Transcript A rich inheritance

A rich inheritance
Heredity and Homologies
All cells from cells
(Omnis cellula e cellula)
• Rudolf Virchow
argued for this
principle based on
microscopic
observations of cell
division. (1858)
• Nuclei were seen to
be duplicated as well,
and nuclei of germ
cells unite at
fertilization.
August Weisman
• Argued that, since the
hereditary material is
contained in the nucleus,
and the nucleus of a cell
comes from the nuclei of
ancestral cells, Darwin’s
pangenesis is impossible:
No combination of
elements from all the
body can be combined
and placed in the nuclei
of the germ cells. (ca.
1887)
Chromatin
• A material in cell nuclei that stained strongly with
new (synthetic) dyes.
• As a cell begins to divide, the chromatin is
gathered in rod-shaped ‘chromosomes’, which
then duplicate (splitting along their length).
• One set of chromosomes goes to each daughter
cell.
• The process is called mitosis; this material,
Weisman concluded, must be the ‘hereditary
substance’.
Meiosis
• Germ cells combine to make a fertilized egg–
each supplies chromosomes, so they must each
carry ½ the usual complement.
• Weisman predicted a division would occur
without duplication, to create germ cells.
• In fact, meiosis duplicates the chromosomes
once, but then divides twice to produce four
germ cells.
• Weisman worked to connect these facts to
natural selection and evolution.
Still more trouble for inheriting
acquired traits
• The cell lineage that leads to germ cells in the
hydra (Hydromedusae) is distinct from somatic
cell lines from very early in development.
• So there is no way for somatic changes to alter
the nuclei of germ cells.
• Weisman tried tail amputations in mice to test
this notion; no inheritance of the mutilations was
found (such inheritance was widely believed in
up to this point).
Translation
• More broadly, Weisman saw no way that
information from the rest of the body could be
received and ‘translated’ by the germ cells to
produce similar effects on progeny.
• This reinforces the notion that variation is
undirected (we often say ‘random’ here, but it’s a
sloppy way to describe it).
• Given undirected variation, natural selection
seems to be the only way to produce adaptive
change over time– Weisman concluded that NS
is the key mechanism in evolution.
Lamarckians
• Lamarckians resisted this conclusion, insisting
that acquired traits could be passed on,
somehow.
• Related to this was Copes notion of
orthogenesis, the idea that there were ‘trends’ in
evolution (towards larger size or altered shapes
of some parts for instance) that were not driven
by natural selection. They could even drive a
species to extinction.
• Cope thought that inheritance of acquired
characteristics could explain these trends.
The Irish Elk
• Largest known member
of the deer family.
• An ice-age animal.
• Cope and others believed
it had been driven to
extinction by an
evolutionary ‘trend’ that
led to ever-bigger antlers,
which eventually made it
unable to avoid obstacles
or escape predators
Patterns of inheritance
• If inheritance was so clear and ‘precise’ a process, surely
there should be patterns of inheritance that we could
detect.
• Bateson and de Vries studied variation and its patterns
of inheritance.
• For Bateson, variation (not selection) was the key to
evolution.
• Bateson thought that sudden, dramatic, discontinuous
variation could explain the discontinuities distinguishing
even closely related species.
• De Vries favoured a ‘particular’ account of inheritance.
De Vries
• Inheritance of one such particle (pangene)
would be independent of inheritance of others.
• So we should be able to follow different ones
through the population, generation by
generation.
• Observations of plants showed a definite pattern
of inheritance; de Vries observed it in 30 or so
species.
• As he prepared to publish, he looked back into
the literature and found that he’d been scooped.
Mendel
• Gregor Mendel was an
Augustinian monk who worked
as a teacher in Brno, Austria.
• He experimented with peas,
breeding them in controlled
ways by hand-pollination.
• The pea plants showed 7
discrete characteristics: tall or
short, with round or wrinkled
peas, etc.
• Selection could make them
breed true for these
characteristics.
• Cross-breeding the results
showed a simple pattern.
Particles
• If separate particles, independently inherited, cause
various traits, and some particles ‘dominate’ others in
their effects, then the pattern Mendel observed is easily
explained.
• The result is that when we cross the pure forms, the
offspring will have one of each particle type (for a given
trait) and will ‘show’ the trait that is dominant.
• But breeding this second generation will produce a mix–
¼ will inherit two dominant particles, ¼ will inherit two
‘recessive’ particles, and ½ will inherit one of each.
• So ¼ of the second generation will display the recessive
trait while ¾ display the dominant trait. More complex
patterns appear, but they all fit this theory.
De Vries again
• De Vries wanted to make his theory of heredity
into a theory of evolution.
• His idea was that sudden, dramatic shifts in the
‘genome’ could lead to a new species forming in
a single step (a ‘mutation’).
• One example seemed to fit: the evening
primrose. The result was the mutation theory of
evolution (like Bateson, the main force in
evolution for de Vries was the force of variation,
not selection).
Further developments
• Thomas Morgan began his career as a disciple of
Bateson and de Vries, and focused on studying variation
and how it arises over time.
• Defenders of natural selection, including Wallace, were
unimpressed: the phenomenon of continuous, slowly
changing variation in wild populations seemed
undeniable to them.
• But in the end it turned out that discontinuous genetic
variation (genotypes), combined with ‘random’ effects
from the environment, could easily produce a continuous
range of phenotypic traits; Johannsen showed this in
1909; more details were revealed by Nilsson-Ehle, in his
multiple-gene analysis of grain colours in wheat.
Smaller discontinuities
• The distance between natural selection
advocates and the geneticists was diminished by
the gradual recognition that genes could
produce small differences, not just major/sharp
ones.
• Morgan’s work reinforced this lesson. Small
mutations in his fruit flies appeared regularly.
This steady supply of small changes allowed
Morgan to study inheritance in great detail,
recognizing many more patterns: links between
genes that tend to be inherited together (but not
always) and other complexities were identified.
Lamarckism’s demise
• The success of this particulate view of
inheritance put the final nail in the coffin of
‘inheritance of acquired traits’.
• Further, it became apparent that there was no
hard and fast distinction between continuous
and discontinuous variation. Mutations could
produce fine steps and small changes as easily
as sharp, large ones.
• And study of selection effects in the wild showed
that small differences could have a significant
effect on survival.
Anatomy and descent
• Meanwhile, detailed studies of anatomy and
development in the vertebrates showed that the
similarities already known were just the tip of the
iceberg.
• Evolution was reinforced by this evidence, which
consistently extended and unified the tree
structure found in anatomy, development and
the fossil record.
• The segmental theory of the head was a major
accomplishment along these lines; observations
of development showed that all vertebrates
develop on the same segmental plan.
Evolution of the head
• Here we see consilience– not just development, but the
evolution of the vertebrate head showed shared patterns
throughout the group, and a gradual transition from one
pattern to another in transitional fossils.
• Transitional forms linking fish to amphibians,
amphibians to reptiles, and certain reptiles (the
‘mammal-like’ reptiles) to mammals were found.
• Jaw morphology is a key line distinguishing mammals
from reptiles; here and in other traits, though, the
transition can be seen in the fossils.
• Development later secured the identification of the two
extra mammalian middle ear bones with the extra two
reptilian jaw bones.
The Branching Process
• The diversity of evolution, the fact that it has no
set goal, no ‘destination’ (such as ourselves)
also became apparent.
• Divergence or radiation, not a linear ladder of
progress, is what we find.
• Groups start out small, with few forms (or one)
that are not very specialized.
• Later (when successful) we find much wider
variation, more striking specializations and many
more forms.
Haeckel’s Tree
Another tree