2. Natural Selection Flashcards
What is the mechanism for evolution?
Natural selection
What do terms like ‘Darwinian’ mean?
Evolution via natural selection
What does lack of understanding of evolution lead to?
Misunderstandings or simplification of Darwinian theory
What is variation?
The differences within a population for certain characteristics
Types of characteristics
Continuous e.g. height, IQ, attractiveness in humans
Discrete e.g. eye colour, blood type
Variation occurs at 2 levels
Phenotype & genotype
What is a phenotype?
Observable characteristics of an individual
How are phenotypic traits created?
By cells (bones, muscle, brain, etc.)
What are cells made from?
Proteins that are strings of amino acids
What do genes do in terms of amino acids?
Genes encode the sequence of amino acids
In natural selection, what is actually selected?
The phenotype
Two types of genes
Recessive & dominant
Alternative forms of genes are called…
alleles
Organisms with two copies of the same allele are called…
haploid
Organisms with different alleles are called
diploid
How are genes assembled?
Genes are assembled into chromosomes in nucleus
How many chromosomes do humans have?
46 in 23 pairs
How is sex determined?
By chromosomes - females have XX and males have XY
What are genes made of?
deoxyribonucleic acid (DNA)
How are genes arranged?
Long chain molecule, two strands bound in double helix
Two strands bound by sequences of base pairs
What are the base pairs?
Adenine (A) pairs with Thymine (T)
Cytosine (C) pairs with Guanine (G)
How many base pairs make up a gene (or allele)
Approximately 27,000
How do amino acids make phenotypic traits?
Amino acids make proteins, and from there make phenotypic traits
Who was the first human to know his entire genetic code?
J. Craig Venter
How many base pairs, chromosomes and genes make up a human?
3.2 billion bade pairs over 23 chromosomes, making approximately 25,000 genes
What % of DNA is genes?
40%
What is 60% of DNA?
Non-coding, such as transposable elements, sequence repeats and junk
What % of our genes do we share with chimpanzees?
98%
What % of our genes do we share with bananas?
50%
How many base pairs difference between humans and chimpanzees?
Approximately 32 million
What characteristics must the 2% difference in DNA between chimpanzees and humans explain?
Bipedalism
Bigger brains
Hairlessness
Language(?)
What are sex cells called?
Gametes
Sexual reproduction
Sex cells (gametes) contain 1 of each chromosome
Split through meiosis, creates sperm and eggs
These then combine, make embryo
Chromosomes pair up, new genotype formed
Why is sexual reproduction relevant?
It is a major source of variation between individuals of a species
Different types of mutations
Single-base substitutions
Single sequence repeat expansions
Insertions
How many new mutations do we carry in our DNA?
100-200
What are mutations caused by?
Mutagens: X-rays Cosmic rays Radioactive substances Various chemicals and even 'mutator' genes
Are mutations good or bad?
Most are either neutral or damaging, but sometimes advantageous
Why is heredity a necessary component for natural selection to occur?
Only traits that are inherited will be exposed to evolution by natural selection i.e. hair colour, but not hair length
How ‘heritable’ are traits?
Different traits vary in heritability
What genes are inherited will be partly based on…
sexual reproduction
What is the study of heritability called?
Behavioural genetics
How can we estimate heritability?
Twin studies
Two types of twins
Monozygotic (MZ) - share 100% of genes
Dizygotic (DZ) - share 50% of twins
What is included in twins’ assumed shared environment?
Diet
Home life
Parental relationships
Schooling
What do concordance rates between twins show?
How much of trait is explained by genes and how much is explained by the environment
If concordance rates are larger between MZ twins than DZ twins, what does this say about the size of the genetic component?
Genetic component will be large
What does little difference between concordance rates in twins suggest?
That environmental influences are more important in determining phenotypic variation
Geschwind et al. (2003)
Measured total volume of cerebral hemispheres in both MZ and DZ twins
MZ correlation was 0.87
DZ correlation was 0.56
Therefore the size of the genetic component (heritability) of size of brain structures is 0.62
How can we use twin studies to calculate the effect of non-shared environment?
1 - concordance rate in MZ
How can we use twin studies to calculate the effect of shared environment?
Concordance rate in MZ - heritability
Problems with twin studies
- Assumes shared environment is the same for twins
- The nature of adoptive families
- Adopted children have 9 months shared environment with their biological mother - prenatal effects (e.g. hormones/diet/drugs) may be large
Greater the heritability, more likely that trait is…
adaptive
TRUE OR FALSE: Traits with heritability >0 can be shaped by natural selection.
True
TRUE OR FALSE: Heritability has to be 100% to say that a trait is genetic.
False
What is the heritability of general intelligence?
Approximately 0.35
What are pre-requisites for natural selection?
Variation and heredity
TRUE OR FALSE: Evolution will only occur when there is some form of selection (i.e. natural, artificial).
True
Selection relies on some form of…
competition
Thomas Malthus
Published “An Essay on the Principle of Population” 1798
European population was growing exponentially
Limited resources - competition to survive
What determines which traits are selected for?
Levels of competition and force of selection
What are K-selected species?
few offspring
high likelihood to survive
long generations
What are r-selected species?
more offspring
lower survival rates
short generations
e.g. fish
How do selection pressures affect K-selected species and r-selected species differently?
r-selected species may be more rapidly and strongly affected
BUT
r-selected species can evolve quicker to cope with extreme selection pressures and can cope with rapidly changing environments
Size of selection pressure in the environment will affect levels of…
competition and selection
Examples of large selection pressures
Major catastrophe e.g. meteor striking the earth or lethal pandemic
‘Endler’s guppies’ experiment: background
Guppies vary from stream to stream
Brightness of males attracts females BUT also predators
Where predator levels high –> drabber males selected for (camouflaged with gravelly floor)
‘Endler’s guppies’ experiment: method
Guppies randomly assigned to ponds with either strong/weak/no predator
Before predator introduced, guppies became more colourful and spotty to attract females
After predator introduced, those in the ‘strong’ condition had less spots
‘Endler’s guppies’ experiment: conclusion
Selection for predator avoidance was greater than selection by females for colourful males (due to size of selection pressure)
What does ‘fitness’ mean?
number of copies of particular allele in subsequent generations
What are the types of natural selection?
Purifying selection
Stabilising selection
Directional selection
3 aspects of adaptation
RELIABILITY: Does trait develop in all members of species in normal environments?
EFFICIENCY: A good solution to a problem?
ECONOMY: Does it do so without great costs to the organism?
Example of an adaptation
Imprinting
The adaptationist stance
If a particular trait is common in a population, then it is the result of natural selection
Which stance does evolutionary psychology take?
The adaptationist stance
