Selection and Adaptation Flashcards
Evolutionary change and adaptation - what is it and how does it happen?
(R)Evolutionary change:
Darwin’s ideas were truly revolutionary. At the time of writing, there was still considerable debate:
1)as to whether animals/plants could change at all
2)How such “modification” could occur
He demonstrated that change does occur, and provided an answer to the second that has underpinned scientific approaches ever since
Variation under domestication:
“I have kept every breed [of pigeon] I could purchase…”
Variation in behaviour as well as morphology
e.g. Dogs domesticated over last ~ 15,000 years
Selective breeding of Russian Silver fox for tameness, since late 1950’s. Tamer population was also associated with physical changes (colour became lighter, tail shorter and curled)
Evolution - Biology to remember: Genes, •Made of DNA •Codeto make proteins Humans have ~20,000 protein-coding genes Alleles, •Variants of a gene •Differ in DNA sequence
Evolution - Terms to remember…
- Evolution: A change in the frequency of alleles within a population. Thus, the allele/gene is the unit of evolution.
- Phenotype: Any measurable aspect (physical/physiological/behavioural) of an individual that arises from an interaction of the individual’s genes with the environment. Genes interact with other genes and with the environment during the development of the phenotype.
- Natural Selection: The process by which genotypes (an individual’s complete set of alleles/genes) with higher fitness increase in frequency in a population. Fitness=“leaving offspring,” thus individuals leaving more offspring in future generations have higher fitness.
- Adaptation: A trait that functions to increase fitness. The only way that adaptations evolve is via natural selection.
Evolution through natural selection:
VARIATION+INHERITANCE+SELECTION
Individuals with characteristics that best solve the recurrent problems of life (finding food, surviving, reproducing and raising offspring to adulthood) will be most likely to pass on their genes to the next generation
These processes lead to:
Adaptations –traits associated with the highest relative fitness in a given environment (morphology, behaviour)
Speciation –divergence and reproductive isolation.
Speciation - how do new species evolve from existing ones?
Speciation is
•the evolutionary process by which populations evolve to become distinct species.
•accentuation of differences between parent and offspring populations
What causes speciation?
1. Having genetic variation… which can arise through:
- Recombination (genes combine in sexual reproduction)
- Mutation (alteration in DNA sequence of a gene)
2. Change in environment, migration;
environment = all aspects affecting the ‘niche’ of a species (climate composition of floral and faunal communities, quantity and quality of competitors, predators and food resources).
3. Reproductive isolation
Allopatric speciation - gene flow is interrupted when a population is divided into geographically-isolated subpopulations (Geographical barrier present). Groups from an ancestral population evolve into separate species due to a period of geographical separation. Occurs through local adaptation and genetic drift.
Allopatric Speciation –Galapagos finches
- Different species on different islands in the Galápagos archipelago
- Reproductive isolation: oceans are a barrier to gene flow
- Over millions of years, each species of finch developed a unique beak that is especially adapted to the kinds of food it eats.
Sympatric speciation -occurs in the populations that live together in the same geographical area. Diverging forms co-exist –same species in the same place that differentiate (No geographical barrier)
Sympatric Speciation –N American apple maggot fly:
•European settlers introduced apple trees about 200 years ago. Flies started to use apples as a food source instead.
•Flies born in apples tended to mate on apples: causing reproductive isolation.
•Over time, the population diverged into two genetically distinct groups with adaptations(arising by natural selection), that were specific for apple or for Hawthorne fruits (e.g flies emerge at different time of the year based on the fruit)
•Leads to reproductive isolation
Isolating mechanisms–mate recognition:
- Specific salient characteristics are selected and develop to maintain reproductive isolation (Specific Mate Recognition Systems)
- Preference for these characteristics can develop if they confer higher reproductive success
- For example, hybrids may have reduced viability or can besterile
Ecological constraints on sympatry:
Principle of competitive exclusion: 2 species cannot share the same niche (Gause’stheorem)
If two closely related species do coexist, then must be sufficiently different:
•Ecologically, through partitioning of resources
•Reproductively
SPECIATION SUMMARY:
For speciation to occur, needs:
1.Genetic variation
2.Environmental change or migration
3.Reproductive isolation
Speciation can involve geographical isolation of populations or not:
- Allopatric –geographically-separate populations change through local adaptation (natural selection) and genetic drift (random sampling of alleles between generations)
- Sympatric–diverging forms coexist in space but occupy differing niches
Natural selection of animal behaviour - examples.
