Mechanisms of Evolution

Mechanisms of Evolution

Genetic Variation • Genes – DNA sequences that code for specific polypeptidesat specific loci (locations on the chromosome) • Most eukaryotic organisms are diploid (have two sets of chromosomes, one set from each parent), but some, like wheat, are polyploid (have more than two sets of chromosomes) • All chromosomes (except sex chromosomes) appear as homologous pairs, where each pair possesses the same set of genes

Allele – particular forms of a gene; most genes have two alleles or more (B for dominant alleles, b for recessive) • Homozygous individuals have two copies of the same allele for a given gene (represented as BB or bb) • Heterozygous individuals have two different alleles for a given gene (represented as Bb)

Differences in genotypes and environmental influences account for differences among phenotypes of individuals • Phenotype – the physical, observable traits of an organism (i.e.: hair colour, eye colour) • Each individual within a species has a different genotype (genetic identity of an individualthe set of all alleles the individual possesses)

Random Change • Allele frequencies change all the time in natural populations due to: • Genetic Drift (random loss of an allele in small populations) • Migration or Gene Flow (immigration and emigration of a population) • Mutation • Non-Random Mating • Selection (adaptive evolutionary changes) • Discussed in its own section (later)

Genetic Drift • causes the frequencies of alleles to be altered in small populations • can have random loss of alleles due to: - Few individuals - Individuals failing to reproduce • tends to enhance differences between populations

Founder Effect • few individuals create a new, isolated population so alleles that they bring have extreme importance • the allele frequencies in the founder population do not necessarily represent those found in the source population • if alleles were rare in the source population, they now make up a significant fraction of the new population’s alleles. • the genetic features expressed today (depending on maintained isolation factor) are characteristic of the founders

Amish community in Pennsylvania– 30 people founded it; one couple had a child heterozygous for Ellis-van Creveld syndrome (dwarfism, shortened limbs, extra fingers and toes. Die within a few months of birth) • Now there is a high incidence of this rare genetic disorder due to the fact that there has not been a lot of genetic variation added. This trait is present in this population at a higher than normal rate (7% as opposed to 0.1%)

Bottleneck Effect • dramatic, often temporary, reduction in population size, usually resulting in significant genetic drift • small founder population becomes the sole source of alleles for a given species • cheetahs in Africa underwent crisis 10 000 years ago which depleted their numbers; within the last century they have been almost hunted to extinction • the decrease in numbers has lowered the genetic variability and has allowed for an increase in fatal recessive disorders, which may cause the cheetah to become extinct

Migration/Gene Flow • movement of individuals from one population to another • if new animal moves to a region and its genetic makeup allows it to survive (adapt) better than individuals already present, then genes will be passed on (if mating is possible) • migration of gametes is difficult to observe but it does happen • male gametes on flowers can be transported large distances • unlike genetic drift, gene flow tends to reduce differences between populations

Mutation • the ultimate source of genetic variation; what makes evolution possible • the ONLY source of additional genetic material and new alleles • most mutations occur in somatic cells, but these cannot be inherited and so do not play a part in evolution • only mutations in gametes have the potential to be passed on • Neutral, harmful or beneficial • All depends on effects on fitness (reproductive success)

Non-Random Mating • individuals with certain genotypes often mate with one another at a higher than normal rate Inbreeding • causes frequencies of particular genotypes to differ greatly from HW principle • does not change the frequency of the alleles, just the proportion of individuals that are homozygous. • Homozygous recessive individuals are more likely to be present more likely to have genetic problems

Selection • The most powerful of the 5 principle agents of genetic change • Stabilizing selection – selection against individuals with traits that deviate from the population average • occurs when the most common phenotypes within a population are most favoured by the environment • Stabilizing selection is the most common form of selection • once a species becomes adapted to its environment, selective pressures maintain their traits

