How Natural Sheep Breeding Reduces the Chance of Inbreeding.

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How "Natural Breeding" Reduces Inbreeding.

Inbreeding is the production of offspring by parents that share common ancestors. Inbreeding will lower the biological fitness of an organism's ability to survive.
Humans cause sheep to inbreed by breeding by two methods:
Breeding a single male ram to a large number of female ewes, and

Confining sheep into small unnatural sized flocks of closely related individuals.

A closely guarded pedigree using expensive registered stock is one solution to escape inbreeding,
But using only registered stock is not practical for running a large production sheep farm.
Nature has a better solution, called "natural breeding."
Natural Breeding is when a large flock of sheep with an equal number of males and females breed randomly.
A dominant male ram with superior genetics may breed more often, but the male ram in Nature must naturally compete with other rams, and so doesn't always breed the majority of female ewes in a large flock.
In Nature, large flocks are not confined into small groups but migrate and periodically meet and exchange members with other flocks, which adds genetic diversity and prevents inbreeding.

The Natural Breeding of Soay Sheep

Soay Sheep are found on the 250 acre Island of Soay, which is 40 miles off the coast of Scotland.
The sheep were likely brought to the remote Soay Island by the Vikings, and are a relict population of the Bronze Age sheep brought to Europe around 5,000 BC.
This small population of Soay sheep have been breeding naturally for hundreds of years without human assistance, yet the Soay sheep do not show obvious signs of inbreeding.
I believe that Natural Selection prevents inbreeding by culling the unfit, and by leaving an equal number of males and females for reproduction, which is Natural Breeding.
Through Natural Selection and Natural Breeding without human assistance, Soay sheep have developed parasite resistance, a trait that has been lost to most modern sheep breeds that depend on dewormers to live.
While Man breeds the unfit, Nature culls the unfit. Every few years, the population of Soay sheep will exceed the available grazing or "carrying capacity" of the island. This creates a famine that will cull 2/3 of the most unfit Soay sheep in just a few weeks.
This lack of nutrition suppresses the immune system in the unfit, with post-mortem examinations revealing that death is not just by famine but also by a considerable parasitic nematode burden.
Any individual Soay sheep that suffers from inbreeding depression and loses parasite resistance, or cannot easily convert grass to energy during a famine will be culled by Natural Selection. Only the most efficient survive.
What is interesting is that Soay sheep are small, with the ewes weighing only 55 lbs, and the rams 88 lbs. Nature is telling us that the most efficient sized sheep for converting grass to meat is much smaller than modern sheep we have bred for size and not efficiency.
Nature lets these sheep breed randomly with an equal number of males and females that survive periodic culling, which produces no obvious problems from inbreeding.
Natural Selection culls the inefficient, which eliminates any inbreeding depression and keeps the Coefficient of Inbreeding to a manageable level.

The Island of Soay, Scotland.

Soay Ewe
(55 lbs.)

Soay Sheep
(55-88 lbs.)

Soay Lamb

The Coefficient of Inbreeding

The Coefficient of Inbreeding (COI) is the probability that through inbreeding, the genetic material at the same place on both strands of the chromosome will be identical, which increases the risk of health issues.
A high level of COI will result in "Inbreeding Depression", which will cause later generations to have stunted growth, abnormalities, and failure to reproduce.

Sheep with deformed limbs caused by inbreeding

Here are some guidelines for COI:

Low COI (0%–6%): Considered safe, indicating minimal inbreeding and maintaining genetic diversity.

Moderate COI (6%–10%): Acceptable with careful monitoring for signs of inbreeding depression.

High COI (>10%): Higher risk; sustained breeding at this level can lead to significant inbreeding depression and reduced overall fitness.

Here is a calculator that I use to help me understand COI:

Coefficient of Inbreeding Calculator

by Sam Kauffman
NORTH AMERICAN BABYDOLL SOUTHDOWN SHEEP ASSOCIATION & REGISTRY

The Coefficient of Inbreeding.

Relationship	Inbreeding Coefficient
Father/daughter, mother/son or brother/sister	25%
Grandfather/grandaughter or grandmother/grandson	12.5%
Half-brother/half-sister	12.5%
Uncle/niece or aunt/nephew	12.5%
Great-grandfather/great-granddaughter or great-grandmother/great-grandson	6.25%
Half-uncle/niece or half-aunt/nephew	6.25%
First cousins	6.25%
First cousins once removed or half-first cousins	3.125%
Second cousins or first cousins twice removed	1.5625%
Second cousins once removed or half-second cousins	0.78125%
Third cousins or second cousins twice removed	0.390625%
Third cousins once removed or half-third cousins	0.195%

How the Male to Female Ratio Affects Inbreeding

The geneticist Ronald Fisher outlined an model called "Fisher's Principle" that explains why the sex ratio is approximately 1:1 between males and females.
According to this principle, if there was no human interference, a flock of sheep existing in Nature would produce an equal number of males and females.
I believe that is the unnatural selection of a single "superior" male ram to breed with up to 30 female ewes violates the Fisher Principle that Nature provides and is the man-made cause of inbreeding.
Nearly every sheep farmer does this, yet I believe the unequal sex ratio is destroying the genetics of their flocks.
Let's make some diagrams to understand this problem:

Ronald Fisher

A Chart of How an Unbalanced Sex Ratio Leads to Inbreeding.

For this chart, let the Blue Square represent a single male ram, and the Pink Circles represent seven female ewes.
Consider that all of the sheep are unrelated, and that a single male (A) is mated with several females (1,2,3,4,5,6,7).

A Single Ram (A) Bred to Many Ewes (1-7)

The Offspring Will Be Related, But Not Inbred.

