Evolution Lecture 3

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Principles of Darwin's Theory
i) Organisms beget like organisms (new species evolve from
ancestral species).
ii) The number of individuals in each generation that survive
and reproduce is small compared to the number initially
produced.
iii) In any given population, there are chance variations among
individual organisms and some of this variation is heritable.
iv) Inherited favourable variation tends to become more
common from one generation to the next through differential
reproduction, a process called Natural Selection.
v) Natural selection leads to the slow accumulation of changes
that differentiate groups of organisms from one another
(speciation).
Darwin's Unresolved Things
-The relationship between natural selection and 'the effects
of use and disuse' (i.e., he could never fully reject
Lamarck's idea of acquired characteristics);
-How inherited characteristics are transmitted from one
generation to the next;
-Why inherited characteristics are not 'blended out' but,
instead, can disappear and reappear in later generations.
-How the variations arise, on which natural selection acts.
Blending
This can never transform a population through the spread of a favourable traits.
Further, it cannot explain the reappearance of traits
apparently lost in the parental generation.
i.e., why recessive traits disappear and reappear, from one
generation to the next
Atavistic
Developmental abnormality or mutations can
sometimes result in the appearance of these traits.
e.g. the capture of a four-finned dolphin in Taiji, Japan is direct evidence that the ancestral cetacean once had a modified hind limb and that the genes for the hind flippers are now vestigial (silenced but not lost).
Blending Inheritance
This inheritance was merely a widespread hypothetical model, rather than a formalized scientific theory, so it was thought inherited traits were determined randomly, from a range bound by the homologous traits found in the parents.
Mendel's Learnings
That the inheritance of a trait is determined by "units" or "particles" (now called genes), which are passed on to descendents unchanged.
That an individual inherits one such unit from each parent for each trait.
That a recessive trait may not show up in an offspring but can still be passed on to the next generation.
Mutation
This was regarded as the only significant force capable of bringing about evolutionary changes, including the origin of new species or forms and more complex organisms
('hopeful monsters').
•The importance of populations as the unit of evolution;
•The central role of natural selection as the primary
mechanism of adaptive evolutionary change, along with
random genetic drift and gene flow;
•gradualism as the explanation of how large changes can
result from an accumulation of small changes over a
long period of time

• e.g., new species arising from ancestral species by the gradual accumulation of genetic changes
Synthetic Theory
The modern theory of evolution, incorporating Darwinian thinking, Mendelian genetics, and an understanding of genes and genetic change at the molecular level.
Hardy-Weinberg Equilibrium
This describes the stability of the frequency of alleles in an
idealized population of diploid organisms where:
1) There is no mutation.
2) There is no movement of individuals with their
genes into or out of the population - no gene flow.
3) The population is large enough that chance alone
cannot affect allele frequencies - no genetic drift.
4) Mating is random or panmictic.
5) There is no difference in the reproductive success
conferred by any alleles - there is no selection.
H-W Equation
This allows us to calculate the expected frequencies of genotypes, given the observed frequencies of alleles.

p + q = 1, for allele frequencies
p2 + 2pq + q2 = 1, for genotype frequencies
Adaptive Radiation
Occasional strong selection of heritable characteristics in a
variable environment (along with geographic isolation) are
the keys to understanding the rapid adaptive radiation of the 13 species of Galapagos
finches.
Beak Depth in Darwin's Finches
Beaked depth is an important character influencing the feeding efficiency of the medium ground finch.
The character shows considerable variation in the population and is shown to be heritable, both
requirements for natural selection.
In wet years (El Niño), birds preferentially feed on small seeds
• average beak depth in the population decreases.
In dry years (La Niña), small seeds are rare and survival
depends on ability to crack big seeds
• average beak depth in the population increases.
Adaptation of the Population
Environmental change + Natural variation in an
heritable trait + Differential survival and reproduction (Natural Selection)
The Modern Synthesis
1. Ample genetic variation arises and is contained in populations by the random process of mutation, recombination, and chromosomal changes, not by directed responses according to the needs of an organism (ie not acquired characteristics
2. Evolution in populations is influenced by natural selection inparticular, as well as by gene flow and random genetic drift, and is characterised by changes in gene frequencies
3. Adaptive genetic variation produces small stepwise changes in phenotypes, which accumulate gradually in evolutionary lineages over long periods of time.
4. The divergence of geographically isolated populations is unimpeded and gradually leads to speciation of reproductively isolated groups
5. The continued gradual accumulation of genetic differences, under the principal guiding force of natural selection, results in new taxa above the species level (genera, families etc.) by the same processes that produce new species.
Population Genetics
The process of change in allele frequencies of a population or gene pool.
Population
An interbreeding group of organisms defined and united by the same gene pool
Species
A group of populations that actually or potentially interbreed in nature and are reproductively isolated from other populations.
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