Chapter 10.3

Question 1

Species

Answer 1

a group of organisms that can interbreed to produce fertile offspring

Question 2

population

Answer 2

individuals of the same species living in the same place and time.

Question 3

Gene pool

Answer 3

the set of all the variations of all genes in any population

Question 4

Genetic drift

Answer 4

random fluctuations in allele frequency

Question 5

When is there less likely to be genetic drift?

Answer 5

if the population has allele frequencies that are already well adapted to the environment, there is often little genetic change because natural selection acts to maintain the status quo.

Question 6

How does new allele combinations occur?

Answer 6

Through crossing over and random orientation of homologous chromosomes in meiosis and the fusion of genetic material from two parents.

Question 7

Natural selection and allele frequencies

Answer 7

Given enough time and a large enough population, alleles that are helpful become more common in the population through natural selection, even though every individual contains a combination of beneficial and harmful alleles.

Question 8

Tips to remember about evolution and allele frequencies

Answer 8

Evolution requires that allele frequencies change with time in populations.

In a population, individuals with different combinations of alleles may show differential survival and reproductive success because of natural selection. Selective pressure leads to evolution of the population.

When populations are isolated from each other, genetic drift and different selective pressures can cause the populations to evolve differently. Over time, when enough differences accumulate, the populations may speciate.

Question 9

Selective Breeding

Answer 9

Through the influence of natural selection; environmental factors can affect the rates of survival and reproduction of certain phenotypes (which are determined by alleles).

Question 10

How is selective pressure caused?

Answer 10

It is caused by both biotic and abiotic factors that change the rate of survival and reproduction of a segment of a population

Question 11

How can selective pressure be weak or strong?

Answer 11

For example, an allele that changes fur colour to make a predator stand out in its environment might be strongly selected against if it can’t approach prey without being detected. If an allele offers slightly better protection against a rare parasite, it might experience a weak, positive selective pressure. Over time, the selective pressures of natural selection change allele frequencies and drive evolution.

Question 12

First way selective pressure can affect phenotypes

Answer 12

1. Stabilising selection

Stabilising selection is widespread. It occurs when the existing variations that are beneficial are already common. Stabilising selection acts against extremes of a trait, for example, the colour of a sand crab. Seagulls prey on the crabs. The more the colour of the crab deviates from the background colour, the less likely it is to survive. New colours may be reintroduced by mutation occasionally, but the stabilising selection exerted by seagulls will maintain the match between the colour of crabs and the sand.

Question 13

Second way selective pressure can affect phenotypes

Answer 13

2. Directional selection

Directional selection occurs when one extreme of a trait offers a survival or reproductive advantage. The sand crabs living on the beach would experience directional selection if the background sand colour changed, for example, if flooding covered the beach with darker-coloured silt. Then darker-coloured crabs would have an advantage in avoiding predators and would survive longer and have more successful offspring. Dark colour alleles would become more common and light colour alleles less common. Directional selection increases allele frequencies at one phenotypic extreme and reduces them at the other. A classic example of directional selection is increasing neck length in giraffes over many generations.

Question 14

Third way selective pressure can affect phenotypes

Answer 14

3. Disruptive selection

In disruptive selection, the most frequent phenotype becomes a disadvantage, and individuals at both extremes have better rates of survival and reproduction. In the sand crab example, perhaps the female crabs inherit a genetic preference for males that stand out against the sandy background. This type of preference is fairly common in animal species. Choosing a mate that can survive even when it stands out to predators suggests a very strong assortment of other alleles. In this case, crabs lighter and darker than the background would have the most success in passing their alleles to the next generation.

Question 15

How are the selective pressures different?

Answer 15

In stabilising and directional selection, the population remains a single group. In disruptive selection, the population may break into two groups. Individuals with extreme phenotypes may also benefit from a genetic preference to mate with similar individuals because their offspring would have less chance of inheriting the disadvantageous intermediate phenotype. Thus, disruptive selection may cause one population to split into two, which may eventually become two species.

Question 16

Allele frequency

Answer 16

A measure of how frequently an allele from a specific gene locus occurs in a population

Question 17

Genotype frequency

Answer 17

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

Question 18

Speciation

Answer 18

A species may gradually change with time to the point where ancestors are so different from their descendants that the two groups would be considered different species

Question 19

What factors turn one species into new species?

