Lecture 11

Lecture 11: Speciation & Reproductive Isolation

Covers Chapter 16 & 17

• Evolution is not a property of individuals, but of populations…in other words, you have to look at a large number of individuals to see evolutionary change, not just a small number of them.

Definitions*

• Species:a group of organisms that is distinct from other species in appearance and behavior AND that regularly breed together producing FERTILE OFFSPRING.

• Population: a group that includes all the members of a species IN A GIVEN AREA that are somewhat independent of other populations and are substantially more likely to mate with each other than with members of other populations within the same species.

Definitions* within populations

• Gene pool: sum of all genes in a population (meaning all of the alleles of all the genes in all of the individuals of a population.

• Allele frequency: sum of all copies of EACH allele in each individual in a population

• Gene Flow: movement of alleles into or out of a population.

• As we already know, “mutations” in DNA result in different alleles (genes)

• The result of these mutations is different proteins that determine an individual’s traits (phenotype)

• So how does evolution happen?

HOW EVOLUTION HAPPENS*

• 1.) Mutation

• 2.) Gene flow

• 3.) Small population size

• 4.) Nonrandom mating

• 5.) Natural selection

1.) Mutation*

• Occurs during DNA replication (cell makes copy of its DNA)

• Most mutations fixed by DNA repair proteins, BUT if a mutation slips by the repair proteins and if it occurs in a cell THAT WILL BECOME AN EGG OR SPERM, the mutation can be passed to offspring and ENTER THE GENE POOL of a population.

How common is that?

• A mutation in the DNA of a sperm cell, in an organism that creates billions of sperm over its lifetime, can only possibly spread to very few individuals (first that sperm has to fertilize an egg) and then that organism has to mate and pass on the mutation to its offspring

• Human example: we have 20,000-25,000 genes, so 40,000-50,000 alleles (2 alleles for each gene). Each allele has 1 in 100,000 chance of mutation. Each newborn has 1-2 mutated genes on average.

• These NEW ALLELES are the material for evolution.

Result of mutation

• The protein that the mutated allele codes for will be altered, but perhaps not so much that the protein malfunctions….perhaps it will make the protein work more efficiently.

• If the mutation does result in a malfuction, or any other change in phenotype that renders that organism LESS able to reproduce, natural selection will likely wipe out that organism and therefore that allele. Conversely, a better more efficient organism will survive longer, thus spreading that mutated allele to its offspring and moving it through the gene pool.

2.) Gene Flow*

• When organisms leave their home population and travel to another population, they take their alleles to a new gene pool.

• Alleles can move through gene pools even if the organisms are incapable of movement. Ex: plants release seeds and pollen that can travel on the wind or on organisms that move (bees)

• The MAIN EFFECT OF GENE FLOW is to INCREASE the genetic SIMILARITY between different populations within a species. The destination population is more similar to the home (source) population

3.) Small population size affects evolution*

• Allele frequencies (how many of each allele are in each population) can change due to events OTHER THAN mutation.

• Genetic Drift: process by which chance events change allele frequencies. Two common events that cause genetic drift:– Population Bottleneck– Founder effect

Population Bottleneck*

– Population drastically reduced as the result of natural catastrophe or over hunting

– BY CHANCE, allele frequencies are changed and there can be reduced genetic variability.

Depiction of population bottleneck

Founder Effect

• Isolated colonies (populations) are founded by a small number of organisms– Small flock of birds becomes separated from

source population in a storm or by wind and starts its own new population in a new place

– BY CHANCE, the new population has allele frequencies that are different from its source population

Depiction of Founder Effect

Drift could result in LOSS of allele

• Genetic Drift occurs more rapidly in a small population than a large one. (less alleles to begin with, change will be less “visible” in a large population)

• Sudden reduction of individuals in a small population COULD remove an allele from that population COMPLETELY AND FOREVER.

4.) Non-random mating

• Non random mating: organisms seldom mate randomly. • They usually mate with others that are in their nearby area. • Organisms MAY mate with their own relatives…called

inbreeding.• This is harmful because relatives are genetically similar,

and inbreeding tends to increase the number of individuals who inherit the same alleles from both parents. This can increase homozygosity for each gene.

