E.6.1 Describe the social organization of
honey bee colonies and one other
non-human example.
The social organisation of the Honey bee
Classification:
Class: Insecta
Order: Hymenoptera
Family: Apidae
Genus: Apis
Species: mellifera
Common name: Honey Bee
The honey bee is a fully eusocial insect species. Colonies of honeybees show the three characteristics of fully eusocial organisation:
Individuals of the same species cooperate in caring for the young
There is a reproductive division of labour with more or less sterile individuals, working on behalf of the reproductives. There is a caste division of into three phenotypes.
An overlap of at least two generations in which the offspring contribute to colony labour.
Originally the honeybees’ distribution was Africa, Europe and Asia however its current worldwide distribution owes much to the activities of man. Colony sizes can reach tens of thousands and their organisation represents the most complex seen in social Insects. Hymenoptera species account for almost all the eusocial Insects with the exception being termites of the order Isoptera.
The advanced nature of the honeybee colony is based on the differentiation (individuals into three castes) for labour along with integration of the community through a communication system.
There are three phenotypic castes within the honeybee colony: IMAGES
Differentiation for task:
The Queen: a diploid reproductive responsible for the production of all the eggs. The honeybee queen has taken this strategy to extremes and does nothing else other than produce eggs.
The queen established the colony after a reproductive flight in which she mated with a male bee called a drone. This is the only mating the Queen will perform and she retains the sperm from this mating, fertilising eggs as required.
The Workers: These are female and diploid. The worker is produced by the fertilisation of an egg from the Queen and sperm from the earlier drone mating.
Within a single colony there will be thousands of workers. The life span of a worker is about 40 days. There is further differentiation of task for the workers as they nurse and tend the brood of larva during the early part of their life and then change to foraging in the latter period of their 40 days.
The foraging workers are the Bees that we see searching and gather food for the colony.
Drones: Drones are male and haploid. The drones are produced from the unfertilised egg of the queen. Drones as the name implies, contribute little if anything to the care of the colony.
Integration through communication:
As eusocial colonies become more complex the ‘power’, the control method shifts form aggressive queen behaviour to the pheromone mediated behaviour typical of honeybee queens. Reproductive control in honeybees is based on inhibitory pheromones exchanged with and between the workers and the developing larvae. 9-keto-deconoic acid is the main chemical pheromone ‘queen substance’ used to both attract males and also to inhibit the building of royal cells (Queen larvae).
Honeybees use a combination of communication forms as already mentioned there is chemical exchange of pheromones but they also use touch and vibration as seen in the famous ‘waggle’ dance.
Social Organisation of the African Elephant.
Class: Mammalia
Order: Proboscidea
Family: Elephantidae
Species; Loxodonta africana
Common name: African Elephant
Family Unit: Herd size:10-20 females headed by the matriarch female.
Relationship: Daughter and granddaughters of the matriarchal female.
Female to calf ratio of about 1to 3 or 1 to 4.
Longevity: 50 years
Range: Family members remain within 1 km of each other.
Control: Matriarch female is the oldest and biggest of the female elephants. The oldest cow takes over when the matriarch dies.
Kinship Unit: Groups of related families remaining in close proximity to each other. Family groups normally divide around the 20-22 individuals and form new families. These are the member families of the kinship unit.
Examples of social behaviour
There are some notable examples of altruistic behaviour.
Any nursing cow can allow any calf to suckle.
Immature cows restraining and organising the calves when moving.
Aggressive defending behaviour from mature females when the herd is threatened.
Males from around 12-13 years are pushed away from the herd and thereafter tend to be solitary or in loose male bands.
Communication between elephants is largely through visual signals although sound and scent play some role.
E.6.2 Outline how natural selection may act
at the level of the colony in the case of
social organisms.
A review of natural selection:
Natural selection is the mechanisms of evolution that was proposed by Darwin.
Natural selection acts on the phenotypes of individuals.
Individuals who survive to reproduce pass on their genes into the next generation.
Successful genes become more frequent in the population.
All individuals within a population contribute their alleles to the ‘gene pool’ of that population. Changes in the frequency of alleles within a population are characteristic of evolution and evidence of the action of natural selection.
