2007 ECOL Lecture 6
Darwinism and DNA
What is a species?
A species is an actually or potentially interbreeding population that does not interbreed with other such populations when there is opportunity to do so
It has been proposed that a genome difference of 2% between individuals is the limit for reproductive compatibility Gene trees and species trees are not the same
Morphological species are based upon phenotypic characters
All taxonomic levels above a species are arbitrary Scientists variously try and group together (lump) or separate (split) species into genus, family, order etc
Phylogenetic species are based upon DNA evidence for evolutionary history
These are also subject to arbitrary judgements including: Back mutation, gene transfer, variable rates of evolution Choice of target gene(s) How phylogenies are constructed
Phylogenetic and Phenotypic trees are not the same Pisces Mammalia
How do new species arise? Speciation
Subspecies P Species A Race X divergence Race Y Subspecies Q Species C Species B
Darwin visited the remote Galapagos Islands in 1835 aboard HMS Beagle
He used observations of 13 endemic finch species to illustrate his theory of natural selection
Darwins 1859 theory of evolution by natural selection
Individuals within a species are variable Some of these variations are passed on to offspring In every generation, more offspring are produced than survive Survival and reproduction are not random
the individuals that survive and go on to reproduce, or reproduce the most, are those with the most favourable variations
They are naturally selected!
Natural selection is NOT evolution
It is just one type of evolutionary mechanism As it was the first to be discovered it is often wrongly used to describe evolution as a whole
An analogy is the way some people describe a vacuum cleaner as a Hoover or a blender as a Kenwood after the first manufacturer
Evolution can be viewed as a two-stage process:
Mutation creates hereditary variation Natural selection, genetic drift and gene flow affect the way these variations are passed on to offspring
Natural selection
Sometimes called adaptive evolution its all about fitness Fitness is a measure of reproductive success Those individuals who create the largest number of mature offspring are the fittest Types of natural selection:
Survival (or mortality) selection Mating (sexual) selection Family size (fecundity) selection
Heritability
Neutral trait Environmental factors Advantageous gene
How does selection affect populations?
Stabilizing selection
Natural selection often works to weed out individuals at both extremes of a range of phenotypes
Results in the reproductive success of those near the mean This maintains the status quo It is not always obvious why both extremes should be disadvantaged!
perhaps sexual selection or liability to predation is at work
In any case, stabilizing selection is common
Directional selection
Here individuals displaying one extreme of phenotype are advantaged Darwins finches again provide an an example!
During a drought in the 1970s few seeds were available to ground finches Only larger tough seeds survived Only finches with larger beaks and bodies could eat these After the drought average body and beak size of offspring remained large = directional selection
beak and body size
Another famous example is that of the peppered moth that experienced severe directional selection for colour during the Industrial revolution in England in the 19th Century By 1849 black moths formed 90% of the population
Disruptive selection
Here individuals at both extremes of phenotype are advantaged
Eg: Plants growing on toxic soil often develop resistance but lose their ability to compete on clean soil with the wild type
This can lead to split gene pools
Genetic drift
Sometimes called neutral evolution An allele may increase or decrease in frequency simply through chance
In the short term allele frequencies would increase/decrease at random creating a high level of polymorphism But the long term effect of genetic drift is a loss in diversity as alleles are lost or fixed such that they are the only variant
This is genetic drift and it is particularly strong in:
Small populations When the gene is neutral (neither helpful nor deleterious)
At any point in time, many genes are on their way to loss/fixation
Frequency
Time
Eventually the entire population may become homozygous for the allele (it becomes fixed) Or equally likely the allele may disappear
Before either of these occurs the allele represents a polymorphism
So at any given time there are numerous alleles on their way to loss or fixation, not yet represented in all individuals hence high polymorphism!
Polymorphism can be reduced in a population by:
Founder effect (single gravid female colonizing a new area) Bottleneck phenomenon (severe reduction in population size)
Example:
Cheetahs seem to have passed through a historical period of small population size with its accompanying genetic drift Examination of 52 different gene loci has failed to reveal any polymorphisms These animals are homozygous at all 52 loci This lack of genetic variability is so profound that cheetahs will accept skin grafts from each other just as identical twins!
Must get a skin graft for this cut on my paw!
Balanced polymorphism can also occur Example: In parts of Africa where malaria is caused by Plasmodium falciparum The allele for sickle-cell hemoglobin is also common
Heterozygous children who inherit one gene for the normal beta chain of hemoglobin and one sickle gene are more likely to survive that either homozygote Homozygous sickle allele children die young from sickle-cell disease Homozygous normal beta chain children are far more susceptible to illness and death from malaria
The main result of genetic drift is a loss in genetic variation With every generation there will be an allele that has become rare by chance that is lost when it is not passed on to the next generation The smaller the population the greater the effect Genetic drift also results in greater genetic difference between populations
Since different alleles become fixed by chance in each population
Genetic drift affects how a population evolves by natural selection
Genetic drift tends to make natural selection less likely It can also counteract the effects of natural selection
It makes all individuals genetically more similar It can cause alleles that lead to low fitness by chance
So it is often assumed that small populations (for example) have lower levels of adaptation than large populations
Other evolutionary mechanisms
Gene flow
Caused by individuals moving between populations Increased genetic diversity within a population since new alleles are introduced to the gene pool But it also makes populations genetically more similar because of this (introgression)
The wolf gray and the coyote
Phylogeny of mtDNA genotypes in gray wolves and coyotes based on RFLP data
(W = wolf genotype, C = coyote genotype, * = gray wolf genotype identical or very similar to coyote)
Hybrids are red wolves
But we have also observed that evolution appears not to occur in a smooth fashion over time
Remember the Cambrian explosion!
