Question

A number of comparisons of nucleotide sequences among hominids and rodents indicate that inbreeding may have occurred more in hominid than in rodent ancestry. When an ancient population bottleneck leaving approximately 10,000 individuals occurred in humans, Knight (2005) and Bakewell (2007) both suggested that this event may have left early humans with a greater chance of genetic disease. Why would a population bottleneck influence the frequency of genetic disease?

Short answer

Answer: The ancient human population bottleneck reduced genetic diversity within the population and increased levels of inbreeding, which in turn increased the likelihood of harmful recessive alleles being expressed. Consequently, this led to a higher frequency of genetic diseases.

Step-by-step solution

Understand the Population Bottleneck Concept

A population bottleneck is an event in which the population size of a species is drastically reduced due to some environmental or human-induced factors. This leads to a decrease in genetic diversity as only a small proportion of the original gene pool survives. As a result, the surviving population will have fewer unique alleles, which contributes to increased inbreeding among the individuals.

Describe Inbreeding Effects on the Population

Inbreeding is the mating of individuals that share a common ancestor, which results in offspring with reduced genetic diversity. Inbreeding increases the chances of expressing harmful recessive alleles in the population, which in turn leads to a higher likelihood of genetic diseases among offspring. Inbred populations, therefore, are more prone to genetic diseases due to their limited gene pool, which increases the frequency of harmful recessive alleles.

Linking Population Bottleneck to Frequency of Genetic Diseases

A population bottleneck, as experienced by early humans, reduces the genetic diversity within the population and increases the likelihood of inbreeding. Consequently, this inbreeding increases the chances of harmful recessive alleles being expressed, leading to a higher frequency of genetic diseases within the population. The ancient human population bottleneck, therefore, would have increased the likelihood of genetic diseases due to the reduced genetic diversity and higher levels of inbreeding.

Key concepts

Genetic Diversity Reduction

Imagine a colorful mosaic consisting of numerous distinct tiles. Now, picture if many of those tiles were suddenly removed, leaving you with a much less varied pattern. This is akin to what happens during a population bottleneck, a critical concept in understanding genetic diversity reduction. A bottleneck occurs when a significant event causes a drastic reduction in the size of a population, leaving only a small number of individuals to continue the species. This small group carries only a fraction of the genetic variation originally present in the entire population.

The danger of genetic diversity reduction is akin to putting all one's eggs in one basket; the resulting lack of genetic variation can mean that the population is less adaptable to environmental changes or new diseases. If an ailment strikes to which none of the limited genes provide resistance, the survival of the entire species can be jeopardized. For students learning about genetics, it's also important to relate this to a loss of potential. Diversity is the wellspring of evolutionary potential, and without it, there's less room for natural selection to shape a species into better forms over time.

Inbreeding Effects

One of the immediate problems stemming from reduced genetic diversity is inbreeding. Inbreeding happens when closely related individuals mate, and as such, it becomes more likely in the aftermath of a population bottleneck. The problem with inbreeding isn't the mating of relatives per se; rather, it is the outcomes of such matings. Genes are like instructions, and harmful ones, or 'mutations,' can sometimes be masked when paired with a healthy copy.

In a diverse population, it’s less likely that two carriers of the same harmful mutation will mate, but inbreeding changes that. It increases the chance that both parents carry the same harmful recessive genes, which can be passed on to their offspring. When this happens, the recessive traits, including genetic disorders, can manifest much more frequently. When explaining this to students, I like to compare it to a backup system failing. If the primary and backup systems are identical and flawed, the entire system's failure becomes more likely.

Frequency of Genetic Diseases

The frequency of genetic diseases within a population can rise significantly as a direct consequence of both genetic diversity reduction and inbreeding. With a shallow gene pool, harmful recessive alleles stand a higher chance of coupling, resulting in genetic diseases becoming more common.

It’s essential to recognize that all populations carry some genetic diseases, but normally, they're rare, widely scattered, or masked by healthy alleles. When the genetic deck is shuffled less – as in a bottleneck with consequential inbreeding – those harmful traits can end up being dealt more frequently. This directly impacts the health, vitality, and even long-term survival of a species. For students, thinking in terms of genetics being like a hand of cards can be useful: a population bottleneck limits the number of cards dealt, and inbreeding prevents a fresh shuffle, increasing the odds of drawing the same undesirable cards over and over again.