Lec 9: Problem with Heredity Flashcards
Preformation Theory
No physical process can build a living body out of disorganised matter. Must have been preformed, e.g. in the ovum.
- > preDarwin, #1 issue is generation (reproduction)
- > how is organism constructed?
- > when “how is organism formed” is more important than inheritance, we tend to see parents as process to construct their offspring
- > generation only occurs through parents building copy of selves
- > heredity is process, where something made by parent transmitted to offspring
Pangenesis
Comes in another book, The Variation of Plants and Animals Under Domestication
Darwin’s theory of heredity, to address swamping.
- > allows production of many heritable individual differences
- > all cells shed minute particles, “GEMMULES”
- > they go to the gonads, multiply, and passed on
1) Differences are function of how gemmules altered by external conditions
2) Shows how acquired conditions are inherited
- >getting more muscles changes the gemmules muscle gives out
3) Blending occurs because you get one set of gemmules from each parent, which mingle
4) Explains things like atavism, prepotency, reversion
- >gemmules usually mix - but sometimes mixture uneven, one parent domintaes
- >gemmules don’t get blended away, they can lie dormant and reappear later
- >they may be transmitted before fully developing, and so the gemmules appear in later generations
The Swamping Problem
Issue with Darwin’s theory & inheritance.
He thought everything was blending inheritance. But then no matter how efficient a variation is, within a few generations, it is swamped out of existence.
->e.g. one genius on an island of dimwits can’t carry
To counter, need many of a kind.
- > but then you need many characters of a kind to bring the the whole group that way
- > hence he pushes for large groups for more variations, and emphasizes inheritance of acquired characters
- > frequent small changes, not infrequent big ones
Two kinds of inheritance? Proponents of each?
Two ways to think of inheritance:
1) Blended Inheritance (darwin)
- >e.g. skin colour is mixed
2) Particulate Inheritance (Mendel)
- >e.g. you generally don’t mix genitalia
- >units of heredity are not broken down
Gemmules
Core of Darwin’s Pangenesis
- > every part of organism throws out minute atoms of contents
- > multiply and aggregate in sex organs
Atavism and Reversion
Explain with Darwin vs Mendel
Atavism
When characteristics skip a generation
Reversion:
Old traits and reappear from nowhere.
Darwin: The gemmules aren’t destroyed in mix, and may reappear later. Or gemmules were not yet mature when transmitted, but will mature later
Prepotency (dominance)
Explain with Darwin vs Mendel
One parent completely masks the other
Darwin: The gemmules weren’t fully formed, or mixed poorly.
Mendel: Recessive unit factors
Gregor Mendel (1822-1884)
The basis of genetics, he gives us HEREDITY.
He proposes heredity is hard wired
- > same characters are always passed on
- > characters do not change in the parent’s bodies (due to habitat, environment)
Spent 8 years with 30k Pea plants. Studied 7 traits
- > ez to grow, interbreed, matures in one season
- > self fertilizing, easily crossbred
In the pea plant, hereditary particles come in pairs (“ELEMENTS”). Each parent gives an element
Gives 3 postulates of inheritance:
1) Genetic factors control by unit factors which exist in pairs
- >3 combs possible: XX, Xx, xx
2) When two unlike unit factors are present in an individual, one is “dominant”, other “recessive”
3) During formation of gametes, each parent has 50% chance of passing either of their unit factors.
How are Darwin’s NatSel, and Mendel’s Heredity linked?
Heredity is the main mechanism for variability in Nat Sel.
While Mendel knew Darwin’s work, Darwin did not know Mendel’s work explained the variations :(
William Bateson
Thought evolution was sudden discontinuous variations, not accumulated small changes.
