BIOL - Lecture 3 Flashcards
Key ingredients for evolution
- Organisms come from different organisms
- Organisms inherit at least some trait from their parents
- Organisms vary in traits within species, and this variation can be heritable
NS also requires one more ingredient
Organisms with some traits produce more offspring or survive longer than those with other traits
of offspring = its fitness
Relative fitness
The time it takes for a trait to increase relative to another depends on the relative fitness of each trait
Frequency-dependent selection
The fitness of a phenotype depends on how common it is in the population.
EX: Rock paper-scissors
- A rare strategy can win when others are common
- If everybody plays rock, it’s best to play paper
- But if everybody switches to paper, it’s best to play
scissors
Does evolution make the perfect organism?
NO
- Historical constraints
- NS uses existing structures
- Evolution does not build new structures from scratch
- Panda’s thumb - a large wrist bone from a meat eating ancestry, evolved to eat bamboo - Adaptations are compromises
- ex: peacocks tail attracts female and predators - Not all evolution is adaptive
- 4 other mechanisms (mutation, gene flow, genetic drift, non random mating) - NS works with available variation
- best rather than ideal
- slight imperfections are a sign of NS
5 Mechanisms of evolution
NS
Mutation
Gene flow
Genetic drift
Non-random mating
These mechanisms interact in all populations to drive evolution.
Population size is a key variable for determining which force is strongest.
Mutation
Happens constantly but at a very low rate.
Is a result from genes changing, due to mistakes in genetic copying
It is the ultimate source of all genetic variation but generally happens ar a low rate for each allele/gene
Gene flow (ie migration)
Movement of an organism from one existing population to another
Transfer of alleles into or out of a population
Often reduces genetic variation between the two groups. And can sometimes lead to two populations becoming one. Mixing of individuals between pops tends to reduce differences between populations over time
May cause population to lose alleles - aka decrease genetic variation - and another population to gain alleles - aka increase genetic variability
Genetic drift
Result of sampling error
Strongest in small populations
It’s not about selecting for adaptive traits, it’s about RANDOMNESS. (ie by chance, only three white flowers leave offspring)
Heavily impacts SMALL populations
Two types of genetic drift
Founders effect - if a small # of individuals colonize a new area. (ie a bird carries flower seeds to a new area)
It is starting a new population rather than happening between 2 existing populations.
Bottleneck effect - follows chance events such as storms, fires, clear-cutting etc.. can also be farming/hunting
The population is extremely reduced.
Effects of genetic drift
Significant in small populations
Allele frequency can change at random over time
Loss of genetic variation within a population
Harmful alleles can become fixed
Conservation implications of genetic drift
Heavily impacts SMALL populations
Endangered species are the “narrow” portion of a genetic bottleneck
Reduced genetic variation
Less able to adapt to change (due to less genetic variation)
Genetic Drift vs NS
The change in allele frequencies is not related to the alleles’ influence on reproductive success.
Non-random mating
Not all individuals have an equal chance to mate with one another
ie: bees tend to visit flowers of the same colour
Many species show assortative mating - non-random mating systems where mates are chosen according to their phenotype.
- positive - mates are chosen based on similarity of phenotype
- negative - mates are chosen based on difference of phenotype
Plays an important role in speciation (sympatric speculation)
Size of population affects the strength of each mechanism
small populations are more affected by random events (genetic drift) and gene flow
Only consistent mechanism of adaptive evolution
NS = chance + selection
Does NS create new phenotypes
No, it edits existing phenotypes
BUT over time it can make new phenotypes
Evolutionary mechanisms can interact
Mutation is the ultimate source of new variation
All traits have been shaped by 1 or more forces - New traits come first from mutations, secondarily from recombination and gene flow
Other forces may act against NS
- some fitter traits may not be selected in the long-term, because of drift (e.g. the ability to make vitamin C)
- mutation can keep rare non-adaptive alleles around
- gene flow can prevent adaption to local
Interaction between gene flow and selection
- Gene flow can slow adaptation down
- New individuals may be less adapted than those already in the population
- But movement can also lead to more adapted individuals finding the habitat and gene flow can bring new alleles, increasing variation
Hardy-Weinberg
HW equilibrium - a population is NOT evolving - the allele frequency remains constant
The frequencies of alleles and genotypes in a population will remain constant unless acted upon by outside agents or forces (such as NS, mutation, gene flow, genetic drift, non-random mating)
Assumptions for HW to hold
No NS
No mutation
No gene flow (isolated population)
No genetic drift (large population)
Random mating
THIS NEVER HAPPENS
HW Equation
genotype frequency
p^2 2pq q^2
allele frequency
p q
1 = p^2 + 2pq + q^2
or
1 = p + q
Why do we care about HW Theorem
It states that variation alone is not enough for evolution, frequencies of alleles and genotypes in a population will remain constant unless acted upon by outside forces
If a population is not in HW equilibrium, at least 1 of 5 evolutionary mechanisms are at work
HW conclusions
It predicts expected frequencies of genotypes given allele frequency
If observed frequencies match expected frequencies - in equilibrium - it is not evolving
If observed frequencies do not match expected frequencies - not in equilibrium - at least 1 evolutionary mechanism is at play