The issue becomes unclear when we consider:
Sterile castes
Altruistic behaviour
There are biologist (Mayr, Gould, Wilson and Darwin) that see the colony as a kind of ‘super organism’, literally sharing food mouth to mouth, workers foraging for others, workers caring of the young belonging to the Queen, workers dying in defence of the colony. The queen is reduced to a reproductive egg machine and drones play little role other than fertilising a queen in flight. An individual’s struggle to survive and reproduce has been usurper by their tasks that promote the greater good of the colony. Individuals such as workers are dispensable and might be view in a similar way to the cells of a multi-cellular organism. When a worker bee dies there are many thousands already present to take up the task and just as many larvae available that to produce another worker is of no real consequence to the colony. Even the queen and drone could be seen as the ‘ovary’ and ‘testes’ of the colony, their function as the reproductive organ of the colony but depend on other parts of the colony for their survival.
How do colonies evolve and what is the unit of selection. There are two polarised views of the situation:
One in which the level of natural selection is that of the colony with the ‘fittest’ colonies surviving.
The second ‘reductionist’ view would suggest that the ‘gene’ is the unit of selection and that there is no need to invoke new levels of ‘colony selection’.
Students wishing to pursue this issue should research and read further perhaps beginning with the work of Stephen Jay Gould (first view) and Richard Dawkins (second view). The exchange of views and the writing of these two great Biologists will keep students busy well beyond their IB examinations and form an important platform for any student seriously considering Biology in Higher education. There are very many other authors involved in this debate but few can match the quality of writing to be found in the above two authors.
E.6.3 Discuss the evolution of altruistic
behaviour using two non-human
examples.
Why should an animal decrease its own chance of survival and reproduction to increase the chance of survival of another animal? This type of behaviour is known as altruism and has been observed in a wide range of organisms including primates and insects, most famously honeybees.
How has altruistic behaviour evolved? To be consistent with the rest of Darwinism there needs to have been a reason why natural selection will select such behaviour in an individual.
The Diploid Elephant example:
In this example we will finally see that it is in the interest of an elephant to show altruistic behaviour towards its closely relate members of the family unit. It can be demonstrated that there is a high fitness value associated with altruistic behaviour and that altruism will be naturally selected.
Consider the relationship between the two elephants, a cow and her calf. The mother elephant as we have already seen will risk itself to protect its young from predators. The mother elephant and her offspring share many genes which can be calculated at 50%. The offspring of the elephant will carry ½ of the mothers genes.
How much of a risk should the mother elephant take to protecting her young? The answer is based on a cost- benefit analysis.
Cost = risk of performing the altruistic behaviour
Benefit= increased fitness form performing the behaviour
When the cost is outweighed by the benefit the behaviour should be avoided and vice versa.
Example:
If the mother elephant dies in the altruistic act the cost was = 1.0
But if two offspring ( 2 x ½ her genes)) are protected in the process then risk as been justified (nearly).
Why should altruistic behaviour evolve? The answer seems to be when the gene for altruistic behaviour provides greater fitness to the altruistic animal.
In most situations the risk would be less than death and the benefits to the animal considerable in comparison. Such small altruistic behaviours such as care, feeding and cleaning should readily evolve.
The altruistic behaviour of parents towards their young is commonly observed. Returning to our social African elephants how far removed can the relative be before the cost of the altruistic behaviour is unjustified.
In 1964 William D Hamilton formulated ‘The genetical evolution of social behaviour’ which included the following ideas that help our understanding of the evolution of altruism and provide a basis to understand the fitness benefit of altruistic behaviour.
Calculation of relatedness:
The relatedness of two individuals is a measure of the genes that two individuals share in common. Of course two individuals of the same species are likely to have a large number of genes in common. Relatedness is the measure of the genes shared above this baseline of common species genes.
Inclusive fitness:
‘In order for an altruistic trait to evolve, the sacrifice of fitness by an individual must be compensated for by an increase in fitness in some group of relatives by a factor greater than the reciprocal of the coefficient of relationship (r) to that group.’
The calculation of r is not required by the syllabus but is an integral part of the theories of how understanding the theory of how social behaviour evolved. It’s not too difficult to carry out the calculation and the fuller understanding it provides is worthwhile. So here we go:
How related are individuals A and B who are first cousins
The formula is r= n(1/2)g
Where n= number of ancestors in common and g= generational distance.
Using the diagram to the left we find g by the following steps.
First find the common ancestor(s). Which is Q and R.
Second start at A and count the number of generations UP to the common ancestor(s) =2
Third count the number of generations DOWN to B. = 2
Generational distance =4
Therefore the degree of relatedness due to Q and R = (1/2) 4 = 1/16
But we must multiple by the n= number of common ancestors = 2
Therefore (r) =2 (1/2) 4 = 1/8
So first cousins share 1/8 of their genes by descent.
