In all human
populations, people usually select mates non-randomly
for traits that are easily observable. Cultural values and social rules primarily
guide mate selection. Most commonly, mating is with similar people in respect to
traits such as skin color, stature, and personality. Animal breeders do
essentially the same thing when they intentionally try to improve varieties or
create new ones by carefully making sure that mating is not
random. When they select mates for their animals based on desired traits, farmers hope to
increase the frequency of those traits in future generations. In so far as the discriminated
traits are genetically inherited, evolution is usually a consequence.
However, the results are not always what farmers
expect. The reasons why will be explained shortly.
Even without the intervention of farmers, most animals select mates
carefully--they do not mate randomly. Charles Darwin
noted this fact in his 1871 book
Descent of Man and Selection in Relation to Sex.
He suggested that mate selection is a powerful force of evolution similar in
its effect to natural selection. This idea was widely rejected in
Darwin's time, but later research showed that he was correct.
In order to
understand the effect of non-random mating patterns, it is useful to first
examine the results of random mating. As
Hardy and Weinberg demonstrated in the early 20th century, the gene pool of a population that is mating randomly and
is not subject to any other evolutionary process will not change--it will remain in
equilibrium. If mating is entirely random, there will be nine possible mating
patterns for a trait that is controlled by two alleles
(A and a).
| AA X AA |
|
Aa X AA |
|
aa X AA |
| AA X Aa |
|
Aa X Aa |
|
aa X Aa |
| AA X aa |
|
Aa X aa |
|
aa X aa |
In a population which has 50% of each of these two alleles, the expected offspring
genotype frequencies with random mating will be 25% homozygous dominant (AA), 25% homozygous recessive (aa), and 50% heterozygous (Aa), as shown in the table
below. They will remain in this ratio every generation that random mating continues
and no other evolutionary mechanism is operating.
|
The number of
children
is
what would be
expected by
chance
if each mating pair
has
4 children.
You can work this out
yourself
by creating a
Punnett Square
for
each set of
parents. |
|
|
Random Mating |
Possible parent
mating
patterns |
Expected offspring genotypes |
|
AA |
Aa |
aa |
|
AA X AA |
4 |
|
|
|
AA X Aa |
2 |
2 |
|
|
AA X aa
|
|
4 |
|
|
Aa X AA
|
2 |
2 |
|
|
Aa X Aa
|
1 |
2 |
1 |
|
Aa X aa
|
|
2 |
2 |
|
aa X AA
|
|
4 |
|
|
aa X Aa
|
|
2 |
2 |
|
aa X aa
|
|
|
4 |
|
Total |
9
( 25% ) |
18
( 50% ) |
9
( 25% ) |
|
Positive
Assortative Mating
The most
common non-random mating pattern among humans is one in which
individuals mate with others who are like themselves
phenotypically for
selected traits. This is
referred to as positive assortative mating
. The
term "assortative" refers to classifying and selecting characteristics. An example
of positive assortative selection would be tall slender people mating only
with tall slender people. Taken to the extreme, positive
assortative mating results in only three possible mating patterns
with respect to genotypes for traits that are controlled by two
autosomal
alleles--homozygous dominant
with homozygous dominant (AA X AA), heterozygous with heterozygous (Aa X Aa), and homozygous recessive with
homozygous recessive (aa X aa).
The net effect
of positive assortative mating is a progressive increase in the number of homozygous genotypes (AA and aa) and a
corresponding decrease in the number of heterozygous (Aa)
ones in a population, as shown in the table below. Each generation that there is positive
assortative mating, this polarizing trend will continue in
the population.
|
Positive Assortative Mating |
Possible parent
mating
patterns |
Expected offspring genotypes |
|
AA |
Aa |
aa |
|
AA X AA |
4 |
|
|
|
Aa X Aa
|
1 |
2 |
1 |
|
aa X aa
|
|
|
4 |
|
Total |
5
( 42% ) |
2
( 17% ) |
5
( 42% ) |
Negative
Assortative Mating
Evolutionary
Consequences of Non-random Mating
Like recombination, non-random mating
can act as an ancillary process for natural selection to cause evolution to
occur. Any departure from random mating upsets the
equilibrium distribution of genotypes in a population.
This will occur whether mate selection is positive or negative assortative. A single generation of random mating will restore genetic
equilibrium if no other evolutionary mechanism is operating on the
population. However, this does not result in a return to the distribution of
population genotypes that existed prior to the period of non-random mating.
