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What is the population limit that makes consanguinity an issue?

What is the population limit that makes consanguinity an issue?


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A recent incident brought in the news one of the last uncontacted people - the Sentinelese:

the Sentinelese appear to have consistently refused any interaction with the outside world.

There is significant uncertainty as to the group's size, with estimates ranging between 40 and 500 individual

If I understood correctly, the Sentinelese have a rather small population for dozens if not hundreds of generations and I am wondering if consanguinity is not issue (e.g. serious childhood effects) for them.

Question: What is the population limit (lower bound) that makes consanguinity an issue?


I don't have a great knowledge in population genetics but I think your question can be answered by the relationship between loss of heterozygosity and the effective population size.

For an ideal asexually reproducing genetically diploid population, the heterozygosity is lost with increasing generations according to this equation:

$$H_t=left(1-frac{1}{2N} ight)^t H_0$$

where $H_t$ is the heterozygosity at the generation $t$, $H_0$ is the heterozygosity of initial population and $N$ is the population size.

If you analyse this equation then you'll note that heterozygosity exponentially reduces at rate inversely proportional to the population size.

For real populations you have to replace the population size with effective population size. Effective population size for a sexually reproducing population would be:

$$frac{1}{N_e} = frac{1}{4N_m} + frac{1}{4N_f}$$

Where $N_e$ is effective population size, $N_m$ is number of males and $N_f$ is number of females.

You can find the derivation of these formulas in Principles of Population Genetics by Hartl and Clark.

Other than that, the probability of extinction is also higher, the smaller the population is.

When would consanguinity be an issue depends on other factors too such as initial heterozygosity, presence of deleterious alleles in the gene pool and other environmental factors. I don't think there is some kind of mathematical/practical lower bound. The minimum viable population is approximated using simulations.


  • The density of a population can be regulated by various factors, including biotic and abiotic factors and population size.
  • Density-dependent regulation can be affected by factors that affect birth and death rates such as competition and predation.
  • Density-independent regulation can be affected by factors that affect birth and death rates such as abiotic factors and environmental factors, i.e. severe weather and conditions such as fire.
  • New models of life history incorporate ecological concepts that are typically included in r- and K-selection theory in combination with population age structures and mortality factors.
  • interspecific: existing or occurring between different species
  • intraspecific: occurring among members of the same species
  • fecundity: number, rate, or capacity of offspring production

The basic components of population change

At its most basic level, the components of population change are few indeed. A closed population (that is, one in which immigration and emigration do not occur) can change according to the following simple equation: the population (closed) at the end of an interval equals the population at the beginning of the interval, plus births during the interval, minus deaths during the interval. In other words, only addition by births and reduction by deaths can change a closed population.

Populations of nations, regions, continents, islands, or cities, however, are rarely closed in the same way. If the assumption of a closed population is relaxed, in- and out-migration can increase and decrease population size in the same way as do births and deaths thus, the population (open) at the end of an interval equals the population at the beginning of the interval, plus births during the interval, minus deaths, plus in-migrants, minus out-migrants. Hence the study of demographic change requires knowledge of fertility (births), mortality (deaths), and migration. These, in turn, affect not only population size and growth rates but also the composition of the population in terms of such attributes as sex, age, ethnic or racial composition, and geographic distribution.


Growth Characteristics

Populations can sometimes be categorized by their growth characteristics. Species whose populations increase until they reach the carrying capacity of their environment and then level off are referred to as K-selected species. Species whose populations increase rapidly, often exponentially, quickly filling available environments, are referred to as r-selected species.

Characteristics of K-selected species include:

  • Late maturation
  • Fewer, larger young
  • Longer life spans
  • More parental care
  • Intense competition for resources

Characteristics of r-selected species include:

  • Early maturation
  • Numerous, smaller young
  • Shorter lifespans
  • Less parental care
  • A little competition for resources

Would you give up having children to save the planet?

These stark figures show why the climate impact of population size simply cannot be thought about in a silo, according to Dr Katharine Wilkinson, co-author of Drawdown, a book which highlights the most effective solutions to tackle climate change.

“If we had just a billion people on Earth but people were wildly consuming fossil fuels and industrial agriculture was growing and people were eating beef five meals a day, you can imagine a scenario where the population’s very small and actually the impact is still really significant,” she says. “Similarly, if we have a large population but consumption comes way down then that’s also a different scenario.”

Those in the rich world who have decided to have fewer or no children for climate reasons often make this argument. “The impact of our children is considerably more than the impact of children in areas where the birth rate is so much higher,” says Olliff.