Remember why we are doing this calculation? We are following the mathematical/ genetic explanation for the evolution of social organisation and in particular altruism. First cousins share on average about 1/8 of their genes. What kind of risk can a first cousin make for another first cousin? Well the ultimate risk (death )= 1.0 , so one first cousin might risk their life for 8 other first cousins. Why? Well the idea is that the 'risk taker' will get more of their genes into the next generation through the survival of their first cousins rather than surviving and reproducing themselves.
Of course animals do not perform these calculations prior to carrying out an altruistic act. Rather those animals that posses a gene that codes for altruistic traits has an advantage over those that do not.
Modeling relatedness in an elephant family unit:
In the elephant herd there can be up to 3 generations with the matriarchal female the ancestor of the calves. We have already noted the altruistic behaviour of elephants such as 'aunts' feeding the calves of their sisters or indeed their mother. The following model uses the information about social organisation of the elephant as given above.
In the diagram there is a family unit with the matriarchal female (M),
M has mated with two different bulls B1 and B2.
Beneath is a table of relatedness between the family unit members
H and E are non related males.
Table of relatedness for a family unit of elephants:
Take a few moments to compare the coefficient or relatedness for different members of this family unit. The idea of parental care is well known and is calculated here at r= ½ .
In the social organisation of elephants aunts (A, B, C and D) are often observed nursing and herding the young (G and F).
The coefficient of relatedness never falls below 1/8.
The matriarchal female never has a coefficient of relatedness less than ¼ .
For non-life threatening risks a gene that encourages altruistic behaviour has a high inclusive fitness value.
For life threatening cost the matriarchal female protecting 2 daughters or 4 grand children balances creates a benefits equal to the risk or cost of a life threatening protection.
Now watch a movie about elephants and constantly look for the inclusive fitness value in an animal’s behaviour.
The more distant a relative the lower the benefit from an altruistic act. Third cousins have base line probabilities of 1/128 which is about the same probability of sharing a gene with a random unrelated individual.
Evolution of social behaviour in the honeybee
Haplodiploid Honeybees
The evolution of social behaviour in the honeybee is not known in any detail. There are comparisons with other bees that can suggest the partial steps taken by solitary species evolving to fully eusocial species. The following analysis will help to explain how natural selection would favour the survival of bees that sacrifice their own reproduction for another, or sting colony aggressors and then die themselves.
Once more the brilliance of W H Hamilton's work and the application of the relatedness and inclusive fitness theory provide a neo-Darwinian explanation. The analysis will show the high selection value of altruistic behaviour in the honeybee.
In the case of the honeybee some of the altruistic behaviours include
Sterile workers promoting the survival of the queens offspring and not their own
Foraging workers stinging intruders as a defence of the colony but dying as a consequence.
The calculations of relatedness breakdown when we introduce sterile workers and if the some individuals are haploid as is the case with the honeybee.
Relatedness in the Honeybee
Remember relatedness is the probability or chance that two organisms may have inherited the same gene. Or the proportion of genes that two individuals share by descent from a common ancestor.
The queen is diploid (2n) having acquired one set of genetic information from her queen mother and one set from the father drone.
Drones (n) are haploid and produced in a colony by the queen. She does not fertilise her egg with her stored sperm so that all the genetic information in a drone comes from the queen.
In terms of relatedness, the probability that the drone has a gene from the queen is 100% or an r = 1.0. However from the queens perspective they share 50% of their genes, r= ½. These are not contradictory facts but a consequence of applying the analysis to a population which is haplodiploid. As far as the queen is concerned the drones or workers are related to her equally at ½.
Workers are produced by the fertilisation of one of the queens eggs with a sperm cell from the drone she mated with prior to forming the colony.
Coefficients of relatedness:
The coefficient of relatedness between a Queen and a drone (from the queen perspective) is 1 / 2.
The drone is haploid and only received one set of chromosomes from the female, there was no fertilisation.
The coefficient of relatedness between a Queen and a worker is½
The worker is diploid (2n) and was produced by a fertilisation between the queen egg and the drone sperm
The coefficient of relatedness between two workers is = drone ( 1 x ½) + Queen( ½ x ½ ) = ¾
Remember that the drone used for fertilisation produces only one kind of sperm cell or the same set of genes (no meiosis) and the queen produces her egg by meiosis.