An example is hummingbirds • Hummingbirds will succeed in drawing nectar from flowers if their bill and tongue length are well adapted for the size of flowers they feed on • Longer bill = more nutrients and energy to grow, as well there’s more weight to carry around (greater expenditure of energy) • Shorter bill = reduces a bird’s ability to reach food • Ideal bill length also increase the success of flower pollination, enhancing the reproductive success of the flowers as well as the hummingbirds

Environment will select against mutations that produce birds with a bill length that differs from the best-adapted length:

Directional selection – selection that favours an increase or decrease in the value of a trait from the current population average • Occurs when the environment favours individuals with more extreme variations of a trait • When an organism migrates to a new environment or aspects of its habitat change, it will encounter new forces of natural selection

if the hummingbirds move to a new habitat with longer flowers, individuals with bills that were adapted to medium-length flowers will no longer have the ideal phenotype • birds that inherited longer bills will be more successful

Disruptive selection – favours individuals at opposite extremes of a trait over individuals with intermediate variations • Environmental conditions may favour more than one phenotype • the hummingbird population may be able to feed from two different flower species, each with different-sized flowers • each flower species is a good source of nectar, but does not favour hummingbirds with medium-sized bills • birds with longer and shorter bills will be more successful

Sexual selection – favours the selection of any trait that influences the mating success of the individual • Results in sexual dimorphism (striking differences in male’s and female’s physical appearance) • Most common forms of sexual selection result from female mate choice and male-vs.-male competition • Runaway Selection  bizarre extremes of sexual dimorphisms which allow for beneficial mating opportunities but are otherwise detrimental • Fiddler crab (p. 560)

Some males of certain bird species have extraordinarily long tails, while the females’ tails are quite short…Wouldn’t these really long tails be a disadvantage for the males? Why do they have them? • Because the females like it! Females preferentially mate with males with longer tails • This has been proven through experiments where they artificially lengthen the tails of male birds • Birds that were previously unattractive (short-tailed) have dramatically increased mating success (and, one would assume, sex appeal) when their tails are lengthened

The Formation of a New Species • Species - members of groups or populations that interbreed or have the ability to interbreed with each other • isolated populations become different from each other in overall characteristics  start to occupy different niches • If this trend continues they may become so different that they are said to be distinct species • Allopatric speciation (geographically based)

If species from different populations ever migrate back to each other hybridization may or may not occur depending on two isolating mechanisms: • Prezygotic Isolating Mechanisms prevent formation of zygotes and includes geographical, ecological, temporal, behavioral and mechanical isolation. • Postzygotic Isolating Mechanisms prevent proper functioning of zygotes and therefore development to maturity and therein ability to reproduce.

Prezygotic Isolating Mechanisms Geographical Isolation • species simply do not exist together in the same place (tigers and lions) Ecological Isolation • species in same area, but in different habitats and thus will not hybridize with each other • species formed may die due to decreased fitness of living in area, which it is not suited for

Temporal Isolation • differences in timing of breeding periods • Lactuca graminifolia and L. canadensis (wild cabbage) grow side by side and hybrids can be created in laboratory situations but L. graminifolia flowers in early spring and L. canadensis flowers in the summer Behavioral Isolation • mating rituals are different therefore individuals will not interbreed Mechanical Isolation • structural differences that can evolve that may prevent mating (general size and mating organs)

Postzygotic Isolating Mechanisms • Development is a complex process • even though gametes might fuse, creating a zygote, there are still factors that can hinder the development of the organism into a normal, functional, fertile individual • Sibling Species – closely related species that are often distinguished by behavior or other non-evident characteristics. (e.g. leopard frogs – all look the same but will not interbreed, even in laboratory situation) • Sympatric speciation  mutation to tetraploid vs. diploid

If partially differentiated populations have contact the differences between them may become negligible over the course of time • If, however, the offspring that are produced are sterile/not as well adapted to the existing habitats, the hybrids will be at a disadvantage • Hybridization then will be of no advantage and will not be passed on

Mechanisms of Evolution