13- Evolution, The origins of vision Flashcards by Anya Laila

What term is this describing?

The change in genetic composition (and inherited characteristics) of a population over successive generations which may be mediated by mechanisms such as natural selection, inbreeding, hybridization or mutation.

Evolution

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What 4 processes fall under Evolution?

natural selection
inbreeding
hybridization
mutation

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What term is this describing?

the changes in genetic composition that enhance reproduction are retained in successive generations, i.e. traits that offer a competitive advantage are passed on. Can act at the level of genes, cells, individuals, groups of organisms and species.

natural selection

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What term is this describing?

The transfer of genetic material between different populations or species through interbreeding. It can introduce new genetic variations into a population, which can affect its evolutionary trajectory.

Hybridization or Gene Flow

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What term is this describing?

When certain types of mutations occur more frequently than others. Mutation bias can influence the direction of evolution by making some genetic changes more common than others.

Mutation Bias

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What term is this describing?

A mechanism of evolution that involves random changes in the frequency of alleles (gene variants) in a population. Genetic drift is more pronounced in small populations and can lead to the loss of genetic diversity over time.

Genetic Drift

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What term is this describing?

Also known as a selective sweep, this occurs when an allele increases in frequency because it is linked to a beneficial mutation that is being selected for. As the beneficial mutation spreads through the population, nearby alleles “hitchhike” along with it.

Groups of alleles are inherited as a group

Genetic Hitchhiking

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What term is this describing?

The process by which genetic material is exchanged between different chromosomes or different regions of the same chromosome during meiosis (a type of cell division important for sexual reproduction). Recombination creates new combinations of alleles, contributing to genetic diversity in offspring.

Recombination

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No evidence of eyes in the fossil remains of pre-cambrian organisms (first fish, first arthropods evolved)

Oldest eyes date back to which period?

Cambrian period (530-540 million years ago)

-The Cambrian Explosion resulted in an entire fauna, including virtually all animal phyla we know today

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The abrupt appearance of a wide range of organisms (including trilobites and arthropods abundant) is known as?

The Cambrian Explosion

-Visually guided predation may have been trigger for ‘evolutionary big bang’

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The evolution of vision - when did we see the light
Cambrian Fauna

What is this?

Trilobites (extinct marine fauna)

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which animals had early compound eyes similar to those of flys?

Trilobites (extinct marine fauna)

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Fossil evidence suggests a large variety of visually guided animals evolved in a short space of time (~5 million years), This led to which hypothesis?

Body size increased during Cambrian period and skeletons and rigid protection seem to have evolved at the same time
Improved mobility and visually guided predation introduced selection pressure to develop protection (e.g. eyes, body armour)

Parker’s ‘light-switch’ hypothesis:
evolution of vision key driver for diversification - visually guided predation may have been a trigger for ‘evolutionary big bang’

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Which hypothesis describes evolution of vision being a key driver for diversification - visually guided predation may have been a trigger for ‘evolutionary big bang’?

Parker’s ‘light-switch’ hypothesis

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During the Cambrian period, what seemed to have evolved at the same time?

skeletons and rigid protection

-Improved mobility and visually guided predation introduced selection pressure to develop protection (e.g. eyes, body armour)

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During the Cambrian period what was the top preditor?

The Anomalocaris

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How did early visual systems evolve?

In order to ‘see’, the species needs something to harvest light energy and an effective signaling system.

What are the 2 processes that occur?

1- All animal photoreceptors use opsin proteins, bound to a light sensitive vitamin A derivative (chromophore), to detect light energy.
(something that harvests light energy or photons out in the environment such as electromagnetic radiation)

2- Light sensitive opsins signal via a G-protein cascade and this may evolved very early in animal evolution.
(a visual pigment that we find in the outer segment of receptors) (the G-protein cascades and triggers a chemical event inside the cell telling animal that there is light around)

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Animal opsins may have originated as a modification of a … (something that generates a biological signal in response to a chemical) early in animal evolution.

chemoreceptor protein
-this can also signal whether something is salty or acidic in their environment
-it has developed to respond to light energy not just a chemical signal in the environment

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What factors shaped the evolution of vision:

We have certain genes which will allow expression of Morphology (the form and structure of organisms and their specific structural features), this creates certain types of behaviours which will moderate the fitness of that animal
(if the behaviour is benificial to the animal it will be selected for, if not selected against)

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Both factors are linked with respect to evolution:
Explain Causality

dictate the morphology and physiology
which in turn generates behavior guided by visual information
this in turn generates the fitness that selection acts upon

Both factors are linked with respect to evolution:
Explain Requirement

genetic modifications are driven by modified requirements on the morphology and physiology
which in turn are driven by modified requirements on visually- guided behaviour
and finally by requirements for improved fitness

Behaviour is the causal or required evolutionary link between fitness and the morphology /physiology of sensory systems?

causal

To understand the evolution of the eye we need to consider the evolution of what? Eyes are well matched to the visual tasks they serve.

visually-guided behaviour

State the 3 functional requirements of vision?