The advantage of being a sterile worker and a care giver
This remarkable fact means that workers are ¾ related to each other and to the egg brood produced by the queen. Their relatedness to the queen is only ½. The workers are more related to each other than to the queen and they are more related than would normally occur in a purely diploid species (like elephants or humans)
If the workers reproduced they would have r = ½ relationship with the next generation. Not reproducing means that they have r= ¾ relationship with the new generation, which they are taking care of! So there was greater fitness value in the workers being sterile and taking care of their sister brood than in participating directly in the reproduction themselves.
Natural selection will select the sterile workers and their caring for the brood at the expense of their own reproduction can be seen to have a very high fitness value.
Evolution of the colony ratios.
The discussion can be taken a little further in understanding not only the evolution of altruism in the colony but also the dominance of the workers in the colony. As we have seen it is very much in the workers best interests that the queen produces lots of workers. Richard Dawkins remarks in the Selfish Gene ‘..this might predispose a female (the worker) to farm her mother as an efficient sister making machine.’
One further insight was developed by Trivers and Hare. They noted that the relatedness between workers and drones is only ¼. If the queen is allowed to determine the sex ratio at 1:1 then the benefits of caring for ¼ related males will seriously reduce the fitness and benefit to the workers (Inclusive fitness). So it is now in the interest of the workers to manipulate the colony to be mainly workers.
Trivers and Hare reworked the theory on sex ratios and found that for sisters the best ratio is 3:1 but that for the queen it is 1:1. So we have two groups the worker and the queen each with a different agenda, as far as sex ratios are concerned. The ratios (developed by RA Fisher) actually apply to the quality of investment into females and males rather than the actual numbers. Further experiments have tend to suggest that the ratio for social bees and the honeybee in particular lies close to the 3:1 ratio if the quality of investment is considered. That the large numbers of workers are representative of the quality of investment required to match the 3:1 ration. At this point the classic notion of the queen controlling the colony is starting to look precarious, who is in control of the colony, the queen or the workers?
Summary:
Altruistic behaviour evolves in animal species because they have satisfied the criteria for inclusive fitness.
Whether the organisms is diploid or haplodiploid the same 'genetical theory of social evolution' applies.
It is in the interests of matriarchal elephants and worker honeybees to sacrifice themselves for the survival of close relatives as this greatly increases the representation of their genes in the next generation.
Remember that not all altruistic acts result involve the risk of death, small acts that improve the survival of relatives will often accumulate to significantly improve the survival of the offspring or relative.
E.6.4 Outline two examples of how
foraging behaviour optimises food
intake, including bluegill fish foraging
for Daphnia.
Foraging is finding food.
All organisms need to find food to survive and reproduce.
Natural selection will favour strategies that minimise the costs of the search and maximise the benefits.
Foraging Theory suggests that the food choice of the animal will maximise the energy obtained.
The food choice is a consequence of:
1. Cost = energy used to pursue, capture and consume the food.
2. Benefit = energy from the individual food item.
Example 1 of the Blue GIll Sun Fish (Lepomis macrochirus) Optimal foraging strategy:
Predator Blue Gill Sunfish Prey:
Water fleas (Daphnia) Studies by Werner and Hall tested optimal foraging strategies in the Bluegill Sunfish (Lepomis macrochirus).
The hypothesis to be tested is that: Blue gill fish will, if the prey availability is altered, change their feeding choices to maximise the energy benefits and reduce costs
.Phase 1: Cost benefit analysis
The costs and benefits from the feeding are calculated including Energy content of different size water fleas
Time and energy required to capture the different size of water fleas
How often prey are encountered under different densities of water fleas
The independent variable = different densities of prey with different ratios of daphnia
The dependent variable was the selection of prey during the three different trials
The prey are set up at three different densities.
The prediction made included: High density only large daphnia will be eaten Low density the feeding will be equally distributed across the sizes of prey.
Results:
At a high density the large daphnia are the prey of choice but there is still some medium and small prey selected.
At low density the prediction met with a little variation between prey sizes selected Note that when the food supply changed then the Bluegill Sun fish had a foraging strategy which allows it to respond and change it s behaviour in the short term.
There is a high fitness value to be associated with such behaviour.
Example 2: The Northwestern Crow (Corvus caurinus)
The optimal foraging theory states that the costs of the feeding behaviour are less than the benefits
.On the islands off the British Columbia the northwestern crow’s choice of prey is the gastropod called a whelk.