A comparison of the 2nd and
5th generations in the table
below illustrates this fact.
|
Effects of non-random mating on a population’s gene pool |
|
Generation |
Parent
mating pattern |
Offspring
genotype frequencies |
Effect on
genotype
frequencies |
|
AA |
Aa |
aa |
| 1 |
random |
50% |
30% |
20% |
equilibrium |
| 2 |
random |
50% |
30% |
20% |
equilibrium |
| 3 |
negative assortative |
45% |
40% |
15% |
change |
| 4 |
negative assortative |
40% |
50% |
10% |
change |
| 5 |
random |
40% |
50% |
10% |
equilibrium |
| 6 |
random |
40% |
50% |
10% |
equilibrium |
| 7 |
positive assortative |
43% |
45% |
12% |
change |
| 8 |
positive assortative |
48% |
34% |
18% |
change |
NOTE: genotype frequencies in an actual population may differ somewhat from
those in
this
table, but
the direction of change from generation to generation will be
the same. |
Plant
and animal breeders usually employ controlled positive assortative mating to
increase the frequency of desirable traits and to reduce genetic variation in
a population. In effect, they try to guide the direction of evolution
by preventing some individuals from mating and encouraging others to do so.
By doing this, farmers, in a sense are acting in the place of nature in
selecting winners and losers in the competition for survival. This
method has been used to develop purebred varieties of laboratory mice, dogs,
horses, and other farm animals. The amount of time it takes for this process can be
much shorter than one might imagine. If brothers and sisters are mated
together every generation, it will only take 20 generations for all
individuals in a family line to share 98+% of the same alleles—they essentially will be clones, and breeding results will be close to
those resulting from self-fertilization. Commercially sold laboratory
research mice have been mated brother to sister for 50-100 generations or
more. The downside of this practice is that positive assortative mating
results in an increase in homozygosity of harmful alleles if they are present
in the gene pool. The high frequency of
hip dysplasia
,
epilepsy
, and immune-system malfunctions in some dog varieties are primarily a
result of inbreeding. The reduction in viability and subsequent loss of
reproductive potential of purebred varieties has been referred to as
inbreeding depression. In contrast, animals that have been crossbred with
mates from very different genetic lines are more likely to have lower
frequencies of homozygous recessive conditions. Subsequently, they are liable
to be more viable. This phenomenon has been referred to as
hybrid vigor or
heterosis
.
Human
mating rarely is as consistently positive assortative as is the case with
purebred domesticated animals. As a consequence, inbreeding depression is
rarely a problem except for some reproductively isolated small societies and
subcultures. The Old Order Amish
are an example. This relatively small
population centered in Pennsylvania and Ohio has been self-isolated by their
religious beliefs and lifestyle for more than two centuries. They mostly
select mates from within their own communities, which results in positive assortative effects on their gene pool. The Amish population has a
comparatively high frequency of Ellis-van Creveld syndrome
, which is a
genetically inherited disorder characterized by dwarfism, extra fingers, and
malformations of the arms, wrists, and heart. The majority of the known cases
in the world of this rare syndrome have been found among the Amish, and 7% of
them carry the responsible recessive autosomal allele.
Consanguineous Mating
Consanguineous mating
, or inbreeding, is the sexual union of closely related individuals,
such as brothers, sisters, or cousins. It is an extreme form of positive assortative mating since close relatives usually are genetically more similar
than are unrelated people who share a few traits. When siblings mate together, it is in effect
positive assortative mating for many genetic traits. Half of the alleles of
brothers and sisters are likely to be shared. If they mate together, their
children would be expected to have a quarter of those alleles in common.
Therefore, when consanguineous mating occurs, the result is significantly less
genetic diversity among the descendants than if the parents had mated with
someone who was not closely related but was like them in terms of selected
traits such as skin color or stature.