This argument is supported by an oft-cited scientific study published in the journal Environmental Research Letters in 2017. The study reviewed the available research on the actions that people in rich countries could take to reduce their climate impact.

“Basically we wanted to know, as an individual in an industrialised country, what can I do that really makes a difference for climate change, that would most reduce my carbon footprint,” says Nicholas, who co-authored the study.

The study found that four choices were consistently high impact in cutting emissions: eating a plant-based diet, living car-free, avoiding flying and having one fewer child. The biggest impact choice of these? Having one fewer child, which would save 58.6 tonnes of carbon per year. The next most effective was living car-free for a year, which would save 2.4 tonnes. “That was basically showing every child that we would choose to create in a high-emitting country has a huge carbon legacy,” says Nicholas.

Assigning responsibility to a parent for a child’s emissions in this way is contentious. Some say a child’s emissions aren’t part of their parents’ ‘carbon footprint’, while others note the risk of framing things in terms of ‘too many people’. “There are certainly many deeply problematic and racist and xenophobic and horrible human rights violations that have happened, or been proposed, in the name of ‘solving’ overpopulation,” says Nicholas.

It also puts an emphasis on personal lifestyle choice, which some say comes at the expense of a focus on more systemic changes to tackle climate change. Effort should go instead to tackling the underlying issues of reliance on fossil fuels and overuse of resources, according to this perspective.

Listen to episodes of the Science Focus Podcast about population:

“I’m not a fan of burdening global care to the choices of individuals, who must often make personal choices against their personal self-interest,” says Deonandan. “To me it makes more sense to create economic incentives and disincentives to guide populations into making more sustainable choices.”

But others argue individual action can scale up to bigger changes. “I think you need individuals to feel engaged and empowered in their sphere of influence that what they can do actually makes a meaningful difference, to get enough people activated to actually solve the problem,” argues Nicholas.

Still, as Wilkinson adds, the climate crisis will absolutely not be solved by individual behaviour change alone. “To the degree that people are thinking about individual behaviour change, I think it’s really good to have a rigorous grounding for that,” she says.

The average emissions per person remain highest in rich, industrialised countries like the US. But population growth has also tended to level off in these countries.

In poorer countries earlier along the ‘demographic transition’, such as India and much of Nigeria, rising populations and a growing middle class increasingly need – and, many argue, deserve – increasing resources. But if these economies develop in a high carbon way, this could lead to rising emissions.

This is why many see supporting people in poorer countries who want to have fewer children as key to reducing emissions globally. Educating girls and family planning are two of the most effective measures for tackling climate change, according to Project Drawdown.

Research also shows that women and girls are particularly vulnerable to the effects of climate change because their roles as caregivers, and providers of food, fuel and water puts them particularly at risk if drought or flooding occurs. This increases the stakes for ensuring they are protected yet further.

“If you actually look at their impact together, educating girls and family planning, or as I prefer to talk about, closing the gaps on access to reproductive healthcare, then it actually turns out … to be the number one solution,” says Wilkinson.

Your human population questions:

Access to good quality family planning is recognised by the UN as a human right and is known to benefit the health and welfare of women and their children. It also brings down fertility rates. Similarly, women with a higher education level tend to have fewer, healthier children and manage their own reproductive health more actively.

Education and reproductive healthcare are things that women and girls should have, says Wilkinson, noting that around the world there are still 132 million school-age girls not in school, and 214 million women who say they have unmet needs for contraception.

“They [education and reproductive healthcare] happen to have these positive ripple effects when we start to add up the individual decisions that a woman or a family makes across the world, and over time start to have real impacts at scale,” she says.

At the same time, it is crucial to avoid the dangerous and problematic territory where the reproductive choices of women are controlled or determined for them one way or the other, she adds.

Similarly, for those in richer countries who may be considering having fewer children due to climate change, the most important thing is that it is a personal decision. “It’s got to be something you choose and you’re happy to choose,” says Olliff. “I’m just trying to raise awareness to make more people feel happy about choosing that decision.”


Spring 2021

BIOG 1250: Politics of Sex & Scientific Research

  • 1 cr., S/U, February 8, 2021 - March 26, 2021, Tuesday 2:40 pm - 4:35 pm
  • Faculty: Caitlin Miller ( [email protected] )

What is sex? How has sex been studied historically? How have political and societal structures shaped the way sex is studied, addressed, and interpreted in scientific research? In this course we will examine and discuss the implicit and explicit biases present in the study of sex across the fields of evolutionary, behavioral ecology, neuroscience, and medical research. We will explore historical and cutting edge research in these fields using a case-study approach. The book "Inferior: How Science Got Women Wrong-and the New Research That's Rewriting the Story&rdquo will be a central text for the course paired with primary literature for each topic and case-study.

