Download Chapter 7 Darwin, Mendel and Theories of Inheritance

Darwin & Mendel…
Theories of Inheritance
Chapter 7
Key Concepts
• Neither Darwin nor Wallace knew how variation could be produced
in Nature. What do we attribute this to now?
• The accepted theories of inheritance were the blending of parental
traits in each generation and the inheritance of acquired characters.
Is this true? How does this fit with polygenic traits?
• Darwin continued to hold to Lamarckian inheritance.
• In seeking an alternate theory of inheritance, Darwin revived the
theory of pangenesis in which gemmules traveled from all the
organs to accumulate in the germ cells.
• Discovery of the separation of germ plasm and body plasm
disproved both Lamarckian inheritance and pangenesis.
Key Concepts
• Gregor Mendel’s breeding experiments with pea plants provided a
mechanism of inheritance in particular factors, subsequently shown
to be genes.
• Mendel proposed two laws of what would later be known as
genetics; the principles of segregation and of independent
assortment of hereditary units, now known to be genes.
• The presence of two sexes in many species and the discovery of
varied mechanisms of sex determination demonstrated the role of
genetics, the environment, and environment- gene interactions.
Overview
• How do species transform into other species unresolved?
– Natural selection explained how species survive and changes within a species.
• How do variations arise?
– Need to account for heritability of variation.
• Problem of blending inheritance
– Most 19th century biologists advocated blending inheritance.
– beneficial variations could not preserved by natural selection due to blending
out within a few generations.
– Darwin spent many years seeking an alternative to blending inheritance,
settling on the theory of pangenesis.
• Contradicted by Mendel’s principles
– Segregation and independent assortment
– Genes not sufficient alone as explanation of species transformation
– Phenotypic but not genotypic blending-alleles of genes preserved
• Other modes of inheritance
– Extranuclear (organelle) and sex-linked
Seeking a Mechanism of Heredity
• Resolving the important issue of small versus large
differences was not possible because the mechanisms of
inheritance were not understood.
– Small gradual changes required for Darwinian evolution but most
(even supporters) held that saltations were the most relevant for
evolution.
– Darwin did not know the biological basis of heredity or of
variation. Consequently, his arguments were weakest in these
areas.
Seeking a Mechanism of Heredity
• Lamarckian Inheritance
– At times, Darwin proposed that environmental change, a large increase
in numbers, or some disturbance of the reproductive organs might
enhance variation among a population of individuals.
– At other times, he adopted a version of the Lamarckian view of the
inheritance of use and disuse.
– Both Darwin and Lamarck believed in common descent.
– Lamarck invoked use and disuse and the inheritance of acquired
characters whereas Darwin invoked natural selection to explain why
some features persisted while others disappeared.
– Lamarck and Darwin invoked use and disuse to explain how new
variations arose. Darwin later placed more emphasis on environmental
change rather than use and disuse.
• Darwin saw no directionality imposed by environmental changes but
proposed natural selection as the guiding principle in evolution.
Seeking a Mechanism of Heredity
• Blending Inheritance
– Offspring were thought to represent perfect or blended copies of
their parents’ parts—tend to produce features that average out
those of the parents— just as mixing black and white produces
gray.
• Critics correctly pointed out that NS could not preserve new adaptions
more than a few generations at best.
– Blending inheritance sets variation and inheritance in opposition.
Seeking a Mechanism of Heredity
– 5 replies/responses/rebuttals by Darwin
(think about how you would critique these)
1.
2.
3.
4.
5.
Isolation of the individuals with the new trait (adaptation) as a means
to maintain beneficial trait.
Some characters dominant and appear undiluted.
Variation is common and arises often and so does not dilute out as it
would if it was rare.
Some forms that carry a particular variation pass on to future
generations a propensity for increased occurrence of the same
variation.
Dilution of beneficial traits reduced by natural selection because
enhances reproductive success of favorable while diminishing
unfavorable traits, thus increasing favorable trait frequency.
Seeking a Mechanism of Heredity
• Pangenesis
– Darwin resurrected and developed the concept of pangenesis by gemmules
to help explain the inheritance of traits, which he believed were affected by
use and disuse, and to provide constancy for the determining agents of
inheritance.
• There were presumably many gemmules for each particular trait, and
their numbers could vary during passage from one generation to another
providing a source of variation. Gemmules could be lost but were not
changed by “ blending.”
• Gemmules (pangenes) produced by all the tissues of a parent and
incorporated into the developing eggs or sperm. When fertilization
occurred and parental gametes united, gemmules disperse to form the
tissues of the offspring.
• By suggesting that changes can arise in the frequencies of particular
gemmules while their structure remained constant, pangenesis helped
account for the presumed effects of use and disuse and for the
observation that not all traits become blended.
Seeking a Mechanism of Heredity
• Pangenesis
– Disproved by August Weismann (cut tails off of mice) and Francis Galton—
cousin of Darwin (rabbit blood transfusion.
• Darwin’s response—I never said gemmules travel in blood (ad hoc)
– 1800s-Lamarckians understood “ acquired characters” to be
somatic traits that could be reproduced in each generation
without instruction from germ-line tissue and that could be
transmitted through somatic OR through germinal cells.
