4 Testable Postulates of Natural Selection
& what if they hold true?
- individuals in a population are diff from one another
- differences are, at least in part, passed from parents to offspring
- some individuals are more successful at surviving and reproducing than others
- successful individuals are not just lucky; instead due to variant traits inherited, which will be passed to offspring
*If these hold true, population changes gradually over time (Darwinian Evolution)
the 4 testable postulates of natural selection put differently
- living things have more offspring than sustainable
- constant struggle for existence
- individuals in population vary in phenotypes
some of the variation is heritable (genotypes) - those individuals best adapted to current conditions (fitness) -> most likely to survive & reproduce
- if adaptations are heritable, passed on to offspring
Fitness?
ability of an individual SURVIVE & REPRODUCE in its environment (lifetime reproductive success)
- relative term (relative to other population members’ fitness)
- objectively measured in nature by:
(1) counting offspring over lifetime
(2) observing ability to survive some natural event
natural selection acts on ___, whereas evolution is ____
phenotypes
change in allele frequencies in a population
evolution in population results from ____________
natural selection on individuals
population genetics
study of allele and genotype frequencies and changes in entire populations
loci vs gene vs allele
loci - some part of genome (can be gene, non-coding)
gene - part of genome that can produce protein
allele - forms of gene
adaptation
a trait that increases ability of individual to survive and reproduce compared w individuals without that trait
trait
any characteristics of an individual ranging from outward appearance to molecular characteristics
A Test of Theory (natural selection) example (finch)
species: medium ground finch
credits: Peter/Rosemary Grant, Peter Boag
research benefits for species/location:
- minimal dispersal
- population small enough to study exhaustively
postulate 1: histogram data (beak depth)
postulate 2: variability can be due to env, gene, or both (heritability - proportion of trait variability that is due to variation in genetics among individuals
postulate 3: 1977 - dry year; fewer & harder seeds
postulate 4: conditions favoured larger beaked specimen, who produced larger beak offspring
exaptations
when some biological function takes novel advantage of another function
secondary adaptation
when the adaptive changes divert from original direction
modern synthesis
reformation of theory of evolution based on genetic integration w Darwin’s 4 postulates
grounded on 2 postulates
- gradual evolution results from small genetic changes that are acted on by natural selection
- origin of species and higher taxa, or macroevolution, can be explained in terms of natural selection acting on individuals, or microevolution
modern synthesis - 4 new postulates
(1) as result of mutation creating new alleles, and segregation and independent assortment shuffling alleles into new combinations, individuals within populations are variable for many traits
(2) individuals pass their alleles on to their offspring intact
(3) in every generation, some individuals are more successful at surviving and reproducing than others
(4) individuals that survive and reproduce, or who reproduce most, are those w the alleles and allelic combinations that best adapt them to their env
modern synthesis - summary
- alleles associated w higher fitness increase in frequency from one gen to next
- consider human-induced selection regimes in natural populations (e.g. big horn sheep)
types of selection (3)
- directional (change in average, no change in variation)
- stabilizing (no change in average, variation decreases)
- disruptive (no change in average, variation increases)
example: gall making fly
gall is immune response to fly larvae
- type of wasp will eggs in gall so their offspring eat fly larvae
directional selection (small gall -> wasp predation)
direction selection (big gall -> bird predation)
therefore, actually STABILIZING selection
