Introduction to Evolutionary Theory, Population genetics (Hardy-Weinberg) , Genetic Drift, and mathematical calculations to predict genotype frequencies
Evolution: Process by which inherited traits in populations change over time
- Organisms are exposed to external environmental change
- Genetic innovation/variation is random with respect to direction of subsequent
adaptation
- Adaptation is driven by natural selection
Microevolution: processes at level populations and over relatively short timescale
How do traits become well adapted to tasks of survival and reproduction?
Local Adaptation: Strong selection can produce rapid evolution in large populations
Example: Guppies, small freshwater fish, living in Aripo River (Northern Trinidad)
subject to 2 predators – Cichlid + Killifish
- Many streams flow into Aripo – cichlid found primarily in river and lower
streams (eats guppies of all sizes and ages ,juveniles and adults), killifish found
in many of the smaller streams (eats smaller juvenile guppies)
- In populations with more dangerous cichlid predator: guppies matured earlier at
a smaller size, with less ornamented (less brightly coloured) males, heightened
predator avoidance behaviour when compared to guppies found only with killifish
- Evidence that populations have evolved contrasting adaptations to local
predator conditions
Test the rapidness of adaptation via experimental evolution assay
- Introduce guppies from population adapted to presence of cichlid into a stream
that contained killifish but no cichlids
- After 20 guppy generations – transplanted populations had evolved suit of
phenotypic trait patterns that resembled those of guppy populations that had
long-term co-existed with killifish
Macroevolution: Over longer timescale, processes producing diversity of life now and
during the fossil record
Evolution has practical relevance + concerned with immediate benefits
Disease vectors evolve drug resistance
Insect pests evolve pesticide resistance
Weeds evolve herbicide resistance
Knowledge of past extinctions may help us devise strategies to slow declines of
current species
,Focus of selection is on multiple aspects of fitness – survival & fertility contribute to
individual performance
Adaptation doesn’t necessarily have to immediately benefit the individual
Could be beneficial on a population level (a net effect)
Example: Sexual cannibalism in Australian red- backed spider (Female Latrodectus
hasselti eating a male after copulation – male size much smaller than female)
- Copulation involves male inserting one of two sperm-laden palps into one of the
pair of female genital tracts
- When palp inserted – male somersaults to position his abdomen under female’s
mouth – female starts to digest the male, using poison-laden fangs
- Process repeated for other fang – female then stores male in silk for later
consumption
Individuals are widely dispersed – cannibalism means males contribute indirectly to
the nourishment of their progeny
Males increase their lifetime net reproductive success – more than they would on
average if survived first mating attempt then searched for another mate (could risk
death during search)
Low chance of male surviving mate with 2nd female
History of the Evolutionary Theory
Classical notions (from Plato and Aristotle) – Theory of Forms: species theorised as
strict categories, hence failed to explain evolution as species are fluid entities
Lamarck – inheritance of acquired characters; evolution as striving – but flaw is that
evolution does not anticipate the challenges an organism will face
Darwin + Wallace (1859) – Theory of Natural Selection – competition for survival
meant that only successful organisms could reproduce and hence pass their traits to
offspring
Flaw: lacked satisfactory explanation of mechanisms of variation and inheritance
Provisional theory of pangenesis: features in common with notion of inheritance of
acquired characters and earlier blending theories
Weismann (1880s) – refuted Larmarkism via theory and empirical evidence
Demonstrated that acquired characters cannot be inherited: proposed (correctly)
that cells forming germ line (and so gametes) are set aside from rest of tissues
(soma) early in development.
Gregor Mendel (1870, rediscovered ~1900) – “Atomistic” or “Particulate” theory of
inheritance. Evidence from controlled breeding experiments with seed characters
of pea plants
Blending Inheritance vs. Mendelian Inheritance
, Blending: on a broader context, population has
become more homogenous (fewer colour
morphs)
Newly arising advantageous mutants will likely
be “blended” away and fail to become
established
Mendelian: on broader context – preservation
of variation
Under natural selection, newly arising
advantageous mutants can persist in a
population and become established – they will
not be diluted away
Mendelian mechanism of inheritance (1870)
fits well with Darwin’s theory but no
recognised until 20th century
Neo-Darwinian Theory/Modern Synthesis
Theoretical + empirical approaches combined to reconcile theory of evolution with
mechanism of heredity
Multiple contributions from genetics, palaeontology, taxonomy
Ronald Fisher + JBS Haldane: especially examined effects of selection on changes in
gene frequencies. Looked at consequences of slight differences between individuals and
genes in their response to natural selection.
Sewall Wright: focussed, not only on selection, but also on effects of chance events on
changes in gene frequencies
Population Genetics
Frequency of phenotypic traits in a population linked to relative abundance of alleles
influencing traits
Population genetics: study of genetic variation in a populations over time
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