Example 1 - Xenophobia in molerats:
- Colonies have dominant breeding pair: high genetic relatedness between colony members
- Populations: mesic (moderately moist) or arid (dry/harsh) environments
- Lower rainfall (arid) means scarcer resources, more competition
- Predicted higher aggression towards strangers in arid populations (scarce resources) than mesic (more resources)
Spinks et al. 1998 Behavioural Ecology 9 (4): 354-359
More aggression from arid mole rats -natural selection favours stronger xenophobic responses where resources are limited.
Lower aggression in mixed-sex pairs shows xenophobia is sensitive to costs/benefits: potential mates are worth tolerating
Example 2: When singing crickets don’t sing
Females will not mate with males who do not produce songs.
But: a parasitic fly is also attracted to the song…
Since 2001 there seemed to be a significant decline of crickets on the Hawaiian islands, and singing has become much rarer.
In 2003, Zuk (who was been studying them) only heard one male singing, but there are still lots of crickets! How?
- Most male crickets had modified wings-could not produce song.
- Selection has favoured loss of the serrated file–now 2 morphs
- Flat wing males survive better,but less attractive…(trade-off)
- so they become ‘’satellite’’ males around singing males
- Mutation occurred only 15 –20 generations ago, leading to rapid change in frequency of the two morphs
Example 3: Natural selection and cooperative hunting:
Hunting behavior—alone or in groups—represents one component of a wild dog’s phenotype, typically defined as the observable properties of an organism. An individual’s phenotype is the result of its genotype
—that is, its genetic makeup—and the way that a particular genotype manifests itself in the environment.
Hypothetical scenario: two possibilities among early wild dogs.
Lone hunting:
•Chances of wild dog catching gazelle alone is low.
•Little food = no offspring.
Pack-hunting (cooperative hunting):
•More food due to pack hunting = more offspring produced. helps us to understand why modern wild dogs display this behavior
•Behavioural trait for group hunting will increase in frequency over time.
•Natural selection would favour pack hunting
Higher foraging success will lead to higher reproductive success, so more cooperative individuals pass on more genes
Even a fitness advantage of 1% per generation is sufficient to for one behaviour to replace another over evolutionary time.
If assume (for simplicity) that hunting preference –alone or in packs –is controlled by a single gene.
…in a population of 100 dogs, the allele (a gene variant, one of two or more alternative forms of a gene) ‘for’ pack-hunting would spread to 100% of the population (‘fixation’) in 1,060 generations
….assuming average generation time for a wild dog is 5 years, this would take only 5,300 years.
Natural selection of animal behaviour -examples:
Mole rats: natural selection favoured stronger xenophobic responses where resources were limited and competition high Songless Hawaiian crickets: fast natural selection, driven by a parasite, favoured songless morph with satellite-male behaviour. Wild dogs (hypothetical example): even a relatively modest fitness advantage of cooperative hunting could favourits rapid spread through population
Natural selection resulted in adaptation of behaviour
Darwins Theory of Natural Selection
Charles Darwin studied:
- how evolutionary change shaped the diversity of life
- how the primary engine of that change is a process that he dubbed natural selection (Darwin, 1859).
Darwin argued that any trait transmitted across generations (i.e., is heritable) and provided an animal with a reproductive advantage in its population would be favored by natural selection. Natural selection is then, the process whereby traits that confer the highest relative reproductive success on their bearers increase in frequency over generations.
Whereas natural selection changes the frequency of different behaviors over the course of many generations, individual learning can alter the frequency of behaviors displayed within the lifetime of an organism. Animals learn about everything from food and shelter to predators and familial relationships. If we study how learning affects behavior within the lifetime of an organism, we are studying learning from a proximate perspective. If we study how natural selection affects the ability of animals to learn, we are approaching learning from an ultimate perspective.
Cultural transmission also affects the type of behavior animals exhibit and the frequency with which behaviors occur. While definitions vary widely across disciplines, the Principles of Animal Behaviour book focuses on the cultural transmission to mean a transmission system in which animals learn through various forms of social learning.
