Evolution and Conservation

Given urgent need, limited resources and competing interests, what are our priorities for conservation, and why? What considerations should be brought to bear on decisions that affect biodiversity?

As discussed in the introduction, biological systems evolve. Variables change, because evolution is change over time through descent with modification. So in the field of conservation, history is a critical part of how we think about the complex issue of conservation.

The case studies in this section focus on how an understanding of evolution can inform conservation efforts.

courtesy of UC Berkeley

Scientists estimate that about 1000 nesting Kemp’s Ridley sea turtles, 300 right whales, and 65 northern hairy-nosed wombats survive in the wild, to name just a few of the world’s endangered species.1 But what do those numbers mean? Are 65 hairy-nosed wombats enough to save a species teetering on the edge of extinction? Ignoring evolutionary history, one might answer, “Sure; as long as they can breed, we only need a few individuals to start a new population.” But evolutionary theory tells a different story.

According to evolutionary theory, very small populations face two dangers — inbreeding depression and low genetic variation — that might keep them from recovering, despite our best efforts to preserve them.

courtesy of UC Berkeley

In a small population, matings between relatives are common. This inbreeding may lower the population’s ability to survive and reproduce, a phenomenon called inbreeding depression. For example, a population of 40 adders (Vipera berus, shown above) experienced inbreeding depression when farming activities in Sweden isolated them from other adder populations. Higher proportions of stillborn and deformed offspring were born in the isolated population than in the larger populations. When researchers introduced adders from other populations — an example of outbreeding — the isolated population recovered and produced a higher proportion of viable offspring.

The explanation for inbreeding depression lies in the evolutionary history of the population. Over time, natural selection weeds deleterious alleles out of a population — when the dominant deleterious alleles are expressed, they lower the carrier’s fitness, and fewer copies wind up in the next generation. But recessive deleterious alleles are “hidden” from natural selection by their dominant non-deleterious counterparts. An individual carrying a single recessive deleterious allele will be healthy and can easily pass the deleterious allele into the next generation.

When the population is large, this is generally not a problem — the population may carry many recessive deleterious alleles, but they are rarely expressed. However, when the population becomes small, close relatives end up mating with one another, and those relatives likely carry the same recessive deleterious alleles. When the relatives mate, the offspring may inherit two copies of the same recessive deleterious allele and suffer the consequences of expressing the deleterious allele, as shown in the example below. In the case of the Swedish adders, that meant stillborn offspring and deformities.

For Swedish adders, the solution to the inbreeding depression problem was simple—introduce adders from other populations. But if the northern hairy-nosed wombat suffers from inbreeding depression, there are no other populations that can rescue it. Understanding the evolutionary history of a population and the likelihood that it carries recessive deleterious alleles, suggests that we should not allow population sizes to dip too low in our conservation efforts, or inbreeding depression may jeopardize the survival of the species.

courtesy of UC Berkeley

Genetic variation is the raw material of evolution. Without genetic variation, a population cannot evolve in response to changing environmental variables and, as a result, may face an increased risk of extinction. For example, if a population is exposed to a new disease, selection will act on genes for resistance to the disease if they exist in the population. But if they do not exist — if the right genetic variation is not present — the population will not evolve and could be wiped out by the disease.

As an endangered species dwindles, it loses genetic variation — and even if the species rebounds, its level of genetic variation will not. Genetic variation will only slowly be restored through the accumulation of mutations over many generations. For this reason, an endangered species with low genetic variation may risk extinction long after its population size has recovered.

Evolutionary theory suggests that, for the long-term survival of a species, we need to conserve not just individual members of a species, but also a species’ ability to evolve in the face of changing environmental variables — which means conserving individuals and genetic variation.

The risk of extinction or population decline because of low genetic variation is predicted by evolutionary theory. Scientists have not yet found any absolutely clear-cut examples of this in endangered species today, but they continue to investigate the possibility. A case study of the cheetah, which has famously low genetic variation, suggests the sorts of dangers that are possible. When the captive felines at an Oregon breeding colony for large cats were exposed to a potentially deadly virus, it swept through the cheetah population, killing about 50% as a direct or indirect result of the virus — but none of the lions even developed symptoms.

Although this example is by no means conclusive, it is possible that the cheetahs’ low genetic variation — unlike the lions’ more extensive variation — meant that none of them had the right immune system gene variants to fend off the disease. Similar epidemics could sweep through other vulnerable species with low genetic variation, increasing their chance of extinction.

courtesy of UC Berkeley

Because of limited resources, conservationists must inevitably make a difficult decision — which ecosystems should we try to preserve? Studying the evolutionary history of the organisms that comprise those ecosystems can help us make decisions that maximize the biodiversity preserved.

By some estimates, we are losing biodiversity at a rate that will halve the number of species on Earth within the next 100 years.1 There are many reasons for trying to slow this rate of loss, but it is certain that, no matter what measures we take, we simply will not be able to save everything. We will have to decide which species and habitats to concentrate our efforts on — and evolutionary history can help us make these difficult choices.

A “saved” species will not be safe for long if its habitat has been destroyed — so conservation efforts have increasingly focused on preserving entire ecosystems, along with the species that comprise them. But how do we decide which ecosystems to preserve? Many scientists argue that we should prioritize ecosystems with the highest biodiversity — and, although there are many other important considerations involved in making these decisions, phylogenetics provides a useful measure of biodiversity.

The following simplified example illustrates how phylogenetics can aid conservation efforts. Imagine that a government only has the resources to create a preserve in one of three river basins (A, B or C below), and a local biologist is hired to help choose the location of the preserve.

She discovers that each river basin happens to support four related fish species, among other organisms, and she constructs a phylogeny for that fish clade.

In this phylogeny, the vertical length of each branch indicates the amount of evolutionary change that occurred in that lineage. The four species in river basin A cluster in a tightly-knit clade, with little evolutionary differentiation among species, while river basin B supports a more diverse clade of fish — note the longer branches for clade B. River basin C, supports fish that are rather distantly related to one another and that have evolved in different directions.

Based on this information, the biologist might recommend preserving basin C since it supports a more diverse assemblage of species. But of course, in real life, many more considerations — such as the status of other organisms, whether the species are found only locally or are widely distributed, and the economic value of the area — are involved in making such decisions. Nonetheless, evolutionary history makes a more meaningful measure of biodiversity than mere species counts, and can help conservation efforts preserve more genetic, morphological, and ecological diversity.

courtesy of UC Berkeley

Understanding evolution helps us solve biological problems that impact our lives. There are excellent examples of this in the conservation field. An understanding of the relationship between population size and genetic variation can give us a better idea of when a population is endangered. Artificial selection in hatcheries can impact wild populations when those hatchery fish are released. Phylogenetics with an eye toward biodiversity can help conservationists make decisions about where to focus their efforts. In these ways, knowledge of evolution guides conservation work and policies.

courtesy of UC Berkeley

Relevance of evolution: conservation

Laboratory for Conservation and Evolutionary Genetics

Conservation Biology

Conservation Biology: Evolution in Action (Google eBook)