Stephen J Gould (1941-2002) proposed a theory to explain this: Punctuated Equilibrium Basically he suggests that when a population finds itself isolated or in new environmental conditions, an initial rapid period of evolution will be followed by a period where individuals vary little from their ancestors
This is in contrast to the more traditional view that changes occur gradually
Evidence for this:
Fossil records seem to show (in some cases) sudden changes between types of adaptation Gould suggests that the absence of transitional species is evidence for punctuated equilibrium
Whales are a good example of transitional species - 4 legged land mammals that returned to the oceans, visible in redundant leg bones in fins
He also argues that the transition to say have woings is not a single step There would have to be intermediates with 5%, 20%, 60% of a wing for example - but that this appendage would be no use So where is the natural selection for these intermediate stages?
So how do these evolutionary factors affect the genetics of natural populations?
Allopatric Speciation
This is basically the role of isolation
Allopatry = other country
Populations begin to diverge when gene flow between them is restricted It is no coincidence that races and subspecies of an organism almost never occur in the same location
Geographical barrier to movement of breeding individuals - and therefore gene transfer
Darwins finches are a good illustration of allopatric speciation The ancestral species arrived in the Galapagos several million years ago It found no predators and few competitors Proximity between islands allowed migration
But was an effective barrier to interbreeding
In isolation changes to the gene pool (genetic diversity of a population) occur via
Natural selection Genetic drift
Founder effect (in this case)
Sometimes a change in only one or a few loci can give rise to sufficient change for a new race/subspecies
If subspecies are reunited in the same location it is possible to resolve their status
If they reunite but fail to resume breeding then they are separate subspecies
Eg: C.pauper is found only on Floreana island, where it shares territory with the C.psittacula found on all central Galapagos islands
Isolating mechanisms that prevent subspecies interbreeding when they are reunited can be:
Prezygotic Factors relating to phenotype prevent mating
Eg: beak size (in mate selection), behaviour
Postzygotic Mating does occur but offspring are not viable
Eg: sterile offspring (mules)
The opposite scenario is where two subspecies reunite and interbreed to create a new hybrid species (hybridization)
This is particularly common in flowering plants
Eg: genetic recombination of parental genomes in the sunflower species H.annulus and H.petiolarus produced three hybrid species that grow in conditions distinct from either original species and cannot reproduce with wild types
Sometimes a reunion creates intense competition and this increases the rate of speciation One species is eliminated or character displacement occurs to lessen competition Eg: insectivorous C.psittacula and C.pauper have similar beak size as they eat similar prey but on Florean island P.psittacula has evolved a larger beak so the two species do not compete for food
When a number of diverse species form from a single ancestor species it is termed adaptive radiation A more recent example demonstrated this phenomenon at the genetic level in the house mouse
The island of Madeira off the northwest African coast supports 6 distinct subspecies of house mouse isolated in different valleys They have different karyotypes (chromosomal complements) Whilst there appear to be no prezygotic barriers to mating, this would prevent offspring arising between subspecies
Sympatric Speciation
Sympatric speciation refers to the formation of two new species from a single ancestral species all occupying the same geographic location Some evolutionary biologists do not believe this occurs!
They feel that interbreeding would soon eliminate any genetic differences that might appear
But there is strong (but indirect) evidence that sympatric speciation does occur
Mechanisms: Disruptive selection favours individuals at extremes of an expressed trait, so intermediate phenotypes are discriminated against Assortative mating individuals prefer mates similar to themselves, and so mating with wild type prevented
Two different species of three-spined sticklebacks inhabit five lakes in BC Canada
Large benthic species with a large mouth that feeds on large prey in the littoral zone A smaller limnetic species with a smaller mouth that feeds on the small plankton in open water
Each lake was colonized by a single ancestor The two species in each lake are genetically more closely related to each other than they are to any of the species in other lakes
But the two species in each lake are reproductively isolated neither mates with the other Aquarium tests showed that The benthic species of one lake will spawn with the benthic species of another lake Even though they are less related to them than their limnetic cousins!
Parapatric speciation
This is another possible way for new species to arise in the absence of geographical barriers
Again indirect evidence only
Where a population ranges over a vast area with geographical gradients
Individuals can disperse over a relatively small part of this area due to distance Genetic isolation arises simply from the great distance separating subpopulations
This has been proposed to explain the enormous diversity in the Amazonian basin
Ecological speciation
Level of reproductive isolation 0% Allopatric phase
Speciation occurs as a by-product of adaptation to alternative environments
100% Sympatric phase
Speciation occurs through reinforcement, due to pre-mating isolation caused by low fitness of hybrids
Adaptation to specific habitats: Niche-variation model
predicts specialized organisms in a narrowly-defined niche will exhibit low genetic variability
Fine-coarse grain model
predicts fine grained environments (seasonal variation but predictable over time) will support organisms of lower heterozygosity than coarse-grained (vary randomly between generations)
Indeed, low levels of heterozygosity in fossil animals such as the American alligator (Alligator mississippiensis) may reflect their stable environment over time
Directed Reading
Evolution, S.C. Stearns & R.H Hoekstra, Oxford University Press (Ch2 & 3) Main Library call number 576.S79