EVOLUTIONARY PSYCHOLOGY
, LECTURE I- Evolution and natural selection
1-What is evolutionary psychology?
The study of human nature in the light of evolutionary thought, what can evolution tell us about psychology or human nature.
We can start with the question: are we blank slates (tabula rasa) moulded by culture or have evolutionary pressures shaped
us?
There are philosophers in the past that have argued that we are blank slates when we are born, that we are completely blank
and what we go through when we grow up is what makes , us who we are, like for instance St Francis Xavier, who said “Give
me a child until he is seven and I will give you the man”, which meant he can breed that child out in different ways in order
to make that child the kind of person that you want that child to be (through punishments, rewards, etc.), so you can make
anybody depending on the environment and culture they are growing. Other philosophers like John Locke also agreed that we
are product of our environment and that´s why we have the behaviours and personalities that we have.
On the other hand, a good chunk of our body is shaped by evolution, like the eye, could not be the case that the mind is also
shaped by evolution? So, the goal of evolutionary psychology in answer why we are the way we are, or in other words, what
psychological traits can be traced to our evolutionary past, and what to culture?
2-Human nature
From a biological approach, human nature is the set of species-specific traits that distinguish humans from other species.
Some of these traits are intelligence, language, morality, sexuality, cooperation, culture, society, science and technology, art
and beauty and fashion, etc.
3-Evolution
3.1-What is the evidence for it?
1. Geological stratification of fossils: digging into Earth you can find different layers of rocks
and nature materials from different times points. You can see fossils in a layer that you do
not find in any other one either after or before that, so they are geologically stratificated,
Earth was really different in past times so the living beings in that times were different too.
2. Transitional fossils: “Missing links”. In many species cases it is possible to link those old
fossils to what kind species that exist now (like dinosaurs to birds). We can have similarity
of fossils to living species, so that means things have gradually changed, not randomly
appeared and disappeared. We can trace the evolution species have gone through to reach
todays stage.
3. Shared traits across different kind of species: which means we come from a common descent. One example is body
homology or the sharing of a common body plan between many species (we share similar body structures, like arm
structure in humans, cats, whales, birds, etc.: one strong humerus, two forearm bones and kind of fingers). The fact that
even being species from completely different means have the same underlying structure, can indicate that they all come
from the same ancestor whom they have inherited these structures from, to then adapt them to the environment. We also
share common developmental pathways (embryology): many animals tend to have a similar embryonic pathway at least
in the first parts of pregnancy. Evolution has changed embryos a little bit to create new species, but only a little bit
because embryos are very sensitive to environmental forces so tries to preserve the same pathway as much as possible.
4. Vestigial traits: traits that serve no function at all, like fishes living in dark areas that still have eyes or
human muscles in the ears that we do not use anymore or pelvis in python. Their existence makes sense if
there is an ancestor who had them and they are still in the process of getting rid of it.
5. Bad “design”: active evidence for bad design, so many organisms have very poor forms, like human
knees or backs (we all use to suffer from them at some point).
6. Observed changes: in real time we can observe changes in many species like fruit-fly, mice, fern, etc.
3.2-What is it?
Heritable changes in a population over time. It includes any type of change, whether it leads to a more or less complex
situation. But it is not changing within an individual over time and in one lifetime, but hereditable changes.
The modern definition says it is the change in gene frequencies in a population over time (like changing from genes that code
for blue eyes to genes that code for brown eyes), what changes in the genetic composition.
Misconceptions:
, Evolution is progressive: you do not need to go from something simple to something more complex. Evolutions
does not care about that at all. In fact, for half of life existence on Earth, nothing has changed at all, unicellular
organism fitted perfectly in the planet for years and years and are less complex than pluricellular organisms, and
still the vast majority of organisms are still single cellular organisms.
Evolutions has a purpose: it is not goal directed towards “intelligent, stronger or faster beings”.
Evolution is the same as natural selection: evolution is a fact that things have changed, and natural selection is one
of many mechanisms to explain why this has happened. If someone disproves natural selection, he or she is not
necessarily disproving evolution.
Evolution is just a theory that has no evidence: (this applies mostly to the USA) it is just a small set of evidence
comparing to the huge amount of evidences towards the existence of it.
3.3-Major questions
The first question is how does evolution occur? Organism seem to be “adapted” to their environment, why and how are they
so well suited? Evidence like bees are attracted to the flowers they get the nectar from were used to say that they must have a
creator who has created species in such a way that they are so in sink to each other, rather than natural mechanisms coming
up with sophisticated things like this.
