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Summary - evolutionary developmental biology AND human evolution AB_1141
Summary from 23/24 evolutionary developmental biology (EDB), next year (24/25) the course will be named Human Evolution.
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Evolutionary Developmental BiologyChapter 2 Evolutionary Theory
Evolutionary biology provides explanations of 2 phenomena:
1) How a plethora of species emerged from a single common ancestral species: macroevolution
2) How organisms come to be well matched to face the threats and opportunities in the
environment they inhabit: microevolution
Creationism: the belief that a deity of supernatural force intervened in the making of the natural
world, which is in contrast to EB.
Empirical data is difficult to obtain, because evolution takes place over multiple generations. In case
of human evolution, researches are largely restricted to the interpretation of retrospective analyses.
Contemporary evolution: microevolutionary adaptation to new environment, because of a strong
selective pressure.
The diversity between species can appear great, as between a whale and a beetle for example, or
small, as between two species of bacteria. Evolutionary theory provides an explanation for how this
huge diversity of present and past life forms arose from ancestral species. Evolutionary theory is
concerned with the diversification of life on Earth but does not attempt to provide an explanation for
how life originally arose, which remains an important but separate question.
Adaptation: features that make an organism better able to survive, and affects the fitness.
- Must increase fitness: growth and survival.
- Trying to imagine what a trait would be like if it had been selected, can easily lead to
teleological arguments: ‘just-so stories’.
Fitness: the potential for an individual to survive and reproduce successfully.
- Reproductive age (if individual dies before this age, fitness = 0)
- Reproductive rate (number of offspring produced)
Reproductive success: the number of offspring it produces, that are able to reproduce as well.
Relative fitness: absolute fitness / average fitness (mutation has 3 offspring, normal genotype has 2
relative fitness = 1.5). Thus, mutated genotype will increase by 50% in the next generation.
The favored genotype may depend on the environment.
Indirect fitness: by helping relatives survive, an individual can improve the fitness of that genotype
Inclusive fitness: direct fitness + indirect fitness
Darwin and Wallace’s findings:
1) Individuals of a species vary in ways that affect their fitness (variation)
2) Some part of this variation is heritable (inheritance)
3) Differential fitness among species leads to over-representation of successful forms in the
next generation (selection)
4) Selection over generations leads to change in the composition of the population (evolution)
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individual composed of cells of two genetically different types
,In order to evolve, the phenotypic traits must be associated with heritable factors. Thus, selection
must result in changes in allele frequencies in those genes.
Types of variation:
Heritable
- Single nucleotide polymorphism SNP: single base substitution
- Copy number variation + inversion + non-coding DNA: duplication
Non-heritable
- Aneuploidy = deletion of entire chromosome
- Duplication of entire chromosome
The process of recombination can generate high levels of variation in offspring, because it generates
new combinations of alleles on a chromosome. Particularly to have an effect on the phenotype.
Some genes are more likely to recombine (hotspots), and others are more likely to stay linked.
Linkage disequilibrium refers to the bioinformatic estimate of how closely genes stay linked through
recombination, which can identify recent human evolution.
Constraints on variation:
1) Accumulation of genetic variation and its filtering by selection are slow processes, which
makes the response to new selective pressure such as environmental changes slow as well.
The organism will look like a ‘poor design’ or mismatch of environment and organism.
2) Variation and selection requires continuous ‘tinkering’ and not allow partial dismantling of
an existing variant of modest fitness to construct a better design. This would reduce the
fitness of intermediate generations.
3) There is a limited amount of genetic variation in any generation. A population may fail to
meet an environmental challenge simply because it lacks the genetic variation on which
selection could work. (More likely in bottleneck populations)
a. Bottleneck = an event that drastically reduces the size of population, which causes
the population to have a small range of alleles
4) Processes that reduce the chance of the phenotype being expressed, ex. Distinct genotypes
produce the same phenotype.
a. Canalization = a self-correcting mechanism to ensure that a relatively narrowly
defined phenotypic outcome develops. Some novel mutations remain silent, and
they will not be exposed to selection. This causes the traits to accumulate if not
exposed by stressors.
Types of selection:
1) Artificial selection = breeder identifies a characteristic and chooses to mate particular
individuals to try and amplify (positive selection) or remove (negative selection) that trait
from the lineage.
a. Only successful if the trait is genetically determined and not constrained by factors.
2) Natural selection = some traits are more likely to survive and to reproduce successfully, and
their offspring would also be more likely to survive/reproduce. No external agents required.
a. Speed of selection depends on the genetic determinants of the trait and the strength
of selection pressure created by the environment.
b. The principle that leads to changes in allelic frequencies across generations.
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individual composed of cells of two genetically different types
, c. The preferred allele is different in every environment.
d. Balancing selection = different alleles can stay if they have differential advantage in
relation to the environment.
3) Sexual selection = differential fitness created by intraspecies competition affecting which
members of the species could mate.
a. Same sex: males compete and a certain male phenotype is favoured and reproduced.
Not necessarily physical traits, can be singing or finding objects.
b. Other sex: member of one sex chooses their mate through attractiveness.
c. Can lead to relatively rapid evolution of traits which improve reproductive
attractiveness or competitive abilities.
Extended phenotype = everything that the genes can influence to gain advantage
- Social behaviour, manipulative effects of parasites and pathogens, creation of artifacts
(nests), engineering of the environment by building structures (dams).
Niche construction = organism changes environment by building structures that influences selection
Gene-culture coevolution = culture creates selective pressures and thus may change the direction of
evolution, which may change culture.
Meme = true unit of cultural inheritance, capable of self replication with variation due to errors in
duplication, which would have a different fitness. Ex: tunes, catch-phrases, clothes, architecture.
Teleology = if a feature of an organism performs a function, then it has been chosen to achieve some
goal or purpose. It’s too easy to suppose that every trait is an adaptation.
Limitations of the Adaptationist Argument:
1) Evolution has no direction: it doesn’t go from ‘lower’ to ‘higher’ organisms. The trend
towards increasing complexity is not in itself an argument for progress. Sometimes evolution
leads to greater simplicity, like the loss of an eye
2) There is no external process ‘selecting’ the fittest individuals. It is just the result of
differential reproductive success among individual organisms.
3) Evolution does not ‘improve’ the species. The reproductive success is only maximized in a
environment that does not change. The match between the species and its current
environment is merely maintained. More importantly, the improvement is defined by
reproductive success, longevity or health is ignored by natural selection.
4) Evolution does not act for ‘the good of the species’. Most selection occurs at the level of the
individual.
Stabilizing selection = extremes in traits in either direction are selected against.
Convergent selection = similar phenotypic traits, serving the same function, evolve independently as
a response to similar environments. Different ‘solution’ same problem.
Neutral theory = random mutations and drift account for most of the variation between individuals
at the level of DNA and amino acid sequences. Many of these will be neutral (no effect on fitness)
because they’re not in a functional region. This way, different genotypes between populations arise
without selection.
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individual composed of cells of two genetically different types
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