On average, St. Croix sheep will have 2 lambs per pregnancy, and produce an equal number of males and females, which are the blue squares and pink circles of A1-A7.
Since all of the offspring (A1 to A7) come from parents that are not related, the offspring are not inbred even though they have the same father and are half-siblings.
If all of these related offspring are sold for slaughter, there is no possibility that they could later inbreed with each other.

This mating will produce 14 related offspring that have no common ancestor and are not inbred. (0% COI)

But Breeding These Half-Siblings Creates a COI of 12.5%

If only a single offspring is sold to a single breeder, then there is no potential for inbreeding.
The problem arises when a breeder purchases both a male and female of these offspring for breeding purposes.
Since a single ram (A) mated seven ewes (1,2,3,4,5,6,7), all of their offspring are half-siblings.
If these half-siblings breed with each other they will produce lambs with a Coeffient of Inbreeding (COI) of 12.5%

Breeding half siblings produces 12.5% COI)

Small Flocks May Accidentally Provide Full Siblings For Breeding.

With a smaller flock, a breeder purchasing breeding stock from the same farm may be purchasing a brother and sister.
Breeding siblings produces a very high COI of 25%.
Since breeding a single male to many females is a standard breeding practice, the best solution for starting a flock is to either:
Purchase your breeding stock from different farms, or

Use Registered Breeding Stock and check their pedigree for inbreeding, or

Find a breeder that uses more males when breeding females. Using a single male for breeding multiple females can easily cause inbreeding.

Breeding full siblings produces 25% COI)

Note that none of offspring in this last generation (A1-A7) can be bred to each other without causing inbreeding with a COI of either 12.5% or 25%.
Using a single male to mate with multiple females is one of the main causes of inbreeding.

Remember One Thing:

This Inbreeding Problem began by mating a single "superior" male to many females to create "better genetics" in the short term, but using a single male ends up producing inbreeding in the long term.

How a 1:1 Sex Ratio With An An Equal Number of Males and Females Prevents Inbreeding.

Let's make another chart with 7 Blue Squares representing 7 male rams, and 7 Pink Circles representing 7 female ewes.
Seven Rams are equally mated to seven ewes.
All of the male rams and female ewes are unrelated.

A 1:1 Sex Ratio Produces More Genetic Diversity

By mating an equal number of rams to ewes, all of the offspring are unrelated.
The exception are the full siblings (brother/sister).

Offspring from an equal number of males and females can mate randomly with a lower chance of inbreeding.

Let's consider the amount of COI produced by a single ram that mates randomly.
For example, ram A1 has a 1 chance in 7 (a 14% chance) of randomly mating with his full sibling, which could create a COI of 25%

With a larger flock, a higher chance of mating with unrelated ewe.

But Ram A1 has 6 chances in 7 (an 86% chance) of randomly mating with a completely non-related ewe, which produces a 0% COI.
If this were a large flock that produced 100 ewes, he would have a 99% chance of randomly mating with a non-related ewe.

The Average Probability Through Random Mating Creates a COI of 3.57%.

With this small flock, we need to average the chance of the random mating of Ram A1 with his sibling and producing a 25% COI, with the chance of Ram A1 randomly mating with the other 6 ewes and produce a 0% COI.
If we add the one mating of 25% COI to the six matings of 0% COI, and divide by 7, we get:
Average COI is 25% / 7 = 3.57% COI.
At this point, the younger males in this flock can be allowed to randomly breed to create a low COI, something not possible if we started the flock with only 1 male and 7 females.

Conclusions About Natural Breeding:

Natural Breeding is the random mating of sheep using a 1:1 sex ratio. Breeding an equal number of rams and ewes lowers the chance of inbreeding. Many farmers use a 1:30 sex ratio where a single ram is bred to up to 30 ewes, which almost guarantees inbreeding and a lost of genetic diversity.

Random natural breeding in large flocks creates a lower chances of inbreeding than in smaller flocks. Commercial sheep flocks need to contain at least 50 to 100 sheep.

Random natural breeding eliminates males with low reproductive abilities. Nature does not select for size, it selects for biological fitness. When men select breeding rams for size, they may be passing on genetic defects in other areas, like a low sex drive.

Random natural breeding preserves more genetic diversity than controlled breeding. Each mating creates a unique genetic code, of which only Natural Selection should rate which offspring should survive. Men should "cull from the bottom" to get rid of the weak or inefficient like Nature does, instead of "closing the top" to breed only a select few, as much of the good genetic code that needs to be kept may not be fully expressed in every individual.

Only the most efficient Soay sheep that can convert grass to energy can survive the periodic famines. Nature is telling us that the most efficient size sheep for converting grass to energy is between 55 to 88 lbs.

The stress of parasites, famines, and no shelter have produced a Soay sheep that does not need dewormers, grain, or a lambing pen inside a barn to survive. As resources become more limited, self sufficient sheep like the Soay and St. Croix will be the future.

Man's efforts to produce larger sheep in greater numbers is at odds with Nature's efforts to create the most efficient sheep.

Man uses chemicals to help the unfit to live, while Nature is always developing resistance to these chemicals. When man's chemicals and vaccines no longer work, Nature will win, and only the Pure Natural Sheep that have been bred to excel without chemicals will survive.

The best solution is to let sheep breed naturally and live without man's assistance, and follow Nature by culling those that are thin or do not thrive on their own.
By "assisting sheep" to increase the rate of production, Man has created sheep that require continual assistance.
Dewormers were invented in the 1950's, and sheep existed for thousands of years without chemical dewormers. Now every sheep breed must have these chemicals to survive.
We are breeding for the wrong purpose. We are breeding for size and not efficiency.