BIOG 1250: The Age of Contagion: The Rise and Fall of Viruses

  • 1 cr., S/U, February 8, 2021 - March 26, 2021, Friday 2:40 pm - 4:35 pm
  • Faculty: Rachael Fieweger ([email protected])

For millennia, viral pathogens have been infiltrating the human population causing widespread disease and from their history we can better understand the outbreaks that currently burden mankind. This course takes a case-study approach to introduce students to the world of infectious disease by investigating outbreaks that have occurred throughout the globe during the 20 th and 21 st centuries. During this course, students will explore basic microbiology principles from the unique perspective of global health and epidemiology. Key topics cover the emergence, pathogenesis, control, and socioeconomic effects of viral pandemics, including Ebola, HIV, influenza, and SARS-CoV-2.

BIOG 1250: Defending Against Pathogens: Our Molecular Arsenal

Even though as humans we have our own inherent defense against pathogenic microbes, our immune system, against some pathogens this defense is not enough. To protect against the pathogens that plague us, humans have developed a molecular arsenal, mainly vaccines and antimicrobial agents, in order to aid our immune system and successfully defend against harmful microbes. This course takes students through the history of vaccines and antimicrobial agents that have been developed for some of the toughest pathogens, such as smallpox, tuberculosis, influenza, and SARS-CoV-2. Through this course students will explore the process it takes to develop these tools, the biology behind them, and the implications they have for public health.

BIOG 1250: Keep Calm & Be Science Literate in the Pandemic

Unsubstantiated claims about COVID-19 and vaccines circulate as rapidly as the SARS-CoV-2 virus, and they can be just as serious. How can you distinguish pseudoscience from real science? How can you make well-informed decisions about your health if you don&rsquot know immunology? The answer is that you become science literate &hellip capable of asking questions, finding scientifically reputable sources, determining answers, and engaging in productive social conversations based on your informed views. This course will teach students how to evaluate scientific claims and make science-informed views. It will provide a foundation of basic understanding of the immunology of vaccines and the COVID-19 pandemic. Students will apply science literacy skills by exploring a sociocultural issue of the pandemic, such as vaccine hesitancy, and communicating their informed views.

BIOG 1250: Pathogens of Mice & Men: Animal Models in Medicine

From the current COVID-19 pandemic to Romaine lettuce recalls, pathogens make the news when there are outbreaks, but researchers are always studying these disease-causing microorganisms and rely upon a variety of cell-based and animal models to do so. In this course, students will explore the basics of bacterial pathogenesis, the models that scientists use to learn about pathogens, and the benefits, limitations, and ethics associated with these models. As students explore these topics week by week, they will also learn to read and understand primary literature as we work through relevant sections of a representative primary research article in class. By the end of the course, students will be equipped to present a primary research article of their choice, addressing the topics covered in the course.


Iguana nightmare: Massive iguana population turns Florida into 'Jurassic Park'

Experts believe the Florida heat provides the right situation for the iguanas to thrive and breed, Elina Shirazi reports.

MIAMI –– Hundreds of thousands of unwanted visitors are invading Florida homes and public areas. While these green iguanas seem to be enjoying the sun, residents are comparing their takeover to "Jurassic Park."

"My daughter was home visiting school in the summertime and she took out her camera and literally played the [theme to] 'Jurassic Park' and started videotaping as they were approaching the greens … they will just go running, and you see what looks like little dinosaurs running away from you," Dawn Braeseke, the general manager of Cooper Colony Golf and Country Club, said.

Braeseke said she spends hundreds of dollars each year trying to control the herd of invasive iguanas littering her property.

Trappers say they use special equipment to capture up to a hundred iguanas each day. (Elina Shirazi)

"Here on my property, we're having an issue with some of our pavement being deteriorated by the iguanas that try to burrow underneath the bridges. My sand traps … what used to be kind of fun to see one or two, when you see hundreds a day as you're driving around our golf course, yeah, that's just not fun anymore," Braeseke said.

The iguanas are not native to Florida, but they have been present in the state for at least six decades. Scientists believe they were first shipped as pets from their native habitats in the Caribbean, as well as Central and South America. According to the Florida Fish and Wildlife Conservation Commission (FWC), the iguanas can weigh up to 17 pounds and grow to more than five feet in length. They also have a relatively long life expectancy, surviving for up to 10 years in the wild and 19 years in captivity.