1- integration time

2- detecting the angle of incoming light

3- detection accuracy

Which functional requirement of vision is this describing? The time period for receptors to catch photons. Long integration time required for monitoring daily light cycle, but short integration is needed for avoiding motion blur

integration time

Which functional requirement of vision is this describing? Unshielded receptors in transparent animals are non-directional and are useful for monitoring ambient radiance, but phototaxis responses require directionality (to know where in your visual field the light is coming from)

detecting the angle of incoming light

Which functional requirement of vision is this describing? Poor detection threshold (~30% change) may be sufficient for detecting the onset of dusk, but not for discriminating boundaries of an object in a visual scene (~3% change) which is much smaller

detection accuracy

What are the 4 groups used to classify visual behaviour?

class 1- Behaviours controlled by non-directional monitoring of ambient light class 2- Behaviours based on directional light sensitivity class 3- Visual tasks based on low spatial resolution class 4- Visual tasks based on high spatial resolution

Which class of classifying visual behaviour does this description belong to? Control of circadian rhythms Monitoring water depth Avoiding harmful levels levels of UV radiation Shadow detection to avoid predators Surface detection for burrowing Monitoring light levels for reproduction

class 1- Behaviours controlled by non-directional monitoring of ambient light

Which class of classifying visual behaviour does this description belong to? Phototaxis responses Control of body position predator detection Phototaxis - movement towards or away from light a large field is sufficient (180 deg.). Spatial information is gathered by moving the field (scanning) This requires much faster responses from the photoreceptors Intensity differences from scanning are small - need better ability to detect intensity change

class 2- Behaviours based on directional light sensitivity

Which class of classifying visual behaviour does this description belong to? Detecting ego or self motion Anti-collision mechanisms Selecting a new habitat Orientation towards large landmarks or celestial objects Coarse spatial resolution is sufficient (5-25 deg.) Stacking of photoreceptor membrane is required to increase photon catch by adding multiple photoreceptors Reduced angle of capture for each receptor

class 3- Visual tasks based on low spatial resolution

Which class of classifying visual behaviour does this description belong to? Detecting prey and pursuing them Evasion of predators Mate selection Orientation or navigation using fine-scale landmarks Recognition of objects and individuals of same/different species Visual communication Require fine spatial resolution (1 arc min or 1/60 deg.) Needs an optical system that can focus light Very small angle of light capture for each receptor

class 4- Visual tasks based on high spatial resolution

Which class tasks of classifying visual behaviour can be performed at very low light intensities (starlight) with unaided and morphologically unspecialized photoreceptors (i.e. only opsin and signaling system to transfer information)?

Class 1 tasks/behaviours controlled by non-directional monitoring of ambient light

Which class tasks of classifying visual behaviour require the addition of a screening pigment or photoreceptor shielding - this makes them directionally selective? (Eye spots, or ocelli)

Class 2- Behaviours based on directional light sensitivity

Which class tasks require multiple photoreceptors that monitor light changes in different directions. (Pigment cups or pits)?

class 3- Visual tasks based on low spatial resolution

Which class tasks require the evolution of a lens (or other focussing system) and integration times need to be short to avoid motion blur (because with longer motion times blurring is greater)?

Class 4 - Visual tasks based on high spatial resolution

Info slide: IMPORTANT LEARN Minimum intensities required to support the four classes of visual tasks, plotted with the daily variation of natural luminance and daylight intensities at different depths in clear water. Explain what happens at each class:

Each higher level of task requires faster integration times, narrower angular selectivity and higher contrast sensitivity. Therefore, system must capture more photons per unit time. This is aided by membrane stacking and focussing optics

Visual systems evolve through the gradual acquisition of complex visual behaviours. Eyes of most species evolved rapidly during the ... period, coinciding with increases in body size, improvements in mobility and visually guided predation.

Cambrian

IMPORTANT: Name the Visual behaviours that correspond to the sequential stages of eye evolution

A. unshielded photoreceptor cells B. pigmented ocelli with broadly directional photoreceptors C. pigmented pit or cup eyes with low spatial resolution D. focussing optics and high spatial resolution

What do membrane stacking and photo optics allow the eye system to do?

Allow the system to capture more photons at a time and move up to higher functions

What are the 2 ways spatial vision can evolve?

1- More photoreceptors are added to the same pigment shield or cup, then a single focusing system is added to that eye (Left - camera eye) 2- The entire organ is multiplied as the pigment shield is duplicated and add an optical system to each of them (Right - compound eye)

Different types of eyes (size, design, placement etc.) can evolve from range of different tissues types but when are there no further adaptations need take place?

once optimized to the visual needs or behaviours of the animal

Name the 3 different type eyes which are modes of image formation used in both chamber and compound eyes

shadow: control direction of light by regulating the lens of the eye (camera) so light falls directly onto the receptor Refraction: bending of light as it moves through indexes entering the eye, brought to a focus on an individual receptor Reflection: light enters bouncing off reflective material at the back in mammals, helping the animal catch photons where little are

Eye evolution and visually-guided behaviour are closely linked: animals have acquired visual systems that detect ... and can support low and high ... behaviours.

light resolution

Structures responsible for spatial vision in vertebrates, cephalopods and arthropods have evolved independently: which type of development is different in each group?

embryonic development

Evolution has found different eye solutions to the same problem: need for increased spatial resolution can be achieved by which type of eyes?

camera-type eye and compound eyes

Eye evolution could have happened in a relatively short period of time (on a geological time scale) - around how many years?

half a million years

As humans have very high spatial resolution in the centre of our retina (due to the packed tightness of cone formation) which communicated with a single RGC with private line arrangement and that info is then carried, these small receptive fields give high spatial resolution/acuity. This high spatial resolution is only high in the centre of visual field and it blurs out when reaching the circumference.

The need to constrain eye size makes the camera or chamber eye a better solution, overlap in visual field is important for predators to perceive depth of objects (hunting for food)

How long does it take for an eye to evolve? Researchers started with a light-sensitive pigment shield

Assuming a continuous selection for improved spatial resolution, a patch of light-sensitive epithelium can be transformed into a fully focussed camera-type eye in less than 400,000 generations (or similar years in small invertebrate)