Answer 19

genetic variation and time (usually with the help of some type of selective pressure).

Question 20

What factors are needed to turn one species into two new species or more?

Answer 20

It is necessary that there be some sort of barrier, or isolation, between groups of the starting species. Without isolation, there is little chance for differences to accumulate and turn only a portion of an interbreeding population into a new species due to sexual reproduction causing variation.

Question 21

How does reproductive isolation lead to new species?

Answer 21

By reducing or eliminating sexual reproduction that leads to fertile offspring, reproductive isolation reduces or eliminates the flow of alleles between gene pools. Changes, such as the introduction of new alleles through mutation, remain in that population’s gene pool only. Over time, genetic differences accumulate that can eventually prevent reproduction between populations and establish permanently separate species.

Question 22

Reproductive Isolation

Answer 22

the failure of individuals from two populations to mate and produce fertile offspring, resulting in the reduction or elimination of gene flow between the populations.

Question 23

What types of barriers are in reproductive isolation?

Answer 23

Some barriers may involve physical separation of populations, but an isolating barrier can be anything that reduces the chance of fertile offspring between two groups. There are several main ways a population can become reproductively isolated, three of which are discussed here: geographic isolation, behavioural isolation and temporal isolation.

Question 24

Explain geographic isolation

Answer 24

A physical barrier that keeps them in two separate places. Depending on the nature of the species, geographic barriers could be entire oceans between continents or a small strip of land between neighbouring ponds. Sometimes a small population becomes isolated after a storm, and sometimes an established population is divided by a natural phenomenon like a landslide or a river changing course. Occasionally a species has a large enough range and slow enough gene flow that populations at the extremes of the geographic range cannot interbreed, as seen in the Ensatina salamanders.

Question 25

Geographic Isolation

Answer 25

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

Question 26

Allopatric speciation

Answer 26

When a new species emerges during geographic separation of a population from other populations

Question 27

Example of geographic isolation: Salamanders

Answer 27

As Ensatina salamanders from the ancestral population in the north expanded their range southward, salamanders on the coast experienced different selective pressures than inland salamanders, and salamanders in the south experienced different pressures than more northern salamanders. Today, even though most neighbouring species show some degree of hybridisation (and are therefore not fully separate species), the southernmost species cannot interbreed with the ancestral population

Question 28

Explain behavioural isolation

Answer 28

Behavioural isolation occurs when genetically-influenced differences in behaviour have the effect of reducing or preventing mating between different portions of the population. For example, a variation in production of the mating song in male crickets may be mirrored by a preference for different variations in female crickets. Although mating would produce fertile offspring between any pairing in the entire population, individuals mate primarily with their own song-group. New alleles that occur in one group are unlikely to spread to the other due to lack of interbreeding. Differences accumulate between the song-groups, making it increasingly unlikely and eventually impossible for successful interbreeding to occur.

Question 29

Sympatric speciation

Answer 29

the divergence of a population into two new species while in the same geographic area.

Question 30

Explain Temporal isolation

Answer 30

Temporal isolation occurs when portions of a population are reproductively active at different times. Portions of the population may mate or flower at different times of day or during different seasons. For example, Anaxyrus americanus toads and Anaxyrus fowleri toads have some sympatric populations. In laboratory settings, they easily hybridise and produce mostly fertile offspring. However, in nature, they rarely interbreed because A. americanus mate only during a few days in early summer, while sympatric A. fowleri mate a few weeks later. Temporal isolation can also be seen during a single day, for example in populations that are active primarily at dawn versus dusk. Another example is migratory birds that select mates and nesting sites when they arrive at their mating grounds. Early arrivers tend to mate with other early arrivers, and, because arrival time is likely to have a genetic influence, over time early arrivers may accumulate differences from the late-arriving portion of the population. Eventually temporal isolation can lead to sympatric speciation.

Question 31

Behavioural isolation

Answer 31

Any behaviour that acts to reduce or eliminate gene flow between portions of a population, such as variations in courtship song or location.