It has long
been assumed by the general public in western nations
that children of inbred parents
inevitably have a high probability of inheriting mental retardation and other
serious genetic defects. This is not necessarily true. If a harmful allele
is present in a family, it will show up at a higher than normal rate among
inbred children. If inbreeding continues to be the common mating pattern in a
family line, it is likely that homozygosity will increase in frequency and the
family will experience a progressive rise in the
genetic load of the
deleterious allele. On the other hand, if the allele is not present in the
family line, inbred offspring are not likely to have a higher than normal risk
of inheriting a mutation for it. Inbreeding also could potentially increase
the odds of a child inheriting desirable traits. If the family genetic line
has alleles that contribute to advantageous characteristics, such as
intelligence, health, or what their culture defines as beauty, they are more
likely to show up in children resulting from inbreeding if
the parents have these characteristics. Consanguineous
mating also may be an advantage for women who are Rh negative because it would
increase the chances that their children would be Rh negative. As a
consequence, there would be a lower risk of
erythroblastosis fetalis
in the
children. You will learn more about this potentially
fatal condition and its connection with Rh blood types in the next tutorial
of this series.
NOTE:
The word "consanguineous" comes from Latin and literally means "with the
blood". In other words, consanguineous mating is between "blood
relatives". The term "blood relatives" goes back to the time when
people mistakenly thought that what passed between parents and their
children was blood. In fact, it is DNA rather than blood.
The
closer two mates are in generational distance from their common ancestor, the
greater the likelihood of positive assortative effects on the
genomes of their
children. Based on
statistical data from 38 populations in South Asia, Africa, Europe, and South
America, it has been determined that the increased
risk for “significant birth defects” among the offspring of
first cousins is
only 1.7-2.8% above the risk for the general population. The predicted risk for the children of brothers and
sisters or parents and their children is 6.8-11.2% above that of
the general population. Based on these numbers, it would seem that while the
risk is high for very close biological relatives, it
is relatively low for first cousins and more distant
kinsmen. In fact, there is a high probability that the children of
first cousins will not have significant birth defects. In addition, there does not appear to be a
statistically significant increase in the frequency of gross chromosomal
anomalies, such as trisomy-21 (Down syndrome), in the children of
consanguineous unions.
While first cousin marriages are
extremely rare in North America, Europe, and East Asia, they are very common in some
parts of the world due to long existing cultural traditions. Roughly a
third of the marriages in rural India are between first cousins. In
the Arabian Peninsula, the rate is 50% or higher. In both areas, there
are ongoing nationwide educational campaigns to discourage first cousin
marriages. The hope is that if they are successful in reducing the
number of such unions, it will cut medical costs for the nations. So
far there has been some success in changing the marriage patterns of more
educated urban Saudis and Indians, but there have been little inroads into
rural areas where most first cousin marriages occur.
A recent statistical study of 165
years of genealogies for 160,000 couples in Iceland has shown somewhat
surprising results. Married couples who were third cousins (they
shared a great-great-grandparent) had more offspring than did couples who
were less closely related. In other words, marrying third cousins
resulted in greater reproductive success. However, couples who were first or second
cousins had fewer offspring and those children died at a younger age.
Is Love in Our DNA--how
do you choose a mate? (Take a poll to find out.)
This link takes you to an external website. To return here, you must click the
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Summary
We have seen in this and
previous sections of the tutorial that evolution can result from any of
four main processes operating independently or
together. In addition, there are two ancillary
contributing processes.
|
Main processes |
Ancillary processes |
|
|
1. |
mutation |
1. |
recombination |
|
2. |
genetic
drift |
2. |
non-random mating |
|
3. |
natural
selection
|
|
|
|
4. |
gene
flow |
|
|
Mutation is the ultimate source of
new genetic varieties. However, gene flow can be responsible for the
introduction of new alleles into a population and very
rarely into a species. Generally the most rapid
and dramatic evolution is due to natural selection.
Recombination and non-random mating can change the frequencies of genotypes which in turn can be selected for or against by nature.
Genetic drift can also result in rapid evolution of the gene pools of small, reproductively isolated populations. It is highly likely that our ancestors lived in
such small populations for 99+% of the last 2.5 million years during which
our genus Homo was evolving. Subsequently, genetic drift and
other small population size effects must have frequently been a major factor
in our evolution along with natural selection.
Five Fingers of
Evolution--summary of how evolution works by Paul Anderson.
Video
clip from Ed.Ted.com.
To return here, you must click the
"back"
button on your
browser program.
(length =
5 mins, 23 secs) |
Primary source for
human birth defect statistics related to inbreeding: Bennett, R.
L. et al., "Genetic Counseling and Screening of Consanguineous
Couples and Their Offspring: Recommendations of the National Society of
Genetic Counselors", Journal of Genetic Counseling (2002)
11(2):97-119. Source of data on Icelandic couples: "An
Association Between the Kinship and Fertility of Human Couples"
Science (2008) 319: 813-816.