Name: fitness and selection in populations
Uploaded: 2019-01-31T09:27:05.000Z
Duration: 6 min 48 s
Channel: JoVE

K12 Standards

north-carolina-k-12-science-standard-course-of-study

NC K-12 Science Standard Course of Study

9-12

Biology

Biological Evolution- Unity and Diversity

LS.Bio.9.1

Natural Selection

Have you ever heard the phrase, survival of the fittest? When most people hear this phrase, they think of physical fitness features such as speed, strength and endurance. But what do biologists really mean by fitness?
It all started when Charles Darwin published The Origin of Species in 1859 which aroused much debate. Darwin&#39;s Theory of Natural Selection, which he defined as, '&#8230;the principle by which each slight variation, if useful, is preserved.' sparked the interest of a prominent 19th Century thinker, Herbert Spencer. In his book, Principles of Biology, Spencer coined the phrase, 'survival of the fittest', to explain Darwin&#39;s Natural Selection. This phrase resonated with many including Darwin himself and the term fitness became popular. 
However, when biologists think of an organism&#39;s fitness, they don&#39;t think about its strength, speed or endurance. Instead, they simply consider fitness as the reproductive success of an organism relative to others. Take these Snowshoe Hares for example. Organisms with greater relative fitness have adaptations that allow them to have more offspring that survive and reproduce. These adaptations are called phenotypes defined simply as a visual expression of a trait. For example, Snowshoe Hares are brown in summer, but grow a white coat when the temperature drops. The ability to change fur color allows Snowshoe Hares to camouflage. Since hares with white winter coats are less visible to predators in the snow, survival rates are much higher for hares that turn white in winter. As a result, more color changing hares will survive to reproduce in summer. 
To determine whether selection against a phenotype is taking place scientists calculate the fitness of each phenotype known as relative fitness, or 'w'. This is done by multiplying the survival rate - which is the proportion of that phenotype surviving - by the reproductive rate for each phenotype, which is the average litter size per mating season. Then, the product value of each phenotype is divided by the highest product value among the phenotypes.
Once scientists know the relative fitness, they can calculate the selection coefficient, or 's', which is the strength of selection against a phenotype, by subtracting relative fitness from 1. Higher numbers mean stronger selection against that individual phenotype. Scientists can use survival, relative fitness and selection coefficients to figure out if populations may be changing over time. For example, after the Industrial Revolution, the survival and relative fitness of light- colored peppered moths declined and the selection coefficient against them increased. The effect on the population of melanic moths was exactly the opposite, because conditions were favorable to their survival. After the passage of the Clean Air Act, the soot cleared up and the survival, relative fitness, and selection coefficient values for both phenotypes moved in opposite directions - which shows that selection favored the light moths again. 
In general, the majority of biological traits with different potential phenotypes show a normal distribution. Let&#39;s take the spots on this group of ladybugs as an example. We can see that they have a varied number of spots from zero up to 12, but the average trait value is in the middle - referred to as a normal distribution as most of the ladybugs in this population have six spots. The height of the curve indicates the number of individuals in the population displaying the given trait. Now imagine that there is selection against the ladybugs with few or no spots because they absorb less heat and don&#39;t survive so well in cold temperatures. This would be an example of directional selection which favors extreme values of a trait in one direction. 
To understand another type of selection, let&#39;s look at robins. If a female robin lays five or more eggs in a single clutch, the risk of malnourishment increases for the chicks. But if the robins lay three or less eggs, they may not have viable offspring. So here, stabilizing selection is holding a trait, egg clutch size, but an optimum value&#8230;any deviation from which is disadvantageous. Hence, most robins lay four eggs. 
In West Africa, large billed black-bellied seed cracker finches easily break bigger hard seeds of one type of sedge grass while small billed finches effortlessly maneuver and break the smaller seeds of a different type of sedge. Medium billed finches cannot open either seed as efficiently and so are rarely observed. This type of selection is referred to as disruptive - where extremes of a phenotype are optimal which creates a bimodal distribution that deviates from the normal. 
In this lab, you will model four population scenarios using pipe cleaners of varying colors and lengths representing different phenotypes and then determine the type of selection occurring in each scenario.