Viral videos show the problem taking on a life of its own, as iguanas are seen battling each other in plain sight on the streets and showing up unannounced in people's toilets. The problem has gotten so bad, the FWC encourages residents to kill the green iguanas on their properties when possible, or better yet, ask the professionals to do it.

Viral videos show the iguanas battling it out in parking lots, and showing up in people's toilets. (Shannon Moskoff via Storyful)

"We've had instances where we've removed them off of people's barrel tile roofs. They're in their attics. We get calls frequently for them falling into people's toilets, digging underneath homes foundation, defecating by a pool… I have lived in Florida my whole life and I have never seen it this bad," Perry Colato, the co-owner of Redline Iguana Removal, said.

Biologists say these iguanas chew through power lines, causing power outages, and they destroy sea walls. (Elina Shirazi)

Colato said iguana removal experts use special equipment to capture up to 100 iguanas each day.


Example 2: The Carrying Capacity of Grazing Cattle

Overgrazing is another example of carrying capacity coming into play. Farmers must be careful to not let cattle overgraze. Once an area is overgrazed it can take a long time for nutrients to return to the soil, and for grasses to regrow. In the meantime, the area has reached its carrying capacity as it can no longer sustain any new cattle. Perhaps the carrying capacity has even been surpassed if the area cannot fully support some of the existing cattle.


Population Growth Curves | Ecology

In the case of J-shaped growth form, the population grows exponentially, and after attaining the peak value, the population may abruptly crash. This increase in population is continued till large amount of food materials exist in the habitat.

After some time, due to increase in population size, food supply in the habitat becomes limited which ultimately results in decrease in population size. For example, many insect populations show explosive increase in numbers during the rainy season, followed by their disappearance at the end of the season.

The following equation exhibits J-shaped growth:

Here dN/dt represents rate of change in population size, r is biotic potential and N stands for population size.

Type # 2. S – Shaped or Sigmoid Curve:

When a few organisms are introduced in an area, the population increase is very slow in the beginning, i.e., positive acceleration phase or lag phase, in the middle phase, the population increase becomes very rapid, i.e., logarithmic phase, and finally in the last phase the population increase is slowed down, i.e., negative acceleration phase, until an equilibrium is attained around which the population size fluctuates according to variability of environment.

The level beyond which no major increase can occur is referred to as saturation level or carrying capacity (K). In the last phase the new organisms are almost equal to the number of dying individuals and thus there is no more increase in population size.

The S-shaped sigmoid growth form is represented by the following equation:


Genetic diversity helps to limit infectious disease

New research by University of Exeter academics shows that genetic diversity helps to reduce the spread of diseases by limiting parasite evolution.

The idea that host diversity can limit disease outbreaks is not new. For example, crop monocultures in agriculture -- which lack genetic diversity -- can suffer severe disease outbreaks that sweep through the entire population. But why is this?

The study led by the University of Exeter provides an answer. To study the effects of host diversity on disease spread, the researchers used a virus that can infect and kill bacteria. The bacteria defend themselves using a sophisticated immune system, known as CRISPR-Cas, which captures random DNA fragments from the virus. This "genetic memory" protects the bacteria against future infections.

CRISPR-Cas generates lots of diversity because every bacterium captures a different piece of virus DNA. Hence, after virus exposure every bacterium with CRISPR-Cas immunity is unique and diversity in the population is therefore high. This turned out to be ideal to test if and why host diversity limits the spread of disease.

In their experiments, the researchers isolated individual bacteria, and grew them either in monoculture, or mixed them together in diverse populations. Dr. Stineke van Houte recalls: "Viruses could spread on monocultures but when the individual bacteria were mixed together, the virus went extinct very rapidly. This revealed a strong monoculture effect in our experimental system."

Next, the researchers investigated why viruses could persist so much easier on monocultures compared to diverse bacterial host populations. They found that this was because of rapid evolution of the virus, which evolved to overcome CRISPR-Cas immunity of bacterial host monocultures. However, on mixed bacterial populations -- which have much more genetic diversity in the CRISPR-Cas system -- the virus was unable to evolve and therefore went extinct. The ability of viruses to evolve high infectivity was thus shown to directly depend on the level of host genetic diversity. Hence, mixing monocultures together can increase the immunity level of the population as a whole, a feature known as herd immunity.

The conceptual insights from this fundamental research on bacteria and their viruses are likely to be general and the conclusions could therefore have future application in for example agriculture and conservation biology.

The paper, "The diversity-generating benefits of a prokaryotic adaptive immune system," is published in the journal Nature.



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