• Weismann devised the germ plasm theory of inheritance (note that
many organisms also reproduce by asexual means), in which only the
reproductive tissues (testes and ovaries) transmit the heredity factors of
the entire organism; changes that occur in non-reproductive somatic
tissues are not transmitted. Thus, changes in heredity cannot simply be
explained by inheritance of acquired characters or by use and disuse.
Seeking a Mechanism of Heredity
– With respect to heredity and inheritance, the rediscovery in
1900 of Mendel’s work produced a generation of Mendelians,
many of whom were opposed to Darwin, especially to his
mechanisms of natural selection. Evolution meant something
very different to them.
– It would be the 1930s and 1940s before many of the difficulties
Darwin faced were resolved.
• This is when neo-Darwinism really gains a foothold in science and
comes to dominate.
• 80 years of scientific commitment to this paradigm has come at the
expense of research from an Intelligent Design perspective—where
might we be if ID had been given somewhat equal consideration over all
of this time?
Constancy and Variation
• Constancy has the evolutionary significance that all life
processes depend on the transmission of information
from previous generations.
– Like produces like, but also unlike.
• Variation needed in face of changing environment.
– DNA replication requires fidelity, but not perfect fidelity.
– Neo-Darwinian evolution absolutely requires
mutations.
• Firmly holds that the same can be beneficial and innovative in producing
new information both at the micro- and macro- evolutionary levels
Constancy and Variation
• Recall from genetics that there is not a one-to-one
correspondence between genotype and phenotype
– Environment interacts with genotype to produce phenotype
– Rediscovery of Mendel’s work helped to explain these
phenomena and finally provide a mechanism of genetic
transmission.
Seeking a Mechanism of Heredity
• Resolving the important issue of small versus large
differences was not possible because the mechanisms of
inheritance were not understood.
– Small gradual changes required for Darwinian evolution but most
(even supporters) held that saltations were the most relevant for
evolution.
– Darwin did not know the biological basis of heredity or of
variation. Consequently, his arguments were weakest in these
areas.
• Preformationism (homunculus) Box 7.2
Seeking a Mechanism of Heredity
• Blending Inheritance
– Offspring were thought to represent perfect or blended copies of
their parents’ parts—tend to produce features that average out
those of the parents— just as mixing black and white produces
gray.
• Critics correctly pointed out that NS could not preserve new adaptions
more than a few generations at best.
– Blending inheritance sets variation and inheritance in opposition.
Mendel’s Experiments
• Mendel developed two fundamental
principles of heredity:
–Principle of Segregation
–Principle of Independent Assortment
**This is stuff you should recall from
genetics.**
Principle of Segregation
• Factors (genes) are
neither changed
nor blended in the
heterozygote, but
segregate from
each other to be
transmitted as
discrete particles.
Figure 02: Mendel’s results for the
inheritance of seed shape (smooth
or wrinkled) in pea plants
Principle of Independent Assortment
• Genes for different
characters segregate
independently of
one another.
Figure 03: Segregation and
independent assortment of seed
texture and seed color
Adapted from Strickberger, M. W. Genetics, Third edition.
Macmillan, 1985.
Deviations from Mendelian Genetics
• Extranuclear Inheritance
– Some traits do not follow a nuclear pattern of
inheritance but rather transmit through the
cytoplasm of the egg.
• Maternal inheritance
• Sex-Linked Genes and Sexual Reproduction
– When do genes not necessarily assort
independently of each other?
**When they are linked together on the
same chromosome.**
• Sexual reproduction- two sources of
variation
–Recombination can produce different
combinations of genes along a
chromosome and independent
assortment.
–Individuals then incorporate different
beneficial mutations from other
members through mating.
• Why Sexual Reproduction—We expect natural selection to
preserve associations between alleles that benefit the fitness of an
individual. Sexual reproduction (independent assortment),
however, creates new association and genetic recombination
breaks up those alleles.
1.
Originated as a way of overcoming DNA damage by using recombinational
DNA repair mechanisms. Any genetic variation that resulted is regarded as
an accidental by- product.
• Production of genetic variation is likely the primary reason for the
persistence of sex.
2. Removal of deleterious mutations— more mutations accumulate with
asexual than with sexual reproduction.
3. Proposed that sex is advantageous to organisms that exist in variable
environments (see #1 and natural selection)
4. Parasitic elements, such as transposons and plasmids, initiated or
promoted sexual fusion as a mechanism to infect other cells.
5.
Sexual crossing in a population allows single individuals to incorporate
different beneficial mutations from other members through mating and
genetic recombination.
• Without sex, combinations of such beneficial mutations are more
difficult to achieve. Furthermore, mating and recombination in sexual
populations enable relatively rapid combinations of existing mutations.
• In both cases, the advantages achieved include additional genetic
variation allowing a population to persist in a changing environment.
– For example, parasites usually evolve rapidly to counter resistance
in their hosts and hosts usually evolve rapidly to counter infectivity
of their parasites. This “arms race” places a premium on rapid
evolution through sexual recombination.
• Example(s) of “Arms race”
Sex Determination
• Sex chromosomes
– For many organisms, especially mammals, sex
determination is associated with chromosomal
differences between the two sexes, typically XX
females and XY males.
• Autosomes and sex determination
– The sex of an individual is determined by the ratio of
X chromosomes to sets of autosomes (A).
• Environmentally induced sex determination
– Wide variety of mechanisms
– E.g. green spoon worm, Bonellia viridis