Evolution Test #2

Microevolution

A type of evolution that happens within a single population. It occurs when there is a change in a population’s allele frequencies from one generation to the next. E.g. Mosquitoes evolving to grow a resistance to DDT.

Genetic Drift Definition

Chance events change the frequency of traits in a population, NOT an adaption or selection.

Genetic Drift affect on microevolution

It can have major effects, especially in smaller populations, because each individual’s alleles represent a large fraction of the gene pool. Therefore it’s more likely that more genes can be lost from these chance events and affect the population more.

Founder effect

A type of genetic drift when a new population is started by only a small group of individuals.

E.g. The O blood type is almost completely in the South American population because of a high frequency of this gene in the colonizers that first populated the continent.

Bottleneck effect

A type of genetic drift that occurs when a large population is drastically reduced by a disaster like famine, natural disaster, loss of habitat, human hunting etc.

E.g. Cheetahs, who share a small number of alleles. This is due to 2 bottleneck events, the Ice Age and, more recently, overhunting and habitat loss.

Gene flow definition

The migration of fertile individuals or the transfer of gametes between populations

E.g: a bee carrying pollen from one flower population to another

Gene flow affect on microevolution

Extensive gene flow can eventually group neighbouring populations into a single population

Mutation definition

Transmitted in gametes immediately changes that gene pool of a population by substituting one allele for another. The mutation's effect on the gene pool only shows up after many generations.

Mutation affect on microevolution

This rate can be increased if the mutation is chosen through natural selection or genetic drift. In other words, the mutation has to be beneficial and acted upon by natural selection to influence microevolution. Mutations are the only source of new genetic material.

Natural selection affect on microevolution

Selection passes on alleles to the next generation in disproportionate number It is the only agent of microevolution that is adaptive since it accumulates and maintains favourable genotypes Genetic variation makes this possible

Directional Selection

When one extreme of a trait is favoured over the other. E.g: Giraffe neck lengths. The longer the necks, the more advantageous.

Stabilizing Selection

When the ‘extreme’ traits are selected against, and the middle versions of the trait are favoured. E.g. Human baby weight. Too light and too heavy babies are more likely to die and are therefore selected against.

Disrupting Selection

When both extreme traits are favoured, and the medium traits are selected against. E.g. dark and light-coloured oysters. Both dark-coloured and light-coloured oysters have camouflage advantages, but those in-between these colours have no advantages and are therefore selected against.

Hardy-Weinberg Equilibrium

Describes a non-evolving population, and allele frequencies will stay constant unless acted upon by other agents. Alleles are present in the gene pool of new populations at the same frequencies as in the original gene pool. This opposes microevolution which is when frequencies of alleles in gene pools DO change from one generation to the next.

Five conditions necessary to maintain Hardy-Weinberg equilibrium

• Large breeding population

• Random mating

• No mutations

• No immigration or emigration

• No natural selection

Large Breeding population (Hardy-Weinberg equilibrium)

Helps to ensure that chance alone does not disrupt genetic equilibrium. Large populations combat the impact of genetic drift (which disrupts equilibrium) because genetic drift does not significantly affect large populations.

Random mating (Hardy-Weinberg equilibrium)

Mating must be random because in assortive mating, individuals tend to choose mates similar to themselves, which would result in fewer heterozygous individuals than one you would expect in a population with non-random mating.

No mutations (Hardy-Weinberg equilibrium)

For a population to be at equilibrium, there can be no changes in allelic frequency due to mutation because that would change the balance of alleles in the gene pool.

No Immigration or Emigration (Hardy-Weinberg equilibrium)

For the allelic frequency to remain constant in a population at equilibrium, no new alleles can be introduced and no alleles can be lost since it would alter allelic frequencies.

No Natural Selection (Hardy-Weinberg equilibrium)

In a population at equilibrium, no alleles are selected over other alleles. If selection occurs, those alleles that are selected would become more common and mess with the gene frequencies

Hardy-Weinberg equations

p + q = 1

p^2 + 2pq + q^2 = 1

p represents in a Hardy-Weinberg equation

Dominant allele

q represents in a Hardy-Weinberg equation

Recessive allele

Macroevolution

Refers to larger evolutionary changes that result in new species. E.g: The Galapagos finches went through adaptive radiation and diverged into different species through evolution. They can no longer interbreed with each other.

Allopatric speciation

When a population splits into two isolated groups by geographical barrier. This is a physical barrier.

Sympatric speciation

When two population diverge in the same area (they split due to other factors)

Postzygotic barrier

Barriers that occur after zygote formation

Ecological/habitat isolation definition

A type of sympatric speciation in which species occur in the same region but occupy different habitats, so they rarely encounter each other and are therefore reproductively isolated.

Ecological/habitat isolation example

There are two species of garter snakes that live in the same area, but one lives in the water, and the other is terrestrial.

Temporal isolation definition

A type of sympatric speciation in which species that breed during different times of day, seasons, or years cannot mix gametes. They are therefore reproductively isolated.

Temporal isolation example

The eastern spotted and western spotted skunks overlap in range, but eastern spotted skunks mate in late winter, and western spotted skunks mate in late summer.

Behavioural isolation definition

A type of sympatric speciation in which species display unique behavioural patterns and rituals, such as courtship rituals or mating calls, which isolate species and attract mates of the same species. They are reproductively isolated.

Behavioural isolation example

Blue-footed boobies mate only after a courtship display unique to their species.

Mechanical isolation definition

A type of sympatric speciation in which morphological differences can prevent successful mating. They are reproductively isolated.

Mechanical isolation example

Even in closely related species of plants, the flowers often have distinct appearances that attract different pollinators such as two species of monkey flower differ greatly in shape & colour, therefore cross-pollination does not happen

Gametic isolation definition

A type of sympatric speciation in which sperm of one species may not be able to fertilize eggs of another species due to either a biochemical barrier (sperm cannot penetrate the egg) or a chemical incompatibility (sperm cannot survive in female reproductive tract)

Gametic isolation example

Sea urchins release sperm and eggs into surrounding waters, where they fuse & form zygotes. Gametes of different species—red& purple —are unable to fuse.

Prezygotic barrier

Barriers that occur before zygote formation

Reduced hybrid viability definition

A type of sympatric speciation in which genes of different parent species may interact & impair the hybrid’s development

Reduced hybrid viability example

Species of salamanders may interbreed, but most hybrids do not complete development & those that do are frail.

Reduced hybrid fertility definition

A type of sympatric speciation in which if hybrids are vigorous, they may be sterile and chromosomes of parents may differ in number or structure.

Reduced hybrid fertility example

Mules are rigorous but infertile. Horses have 64 chromosomes (32 pairs) and donkeys have 62 chromosomes (31 pairs). Mules have 63 chromosomes.

Hybrid breakdown definition

A type of sympatric speciation in which hybrids may be fertile & viable in the first generation, but when they mate, offspring are feeble or sterile

Hybrid breakdown example

In cultivated rice strains, hybrids are vigorous, but plants in the next generation are small and sterile on the path to separate species.

Gradualism

The gradual divergence over long spans of time. It supports the assumption that big changes occur as the accumulation of many small ones

Punctuated equilibrium

The rate of speciation is not constant with rapid bursts of change. There are long periods of little or no change.