5.) Natural Selection*

• Covered in previous lecture, Natural Selection will favor traits that increase survival to the extent that this increased survival leads to improved reproduction. – Traits that may increase likelihood of living long

enough to reproduce– Traits that may lengthen the life of the organism,

therefore giving it more TIME to reproduce.– These traits, given to the organism by it’s genes/alleles,

are then passed on to more organisms.– (Remember that the result of these alleles is the

phenotype of the organism)

Part of the success is the environment

• All of these alleles/traits that make an organism live longer must also depend on the environment– Climate– Availability of water– Presence of predators/competitors

Example

• Buffalo grass in Wyoming – Roots have evolved to be efficient at taking up water

and nutrients from soil (it can be dry there)

– Roots have evolved to be long and go deep in the soil, thereby making it “better” at taking up water than other grasses growing nearby

– Grass is tough (silica compounds exist in the grass) so that animals are discouraged from grazing

COEVOLUTION

• When one species evolves to adapt to the environment or competition, other species in the area also change…COEVOLUTION.

• Example: wolves & deer• Slow deer are killed by wolves….fast deer are left

which pass on their genes.• Faster deer means that slow wolves won’t catch

them…slow wolves die of hunger, leaving faster wolves to pass on their genes.

Species & Reproductive Isolation Definitions*

• Recall that a species is a group of organisms that regularly reproduces and produces FERTILE OFFSPRING.

• Members of a species are also reproductively isolated….meaning that individuals outside the species cannot reproduce with members of the species.

• How do we insure reproductive isolation?

Isolating Mechanisms*

– Premating isolating mechanisms: prevent mating between species

• Geographical • Ecological• Temporal• Behavior• Mechanical incompatibility

– Postmating isolating mechanisms: prevent formation of any hybrid organisms

• Gametic incompatibility• Hybrid inviability• Hybrid infertility

Premating isolating mechanisms*

• Geographical isolation: organisms LIVE IN PHYSICALLY SEPARATED locations– This also helps the formation of new species

• Ecological isolation: organisms OCCUPY DIFFERENT HABITATS

• Temporal isolation: organisms BREED AT DIFFERENT TIMES OF THE YEAR

• Behavioral isolation: organisms HAVE DIFFERENT COURTSHIP AND MATING RITUALS

• Mechanical Incompatibility: SEX ORGANS OF ORGANISMS DO NOT FIT (Internal fertilization)

Postmating isolating mechanisms*

• Gametic incompatibility (especially important in marine invertebrates and wind-pollinated plants): GAMETES HAVE CHEMICAL OR MECHANICAL STRUCTURES THAT ONLY ALLOW OTHER GAMETES OF THEIR OWN SPECIES TO FERTILIZE

• Hybrid Inviability:HYBRID OFFSPRING FAIL TO SURVIVE TO MATURITY

• Hybrid infertility: HYBRIDS DO SURVIVE BUT CANNOT REPRODUCE (Ex: Mules)

Liger

Zebroid: zebra & horse

Mule: male donkey, female horse

How do new species form?*

• Speciation: process by which new species form

• Depends on two factors:– Isolation of populations: there must be no gene

flow between 2 populations– Genetic divergence: genetic drift or natural

selection causes organisms in one population to become different (new alleles spread through gene pool)

2 pathways to speciation*• Allopatric speciation: two populations of a species become

separated by a geographical barrier (islands, reefs). Natural disasters can cause this separation: earthquakes, volcanoes, floods

• Sympatric speciation: two populations of a species become separated, but not geographically. The two populations occupy same area but different habitats.

• Eventually, after enough time, the two populations become so genetically different that they could not mate with members of other population: therefore they are now two different SPECIES.

Allopatric

Sympatric

What causes extinction?

• Extinction: death of all members of a species

• 99.9% of all species that ever existed are now extinct

• Causes: – Environmental change– Overspecialization– Interaction with competing species

Environmental Change

• Habitat destruction due to natural disasters or destruction by man

• If a species is limited to a specific area, more chance of extinction if that specific area is damaged

Overspecialization

• Some species become so specialized (can only eat certain food, can only live in certain place/temp) that destruction of their area can cause extinction.

Interaction with competing species

• Species unable to compete with other species in same area can go extinct.