Unit 10.3 Gene Pools and Speciation

essential idea

gene pools change over time

evolution

the genetic change in species over time

after mass extinctions

• new species evolve

• take ecological roles left vacant

• natural selection + chance events act on genetic variation causing some to be more or less common

• allele frequencies shift → differences accumulate

Gene pool

consists of all the genes and their different alleles present in an interbreeding population.

• some alleles will be more or less common

• all members of a population share a gene pool

• alleles combine with other alleles in gene pools during reproduction

• ability of species to evolve limited by genetic variations existing

allele frequency

• usually expressed as a percentage or proportion, measures how common an allele is in a population.

• for any gene, sum of all allele frequencies = 100%

• depth, or richness, of the gene pool is measured by the number of alleles and their relative frequencies.

• if allele frequencies are well adapted to environment → little genetic change because natural selections maintain status quo

genetic drift

random fluctuations in allele frequency

mutations

• allow for genetic variation which is prerequisite for evolution

• often harmful, sometimes neutral and occasionally beneficial

• random event

• occurs in all living things

• form alleles → allele frequency changes → evolution

evolution

• mutation → form alleles → allele frequency changes → evolution

• natural selections acts on survival & reproductive success of individuals and thus on alleles of that individua

• new allele combos occur through crossing over or random orientation

• given enough time & large population: alleles that are helpful become more common in the population

selective pressure

• influence of natural selection

• caused by biotic & abiotic factors changing rate of survival & reproduction

• examples

  • strongly selected against: fur color making predator stand out if noticed before prey captured

  • weak positive selective pressure: allele slightly better protection against rare parasite

• three types: stabilizing, directional, disruptive

stabilizing selection

• widespread

• existing beneficial variations already common

• acts against trait extremes

• new colors introduced by mutation but stabilizing selection exerted maintains

• population remains as 1 group

directional selection

• one extreme trait offers benefit

• Directional selection increases allele frequencies at one phenotypic extreme and reduces them at the other

• e.g. giraffe neck length increasing over generations

• population remains as 1 group

disruptive selection

• most frequent phenotype becomes a disadvantage, and individuals at both extremes have better rates of survival and reproduction

• population may break into two groups → speciation

• e.g. Choosing a mate that can survive even when it stands out to predators suggests a very strong assortment of other alleles

  • individuals can benefit from preference to similar individuals because offspring has less chance of inheriting intermediate phenotype

genotype frequency

number of individuals with a given genotype as proportion of the entire population

example:

• 10,000 total individuals

• 4900 individuals with genotype TT

• genotype frequency: 0.49 or 49%

  • 4900/10000

allele frequency math

• calculated by counting the number of T alleles (or t alleles) and dividing by the total number of alleles

example

• 10,000 individuals each carrying 2 alleles for height = 20,000 alleles in population

• 4.900 TT individuals have a total of 9,800 copies of the T allele (4900 × 2) + 4200 heterozygous has 1 T allele

  • f(T) = ((2×4900) + 4200)/20000 = 0.7

convention

• dominant allele represented as p

• other allele represented as q

• hence, p = 0.7 and q = 0.3

  • equal 1.0

ancestral species giving rise to 2 or more species requires:

• more than genetic variation

• necessary for barrier or isolation between groups of starting species

  • otherwise mixing of alleles continues

  • little chance for differences to accumulate

what allows for genetic variation?

• sexual reproduction

  • crossing over in prophase 1

  • independent assortment of homologues in metaphase 1

  • combo of DNA from different parents

reproductive isolation

• failure of individuals from two populations to mate and produce fertile offspring

• resulting in the reduction or elimination of gene flow between the populations

• three ways: geographic isolation, behavioural isolation and temporal isolation.

speciation

involves the formation of one or more new species from an ancestral species. Occurs due to reproductive isolation between populations.

geographic isolation

separation of populations by a physical barrier that reduces or prevents gene flow.

• emerges allopatric speciation

• sea between continents, landslide, land

allopatric speciation

is when a new species develops as a result of part of a population becoming geographically isolated from other populations

behavioral isolation

any behavior influenced genetically reduces or eliminates mating and gene flow between portions of a population

• e.g. variations in courtship song, location, practices, preference or sub-niche

  • alleles accumulated in 1 group unlikely to spread to other due to lack of interbreeding and gradually interbreeding fails

sympatric speciation

when a portion of a population develops into a new species while still living in the same geographic area as the ancestral population. Sympatric speciation can occur due to behavioural, temporal, or other forms of speciation.

temporal shift

any shift in the timing of a behavior that acts to reduce or eliminate gene flow between portions of a population.

• e.g. reproductively active at different times, flower at different times, early arrivers of migrated birds mate with other early arrivers

mechanical isolation

anatomical barrier exists that prevents mating, and hybrid fertility where mating between species occurs and produces healthy but sterile offspring

incipient speciation

occurs when populations that are genetically distinct can still interbreed

can speciation be undone?

Once speciation is complete it can never be undone, but until that time any genetic differences accumulated during isolation could be reversed.

punctuated equilibrium

• generally stable during long periods while stabilising selection maintains the existing phenotype

• these periods ‘punctuated’ by rapid bursts of phenotypic change

  • often result of major upheavals: long-term climate shifts or arrival of new species with a strong ecological impact

  • natural selection may favor phenotypes previously disadvantaged

  • stabilizing selection can shift to directional or disruptive selection

gradualism

• major changes are the cumulative product of slow but continuous minor changes

• mutation or immigration occasionally introduces new alleles to the gene pool → acted on by natural selection → becoming more common or disappearing

• allelic shifts subtle but increase frequency of beneficial alleles

polyploidy

possession of more than two complete sets of chromosomes

• can cause instantaneous speciation

• usually in plants which have higher tolerance for alternate chromosome #s than animals

• caused by non-disjunction during mitosis or meiosis I (all sister chromatids inherited together)

• tetraploids can reproduce with other tetraploids but not with a diploid because offspring becomes triploid

• triploids usually sterile since not all chromosomes have homologous partner

• polyploid will have chromosome numbers that are multiple of haploid (e.g. n=5 so 15, 20, 25…)

polyploidy uses

• when seedless fruit is desired because, since sexual reproduction fails, the embryo-containing seeds do not develop

• large allele variety at same locus — e.g. tetraploid carry 4 different alleles since 4 copies → develop new uses for genes without losing original function / many new &. useful varieties developed

• hybrid vigour: state where hybrid exhibits more desirable traits