Cultural transmission can allow newly acquired traits to spread through populations at a very quick rate, as well as permit the rapid transmission of information across generations. As with individual learning, natural selection can also act on animals’ ability to transmit,
acquire, and act on culturally transmitted information.
Individual Learning and Natural Selection
an important relationship between learning and natural selection.
A role for learning - Imagine a female that mates with different males over the course of time. Such a female might learn which male is a good mate by keeping track of the number of eggs she laid after mating with each male.
It is certainly possible for natural selection to operate on the ability to learn. That is, natural selection might favor the ability to learn which individuals make good mates over, say, the lack of such an ability. If this were the case in the example above, learning would change behaviors within a generation, and natural selection might change the frequency of different learning rules across generations.
Some components of foraging in grasshoppers are learned. Schistocerca americana grasshoppers learned to associate various cues with food sources.
Cultural Transmission and Natural Selection
Jeff Galef illustrates the importance of cultural
transmission and social learning in animals, specifically foraging behavior in rats.
When a rat scavenges in the trash, it may encounter new food items that are dangerous or spoiled and that can lead to illness or even death. Smelling another rat provides olfactory cues about what it has eaten. This transfer of information from one rat to another about safe foods is a form of cultural transmission.
How Natural Selection Operates
To understand how natural selection operates, the first thing any ethologist needs to do is to be specific about which behavior is being studied.
Evolution is the inevitable consequence of inheritance, variation, and selection. Gradually, over many generations, individuals with characteristics that made their ancestors best able to survive and
reproduce will come to predominate.
Once a trait is specified, the process of natural selection requires three prerequisites to be met:
• Variation in the trait—different varieties of the trait. There is variation among individuals within the same species, even when they are closely related
• Fitness consequences of the trait—different varieties of the trait must affect reproductive success and/or longevity differently. What is the behavior
good for; what is its survival value?
• A mode of inheritance—a means by which the trait is passed down to the next generation.
for example, how quickly an animal approaches a novel object in its environment?
Approaching novel objects can be dangerous (if they turn out to be predators), but it might also yield benefits (if the novel object is a new type of prey).
Variation - For natural selection to act on a behavior,
behavioral variation must be present in the population under study. In the case of novel object approach behavior, different birds may approach an object they have not previously encountered slowly (taking 120 seconds to reach the object), moderately slowly (taking
60 seconds to reach the object), or quickly (taking 30 seconds to reach the object). what factors are involved in producing variation? mutation, genetic recombination, and migration.
Fitness consequences of the trait - If approaching novel objects quickly enables a bird to be the first to reach a new food source, this may contribute to its
survival, as it may get more of that food than birds with slower approach times. This variation in approach time will have fitness consequences if it leads to those approaching quickly having more eggs and hence more offspring.
Mode of Inheritance - Without a mode of inheritance, individuals that have low approach scores are
no more likely to produce offspring with low approach scores than are individuals that have high approach scores, and vice versa.
One way for measure narrow-sense heritability is through a truncation selection experiment and through parent-offspring regression
Natural vs Artificial Selection
artificial selection: the process of humans deliberately choosing certain varieties of an organism over others by implementing breeding programs that favor such varieties.
natural selection: the process whereby traits conferring the highest reproductive success to their bearers increase in frequency over time.
For more than 10,000 years, humans have used the process of artificial selection to shape the way that animals and plants look, and the way that animals behave.
Artificial selection on herding behavior:
An example of how herding behavior might be selected in dogs. In each generation, the dogs that displayed the herding behaviors that a breeder was interested in would be allowed to breed, with preferential access to breeding given to the best herders. Over many generations, breeding can lead to dogs that are excellent herders—dogs that will circle around a flock of sheep, keeping the sheep together and also keeping predators away from the flock of sheep.
Darwin came up with his theory of natural selection before Mendel’s work on genetics was disseminated. But Darwin didn’t need to know about genes per se for his theory to work; all he needed to realize was that behavioral traits that affected reproductive success were passed from parents to offspring
SELECTIVE ADVANTAGE OF A TRAIT
When nature is the selective agent, traits, including behavioral traits, increase or decrease in frequency as a function of how well they suit organisms to their
environments. If one variety of a trait helps individuals survive and reproduce better in their environment than another variety, and if the trait can be passed down across generations, then natural selection will
operate to increase its frequency over time.
