Lecture 3: How Mendel rescued Darwin (without either ever
knowing)
Why was genetics important?
1. It was needed to clarify why selection works and
overturn a theoretical objection to evolution by natural selection.
2. It provided a rigorous (i.e. mathematical) basis for
understanding how evolution works.
3. It enormously enriched our understanding of how
evolution works, providing often counter-intuitive, but
empirically correct, results.
4. It tells us what we should be studying – alleles…
1. Blending inheritance and the problem of the vanishing variation
Following the publication of The Origin (1859) there were many critiques. Possibly
one of the most profound was that of Jenkins (1867). He noted that natural selection
as an idea was flawed owing to the disappearance of variation due to blending
inheritance. At the time just about everybody (except Mendel but his work was
unknown) thought that inheritance was blending: mate a red and a white flowered
plant and you get pink. Mate two pinks they thought and again you would get pink.
Jenkins noticed that this would lead to the mode of inheritance destroying variation – a
red plant comes into a white population and next generation leaves only pink
offspring. These will dominantly mate with whites leaving pale pinks and so on, until
simply due to the pattern of inheritance all variation is lost. And without variation
there can be no selection. At the very least selection must have to act in a very strong
manner to be able to oppose the tendency for the trait to be lost due to blending.
However, blending inheritance we now know is not found. Instead, we know
that alleles are inherited in a Mendelian fashion. This makes a profound difference to
our understanding of natural selection. It took however until the rediscovery of
Mendel’s work at the start of the 20th century (originally the work was done in 1870s)
before the rescue of the idea of natural selection was realized. To see this you need
only ask one question:
, 2. What happens to trait frequencies if no forces act on allele frequencies and if
inheritance is Mendelian?
Remember that under blending inheritance if no force acts variation is lost simply due
to inheritance. Let us contrast this with Mendelian inheritance. Consider the simplest
case. Consider a locus with two alleles, A and a. There are three genotypes at the
following frequencies:
AA at x
Aa at y
aa at z
x+y+z=1
Allele A must exist at frequency x+ y/2, likewise a is at z+y/2.
What happens when mating occurs? If it is random then:
Progeny
father mother frequency AA Aa aa
AA AA x.x 1 - -
Aa x.y 1/2 1/2 -
aa x.z - 1 -
Aa AA x.y 1/2 1/2 -
Aa y.y 1/4 1/2 1/4
aa y.z - 1/2 1/2
aa AA x.z - 1 -
Aa y.z - 1/2 1/2
aa z.z - - 1
So if we let x' equal the frequency of AA in the next generation (similarly for y', z')
then:
knowing)
Why was genetics important?
1. It was needed to clarify why selection works and
overturn a theoretical objection to evolution by natural selection.
2. It provided a rigorous (i.e. mathematical) basis for
understanding how evolution works.
3. It enormously enriched our understanding of how
evolution works, providing often counter-intuitive, but
empirically correct, results.
4. It tells us what we should be studying – alleles…
1. Blending inheritance and the problem of the vanishing variation
Following the publication of The Origin (1859) there were many critiques. Possibly
one of the most profound was that of Jenkins (1867). He noted that natural selection
as an idea was flawed owing to the disappearance of variation due to blending
inheritance. At the time just about everybody (except Mendel but his work was
unknown) thought that inheritance was blending: mate a red and a white flowered
plant and you get pink. Mate two pinks they thought and again you would get pink.
Jenkins noticed that this would lead to the mode of inheritance destroying variation – a
red plant comes into a white population and next generation leaves only pink
offspring. These will dominantly mate with whites leaving pale pinks and so on, until
simply due to the pattern of inheritance all variation is lost. And without variation
there can be no selection. At the very least selection must have to act in a very strong
manner to be able to oppose the tendency for the trait to be lost due to blending.
However, blending inheritance we now know is not found. Instead, we know
that alleles are inherited in a Mendelian fashion. This makes a profound difference to
our understanding of natural selection. It took however until the rediscovery of
Mendel’s work at the start of the 20th century (originally the work was done in 1870s)
before the rescue of the idea of natural selection was realized. To see this you need
only ask one question:
, 2. What happens to trait frequencies if no forces act on allele frequencies and if
inheritance is Mendelian?
Remember that under blending inheritance if no force acts variation is lost simply due
to inheritance. Let us contrast this with Mendelian inheritance. Consider the simplest
case. Consider a locus with two alleles, A and a. There are three genotypes at the
following frequencies:
AA at x
Aa at y
aa at z
x+y+z=1
Allele A must exist at frequency x+ y/2, likewise a is at z+y/2.
What happens when mating occurs? If it is random then:
Progeny
father mother frequency AA Aa aa
AA AA x.x 1 - -
Aa x.y 1/2 1/2 -
aa x.z - 1 -
Aa AA x.y 1/2 1/2 -
Aa y.y 1/4 1/2 1/4
aa y.z - 1/2 1/2
aa AA x.z - 1 -
Aa y.z - 1/2 1/2
aa z.z - - 1
So if we let x' equal the frequency of AA in the next generation (similarly for y', z')
then:
