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Assumptions of the Hardy-Weinberg principle

The Hardy-Weinberg principle requires that there be:

No migration

No mutation

No selection

Large population

Mating is random

Usefulness of the Hardy-Weinberg principle

Hardy-Weinberg provides a theoretical benchmark against which real populations may be compared.

Departures from the assumptions occur: Hardy-Weinberg provides a point of reference for evaluating the causes and consequences of the departures.

Genetic drift: random changes in gene frequencies

Genetic drift means the random change of gene frequencies in a population.

Some such changes are "neutral": changes in allele frequencies when the alleles have no immediate consequence to the biology of the population. Example: synonym codons code for the same amino acids and thus make exactly the same protein.

Examples of genetic drift

Population bottleneck. Species temporarily reduced to very low number lose genetic diversity. Examples: cheetahs--low population during Pleistocene; elephant seals--hunted to near extinction during 19th century.

Founder effect. Populations founded by just a few individuals have unusual gene frequencies.

Significance of genetic drift

Founder effect may start a new population with unusual gene frequencies which become the basis of new adaptations.

Bottleneck causes reduced genetic diversity.

For neutral alleles, genetic drift occurs in all populations and species. As a consequence, separated populations (and species) accumulate genetic differences.

Gene flow

Gene flow means the movement of individual organisms from one population to another, or simply the movement of gametes (e.g. pollen).

Gene flow brings the gene frequencies of adjacent populations closer together. Gene flow has the opposite effect of the founder effect: if it occurs, it prevents the accumulation of genetic differences.

Significance of gene flow

If it occurs, gene flow keeps adjacent populations tied together.

If populations are to separate enough to be considered separate species, there must be barriers to prevent any significant gene flow.

Mutation

Mutations are spontaneous changes in the genetic material. These changes include:

Point mutations: changes in a single base pair in the DNA

Frame shift mutations: deletion or insertion of a single extra base pair (codon=3 bases).

Chromosomal changes: duplication, deletion, inversion, translocation.

Significance of mutation

Mutations introduce new alleles. Usually, the new alleles are deleterious. Some few, in a new environmental context, turn out to be beneficial. (Maybe not right away!)

Some chromosomal mutations (e.g. inversion) produce barriers to reproduction between a new chromosomal arrangement and the ancestral arrangement.

Non-random mating

The Hardy-Weinberg principle assumes random mating: mate selection without regard to genotype.

Non-random mating means that mate selection is influenced by phenotypic differences based on underlying genotypic differences.

Example of non-random mating: Sexual selection

In some species, males acquire harems and monopolize females. (Elk, elephant seals, horses, lions, etc.) Commonly, the males of such species are much larger than the females.

In some species, females choose more attractive mates. (Peacocks, Woodducks, Picture-wing fruit flies, etc.)

Significance of non-random mating.

Sexual dimorphism (conspicuous differences between the two sexes) result from non-random mating. The process is a special case of natural selection known as sexual selection.

Sexual selection may serve as a barrier to reproduction between closely similar species. Example: courtship rituals.

Summary of exceptions to H/W assumptions.

Genetic drift--random changes (founder effect, bottleneck, and neutral genetic drift).

Gene flow--movement of alleles.

Mutation-- new genetic material.

Non-random mating--sexual selection, etc.

Natural selection--adaptive changes in the gene pool.

Hardy-Weinberg helps identify natural population processes.

Each type of departure produces characteristic deviations from Hardy-Weinberg predictions.

Example: selection produces changes in expected gene frequencies between new-born individuals and adult survivors.

Hardy-Weinberg is the statistical "null hypothesis" used for testing population genetics data.

Evolution, natural selection, genetic drift

Evolution is: changes in the gene frequencies of a population over several generations.

Natural selection is a process: that occurs if a population has variation, fitness differences, inheritance.

Genetic drift is: random changes in gene frequency from one generation to the next.

Evolution can be the result of....

Natural selection, if the environment changes. Natural selection is responsible for adaptive evolution.

Genetic drift, if random changes in gene frequencies occur. Genetic drift does not produce adaptive evolution. Neutral alleles change because of genetic drift.

What is a species?

Individuals which belong to the same species are "similar" (but what about sexual dimorphism? conspicuous phenotypic differences?, ...)

A biological species is defined as a population or group of populations whose members have the potential to interbreed and produce fertile offspring.

Species: tied together by a common gene pool

Mules are robust individuals produced by a cross between individuals from two different species: Horse x Donkey. But mules are sterile--hence the two species remain separated in spite of interbreeding.

Eastern and western meadowlark look nearly the same, but courtship song is very different--they don’t interbreed.

A species is...

A group of individuals which interbreed and therefore represent a common gene pool.

If there are reproductive barriers that prevent (permanently) two populations from interbreeding, they belong to separate species.

An aside about spelling

The singular of species is....

Species

The plural of species is...

Species

Similar species are grouped together as a genus (singular). The plural is genera: two or more genera.

Speciation: the division of a species into two or more species.

A variety of mechanisms have been discovered which can cause speciation--the division of one species (ancestral) into two or more species (descendant).

The key is reproductive isolation. Mechanisms introduce barriers to reproduction. The barriers may be increased by selection, or erased by interbreeding. Time will tell which.

Significance of reproductive barriers

The significance of reproductive barriers is that they maintain genetic isolation between two populations. If such barriers are complete, the populations represent distinct species.

Barriers may arise by a variety of different means. Example: geographic isolation followed by drift, mutation, or selection until reproductive isolation is complete.

The process of speciation

Many different mechanisms have been studied.

Two examples

*Allopatric speciation--speciation based on geographic separation, and.

*Polyploidy--speciation based on a chromosome mechanism.

Allopatric speciation

Geographic isolation is one of the mechanisms which can bring about reproductive isolation.

Allopatric speciation means: speciation which follows (over time) after geographic isolation. The initial barrier to reproduction is physical separation. Given sufficient time (many generations) sufficient differences may accumulate to make separation permanent.

Example of allopatric speciation

Blue-headed wrasse (Caribbean) and the rainbow wrasse (Pacific) are closely similar. Their ancestral common population was split by the growth of the Isthmus of Panama about 5 million years ago.

Since this allopatric separation occurred, the two species have changed independently.

An ambiguous example

Allopatric speciation is a process which can be interrupted before completion.

Possible example: deermice. There are 4 closely related populations in the Intermountain west. All 4 are distinct in some respects but interbreed, except: two of the subspecies do not interbreed even though they overlap.

So are these species or just populations of the same species?

Two of the populations (in Montana/Idaho) overlap but do not interbreed. Hence, they must be different species.

Both of these interbreed with the other two populations, so genes can flow from one to the other.

The answer: Time will tell. With more divergence, speciation will occur. With more interbreeding, it will not occur.

Speciation: a dynamic process

Speciation is a dynamic process--it is taking place in many places in many populations, but it is being reversed in many places by interbreeding.

We should expect to see: populations with the potential to diverge (e.g. Snail p238), populations which have diverged horses and donkeys), populations which might be in the process (deermice).

Reproductive barriers--many types. (see p241).

Barriers to reproduction may prevent any mating: behavioral (courtship, etc.); habitat (populations choose different habitats, and never meet), etc. Such barriers are prezygotic barriers. No fertilization.

Barriers to reproduction may prevent subsequent reproductive success: sterility (hybrids die or are infertile), etc. Such barriers are postzygotic barriers.

Significance of polyploidy

The occurrence of diploid gametes (rare) can give rise to a polyploid individual after fertilization.

Many plants (e.g. Mendel’s peas) are hermaphroditic.

Polyploidy can give rise to a new species: because of the incompatibility between parent and offspring, offspring are distinct.

Polyploidy: common means of speciation in plants

A common means of developing genetic isolation in plants is known as polyploidy.

In contrast to most animals, extra sets of chromosomes in many plants are not disruptive.

Plants sometimes (rarely) produce gametes with a diploid set of chromosomes. If fertilized, the result is a polyploid plant.

Vocabulary of "ploidy".

Haploid--half set of chromosomes

Diploid--double set of chromosomes (the norm in typical sexual organisms)

Triploid--3 sets of chromosomes (usually sterile, because pairing of chromosomes during meiosis is impossible).

Tetraploid--4 sets of chromosomes. (Meiosis OK for any even number ploidy.)

Wheat: a case of polyploidy and speciation.

Modern wheat is the result of two successive hybridizations (see figure 15.6).

Hybridization 1: Einkorn wheat with a wild wheat. Einkorn wheat and the wild wheat each had 14 chromosomes. The hybrid (eventually) had 28 chromosomes: polyploidy.

The second hybridization brought the chromosome number to 42 in modern wheat

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