cultures to spread and populations to grow. With the increasing loss of biodiversity, some scientists are focused on ensuring that our conservation efforts also include phylogenetic diversity, with a goal of conserving a large, representative portion of the tree of life.

In addition to the sheer loss of habitat and ecosystem degradation by humans, the most pervasive threat to global ecosystems and biodiversity is climate change. When environments change, the first response of organisms is to move to more benign environmental conditions. If environmental change is incremental over long periods of time, many organisms may be able to adapt. For small organisms, with relatively large population sizes and short generation times, natural selection can often lead to adaptations to the new conditions. But when that change is rapid, organisms may not be able to move or adapt.

Anthropogenic climate change is happening very quickly relative to evolutionary time and is having a profound effect on Earth’s biodiversity. This means many organisms die, and species populations decline, because they cannot move away from rising temperatures or from the ecological changes occurring around them (e.g., loss of food supply).

There is an extensive, growing literature examining the evolutionary consequences of climate change. It is important to distinguish true adaptation from plasticity responses. True adaptation involves changes in population genetics or phenotype related to fitness; plasticity responses (e.g., physiological, behavioral) are built into all organisms when they deal with stresses from the local environment. An example of behavioral plasticity might be the daily movement of organisms, marine or terrestrial, from warmer to cooler conditions to avoid temperature stress. Plasticity is a local phenomenon that may mitigate some effects of climate change on some individuals, but it does not prevent mortality from increasing at a broad population level.

There is a large amount of evidence that in addition to these plastic responses to changing climate, organisms are also undergoing adaptive evolution. Globally, plants have had to adapt to major shifts in temperature and moisture, and as a consequence, long-established traits like spring flowering time have shifted over the past decades to accommodate warmer spring temperatures. Likewise in animals, 50 years of steady climate change has resulted in a shift to smaller body sizes in multiple species of salamanders in the Appalachian Mountains. Smaller body size increases their surface area relative to body mass, which helps the salamanders maintain proper metabolism in these warmer environments.

It is important to realize, however, that although local populations can sometimes exhibit adaptive responses to increased warmth at a local level, this does not mean that these populations have a viable long-term future. For any population or species, response to a selection force (e.g., warmer climate) has limits. Thus, genetic variation allowing for an adaptive response is not unlimited. And a population is embedded in its environment, so loss of breeding habitat or food availability because of climate change may lead to extinction: A population might have an adaptive response to increased temperature but not to loss of habitat. All in all, rapid climate change is a relentless killer.

Try a thought experiment. Imagine that the process of accumulating scientific knowledge within a given discipline—say, evolutionary biology—had stopped in 1990. None of the important discoveries I have mentioned in this Lesson's essays would have taken place. We would not know the identity of numerous disease agents or invasive species. Nor would we have discovered, described, or determined the relationships of countless beneficial organisms that are critical for many industries. We would not have knowledge derived from advances in population genetics over the past decades, nor would we know as much about molecular evolution. We would not have the vast trove of whole genomes, from microbes to humans, which provide knowledge that has become central to understanding diseases, including their origins and spread.

The bottom line is that evolutionary knowledge about life on Earth affects our well-being every day, in ways perhaps less apparent but no less important than any of the other sciences. Evolutionary science is crucial for understanding the history of our planet and our role in its future.