That is the challenge that William Paley had thrown as the people wanted to purpose natural mechanisms for evolution. So if
you want to come up with a theory for evolution, then you have to say how is it that organisms are so well adapted not only to
each other but also to the environment.
3.4-What are its mechanisms (for natural selection)
Pre-Darwin: many Greeks (4th cent BC) argued that evolution did not happened. Particularly Aristotle believed in a great
chain of being, there is a ladder of organisms, a hierarchy of organisms with worms in the bottom and humans in the top
and that it is always like this. World was created in such a way that this kind of hierarchy always existed and continues
to the future as well. So, things do not change.
As Enlighten came along, these ideas began to change when looking at archaeology and fossils. For example, Charles
Bonnet (1770) argued that based on fossils things must have changed gradually from one species to another species
while once in a while you have catastrophes which dramatically changes the composition of species and then new
species come and start changing gradually as well.
Jean-Baptiste Lamarck (1809) believed in the transmutation of species by inheritance of acquired characteristics. So,
species acquire characteristics during their lifetime and pass them to the descendance. This kind of traits accumulate
over time generating new species. Once this theory was disproved, there wasn´t a new one until Darwin came with The
origin of the species (1859).
Darwin: proposed a new mechanism called natural selection, which is still one of the best to explain evolution. One
modification of natural selection is sexual selection.
Natural selection was proposed by Darwin (he found more evidence) and Wallace.
Other mechanisms that explain evolution: genetic drift or gene flow.
4-Natural selection
4.1-How it works
Darwin observed that any kind of organisms always over reproduce so the
more organisms are born the more they ensure the survival of the specie.
But one problem of that is that there are limited resources in any given
geological location, so there is always a competition for resources.
Darwin also noticed that not all organisms are created equal, offspring has
different characteristics, so there is a variation in individual traits that can
come from mutations, sex, etc.
So, we have a competition for resources and variation in individual traits, that lead to some winners (in terms of evolutions
means essentially having more offspring, not being able to survive, even being survival necessary to have offspring) and
some losers in this competition for resources: some animals will produce more offspring than others.
If whatever allows the individual to survive is inheritable, these traits will be passed to the offspring and will quickly get
spread within the population. That is precisely what evolution is: the spread of advantageous traits in the population to
achieve adaptation to the environment.
, Natural selection is the most powerful explanation we have of evolution and adaptations in living organisms. Dobzhansky
(1973) said that “nothing in biology makes sense except in the light of evolution (natural selection)”.
4.2-Adaptation
Whom does adaptations benefit? Which level is adaptation acting on? Biosphere, species, groups, individuals, genes? Genes
are actually the ones that gets benefits from adaptation, adaptation works at the level of genes. It is controversial, because
some individuals think it benefits groups, but most of the evidence point to the genes being the ones more benefited by
adaptation and natural selection.
To understand this, we need to understand the concept of Inclusive fitness or the number of gene copies you pass on to the
next generation, so how many copies of genes were there in the one generation and how many copies are passed on the next
generation. The number of gene copies you pass on to the next generation itself is inclusive fitness.
Inclusive fitness comes in two forms:
Direct fitness: through own offspring (the number of genes you pass on to your own offspring). You are passing
half a copy of your genes and your partner the other half.
Indirect fitness: through relatives ‘offspring because we share some genes with our relatives (the number of genes
you pass on to your nephew/niece). You are passing one-fourth copy.
Inclusive fitness: direct + indirect fitness. So, let´s say you have two children and three nieces, the inclusive fitness
is 0.5*2+0.25*3=1.75 copies.
So, the fact that adaptation acts at the level of genes means that:
ADAPTATION (and hence natural selection) FAVOURS TRAITS THAT MAXIMISE INCLUSIVE FITNESS, increases
the number of gene copies going to the next generation whether directly through you or your relatives.
Of course, this is happening in a completely unconscious level, at the level of biology, with anyone directing it.
Most of the time, inclusive fitness correlates with individual survival, but not always, for instance: eusocial insects or
altruism.
Example of adaptation: the beak of finches increased size during droughts, where the plants produced larger and harder seeds.
But what actually happened was that individuals with larger beaks survived and produced more offspring with larger beaks.
When droughts were over, there was no more need for bigger beaks, so they got back to normal.
So, adaptation can be defined as a trait (structure or behaviour) that shows evidence of having been shaped by natural
selection for enhancing reproductive success (inclusive fitness).
4.3-Sexual selection
Next lecture.