The crow will prise the whelk form its calcareous shell by dropping from a height onto the rocks below.
If the shell is broken then the crow eats if not then repeat drops need to be made.
Flying to break the shell costs energy and detracts from the benefit of the food value.
Researchers dropped whelks form different heights to break the whelk shells. They noted the height of the drop and the number of times required to drop the shell to break.
Combining these two pieces of data produces the average total height flight required to break the shell. This is also a measure of the energy the crow must invest to break the shell. The optimal height for the experimental shell breaking was 5 metres. This was the height that required the least total height to break the shell.
Observation of the crow showed that its maximal foraging strategy gave it a preferred drop height of 5.23 m. This result has a very close approximation to the predicted 5m.
The crows have optimised their foraging behaviour.
E.6.5 Explain how mate selection can lead to exaggerated traits.
1.The male bird is a Peacock well known for its magnificent display of feathers.
2. The feathers have been demonstrated to be associated with courtship and mate selection.
3. The male displays and on the basis of a ‘good’ display he will be chosen by the rather dull female bird for reproduction.
4. The feather display might be regarded as a rather exaggerated trait, where such traits are believed to have evolved by sexual selection.
Natural selection is responsible for the selection of purely utilitarian features such as beaks for feeding, testis for producing sperm or eyes for sight. Sexual selection is responsible for the selection of features purely associated with acquiring a reproductive opportunity.
Sexual Selection or Natural Selection?
If you observe a trait in an organism (like Peacock feathers) and ask, is the feature required for anything other than mate selection? Is there any utilitarian function to the feathers? If the answer is no, then you are dealing with a trait under sexual selection.
In sexual selection the ‘greater’ the display of the trait the more mates are obtained.
The successful male gains a greater the representation of his genes in the next generation including those associated with the 'exaggerated trait'.
How big can the Peacocks feathers actually get before this causes a reduction in other general fitness?
E.6.6 State that animals show rhythmical variations in activity.
Animals often show behaviour which appears rhythmic after some fashion. Examples of rhythmic behaviour might include those such as:
Diurnal (daily cycles)
Seasonal Cycles
Lunar Cycles
E.6.7 Outline two examples illustrating the adaptive value of rhythmical behaviour patterns.
Examples of rhythmic behaviour: Uca (Fiddler crab)
In this case the courtship behaviour of the crab is linked to the new and full moon.
This lunar cycle therefore coordinates the courtship displays to the best tidal periods.
Standing at the front of the burrow the male entices in the female with a display of his enlarged claw.
Once in the burrow the entrance is sealed and the pair mate.
The adaptive value of the behaviour is in linking the display to the best display period between tides.
Example 2 The rhythmic emergence of Cicadas from their underground nymph forms.
In the eastern US there is a genus of cicada called Magicicada. There are six species, 3 of which emerge every 13 years and 3 species that emerge every 17 years.
Most of the life cycle is spent underground as a herbivorous nymph before emerging for just a few days to breed and then die. The new generations enter the soil and continue the cycle.
Notice that all members of the genus choose a cycle length which is a prime number. If we then think of the cycle of reproduction and emergence this is a very difficult number for a predator of adult cicadas to coordinate with.
It would be necessary for the predator to fall into a coordinated prime number breeding cycle and then in synch with the cicadas. When cicadas do emerge it is an event which only covers a few days but one in which millions of adults are present.
This would be a welcome feast for a predator but for their inability to coordinate their own cyclic behaviour to a prime number they have failed to take advantage of this glut of food. The adaptive value of this rhythmic behaviour in cicadas is obviously very successful.
Students might like to consider the following questions:
Why do some Cicadas not emerge earlier than the other to take advantage of a greater food supply?
Why is the significance of a larger prime number cycle like 13 or 17 rather than say 3 or 5? The answer by the way is 221.
What would happen if there was a hybridisation between 13 and 17 year Cicadas?
reference: Monte Lloyd, Gene Kritsky and Chris Simon. 1983. A Simple Mendelian Model for 13- and 17- Year Life Cycles of Periodical Cicadas, with Historical Evidence of Hybridization Between Them.Evolution, Vol. 37, No. 6 (Nov., 1983), pp. 1162-1180
Class: Mammalia
How related are individuals A and B who are first cousins
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1.The male bird is a Peacock well known for its magnificent display of feathers.
In this case the courtship behaviour of the crab is linked to the new and full moon. 