Question 32

Temporal isolation

Answer 32

Any shift in the timing of a behaviour that acts to reduce or eliminate gene flow between portions of a population

Question 33

Examples of other types of isolation

Answer 33

Other types of isolation exist, for example mechanical isolation, where an anatomical barrier exists that prevents mating, and hybrid infertility , where mating between species occurs and produces healthy but sterile offspring. Whatever the type of isolation, genetic differences can accumulate and lead to speciation.

Question 34

Incipient speciation

Answer 34

when populations that are genetically distinct can still interbreed.

Question 35

Can speciation be reversible?

Answer 35

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

Question 36

How does gradualism occur?

Answer 36

Gradualism occurs when major changes are the cumulative product of slow but continuous minor changes.

Question 37

Explain Gradualism

Answer 37

In this model, mutation or immigration occasionally introduces new alleles to the gene pool, which are then acted on by natural selection, becoming more common or disappearing. In each generation, allelic shifts are subtle but tend to increase the frequency of any advantageous alleles. Individuals with the helpful trait survive and reproduce to pass on these alleles to the next generation. Over long periods of time, perhaps millions of years, the cumulative changes in alleles become increasingly visible in the form of phenotypic changes in the population. Evolution by gradualism is a smooth and continuous process.

Question 38

Example of gradualism

Answer 38

For example, two populations of the same species may be in two different environments. Perhaps only one area contains a particular predator or food source. Each population will have a range of alleles to begin with and will experience different random mutations. From the alleles available, any that help to avoid the predator will become common in the population where the predator is present due to selective pressure. At some point the gene pools of the two populations will be so different that, even if reunited, members of the two populations would be unable to produce fertile offspring.

Question 39

Examples of punctuated equilibrium

Answer 39

In examples of punctuated equilibrium, species are generally stable during long periods while stabilising selection works to maintain the existing phenotype of the species. However, the periods of stability, or equilibria, are ‘punctuated’ by rapid bursts of phenotypic change. The rapid phenotypic changes are often the result of major upheavals such as long-term climate shifts or the arrival, through immigration or evolution, of new species with a strong ecological impact. During these times of change, natural selection may favour phenotypes that were previously at a disadvantage. Thus stabilising selection would shift to directional or disruptive selection

Question 40

Compare and contrast punctuated equilibrium and gradualism

Answer 40

Both gradualism and punctuated equilibrium are models of evolution with supporting evidence. It is likely that even during periods of relative stability, gradual change occurs and eventually produces new species. It is equally likely that during times of upheaval, selective pressures change rapidly leading to the abrupt development of new species.

Question 41

What can polyploidy cause?

Answer 41

Polyploidy can cause instantaneous speciation, involves increasing the number of chromosome sets, and usually occurs in plants, which have a much higher tolerance for alternate chromosome numbers than animals.

Question 42

What causes polyploidy?

Answer 42

Polyploidy can be caused by the total non-disjunction of chromosomes during mitosis or, most commonly, meiosis I. The entire meiotic spindle fails and all sister chromatids are inherited together.

Question 43

How are new species formed through polyploidy?

Answer 43

When a polyploid organism forms, it may be instantly isolated from its parent species. Meiosis requires that each chromosome form a tetrad with its homologous partner during prophase I. Both diploid and tetraploid individuals will always have even numbers of homologous chromosomes and can therefore produce gametes for sexual reproduction. Assuming they develop normally, tetraploid individuals face no reproductive barriers with other tetraploids.

Question 44

Why can’t there be a new species through triploid chromosomes?

Answer 44

However, if a tetraploid individual reproduces with a diploid, their offspring will be triploid. Triploid organisms are usually sterile. This is because not all chromosomes have a homologous partner. Since diploid and tetraploid matings do not produce fertile offspring they may be considered separate species immediately.

Question 45

Why is polyploidy useful for crops?

Answer 45

Polyploidy is very useful in crops for a variety of reasons. Triploidy is very useful when seedless fruit is desired because, since sexual reproduction fails, the embryo-containing seeds do not develop. For example, commercial bananas and seedless watermelon are triploid varieties that are cultivated asexually through cloning. Further, polyploid species can have a large variety of alleles at the same locus. For example, a tetraploid individual can carry four different alleles for each gene because it has four copies of each chromosome. This allows the species to develop new uses for some genes without losing their original function. Many new and useful varieties can be developed in this way. Polyploidy often produces hybrid vigour, a state where the hybrid exhibits traits that are more desirable than either parent.