natural selection

<h4>Fitness</h4>
Widespread variation of phenotypes in natural populations provides the raw material for evolution, which is the change in the inherited traits of populations over successive generations. Natural selection is one of the main mechanisms of evolution and requires variable traits to be heritable and associated with differential survival and/or reproductive success. Phenotypes that correlate with greater success will have more offspring that survive to reproduce in the next generation, and will thus be represented at greater levels in each successive generation.
The reproductive success of an organism relative to others is known as its fitness and any heritable trait that contributes to an organism&#39;s fitness is known as an adaptation. Therefore, individuals or phenotypes with higher lifetime reproductive success are considered to be more fit and better adapted to their environment.
An example of differential fitness comes from the snowshoe hare, which is brown in summer but grows a white coat in winter to camouflage in snow. Nonetheless, hares that maintain their brown summer coats throughout the year would be at a fitness disadvantage due to their relatively higher visibility to predators. Therefore, higher predator visibility would decrease the survival rate of the brown coat phenotype relative to the white coat phenotype, consequently reducing its lifetime reproductive success. Climate change, however, threatens organisms that fail to adapt to conditions, thus researchers have been studying how snowshoe hares are coping with warmer temperatures and decreased snow cover in winter. Snowshoe hares that grow a white coat in environments with diminished snow cover become more visible to predators, which reduces their reproductive success. Researchers have found that snowshoe hares did not alter their behavior to adapt to mismatch in their camouflage1 and mismatched hares experienced significant decreases in their survival rates2.
Calculation of fitness relies on the relative survival and reproductive rates of different phenotypes in a population. For a given phenotype, survival rate is the proportion of that phenotype surviving, and reproductive rate is the average offspring produced per individual. The ratio between the products of the survival and reproductive rates for the different phenotypes is known as relative fitness (w). Hence, to calculate w for each phenotype (or trait), first the multiplication product of the survival and reproductive rates are calculated for individual phenotypes, this is then divided by the highest product value amongst them.
w = (SurvivalA x ReproductionA) / (Survivalmax x Reproductionmax)
This equation can be used in turn to calculate the strength of selection on different phenotypes, or the selection coefficient (s), by subtracting w from 1. Therefore, the value of the selection coefficient ranges between 0 and 1; the lower the value, the higher the selection against the specific trait. Thus, a phenotype that has a maximum fitness of 1.0, will be experiencing no selection.
s = 1 &#8211; w
<h4>Selection</h4>
There are three main types of selections: directional, stabilizing, and disruptive. Directional selection favors extreme values of a trait in one direction and can be induced by a change in the environment that the population needs to adapt to. One example is the evolution of differential jaw structures of East African cichlid fish in accordance with the diet source3. Stabilizing selection holds a trait at an optimum value; it is generally assumed that any deviation from this optimum is disadvantageous. For example, human fetus size is subject to stabilizing selection; babies born too small are less likely to survive and babies that are too big cannot pass through the birth canal during natural birth4. Disruptive selection favors two different extremes of a phenotype, creating a bimodal distribution. The evolution of West African seedcracker finch beaks resulted in two common phenotypes, large beaks that easily break hard sedge seeds and small beaks for soft seeds. Finches with medium beaks are not particularly good at consuming either seed type, thus are selected against5.
One of the most important ways natural selection impacts the lives of humans is through the rise of antibiotic resistant bacterial strains in hospitals and due to agricultural practices which utilize significant amounts of antibiotics6. The phenomenon of antibiotic resistance is a sort of inadvertent form of natural selection imposed by humans.
A question moving forward is whether natural populations will be able to adapt to the effects of climate change quickly enough to avoid extinction. This is dependent not only on existing variation in populations, but also on the strength of selection being high enough to affect quick change in the population. In this regard, reduced population sizes of many species around the world are of special concern.
<h4>References</h4>
<ol class="references">
	<li>Marketa Zimova, L. Scott Mills, Paul M. Lukacs, Michael S. Mitchell. Snowshoe hares display limited phenotypic plasticity to mismatch in seasonal camouflage. Proc of the Royal Society. 2014, Vol. 281, 1782.</li>
	<li>Nowak, Marketa Zimova L. Scott Mills J. Joshua. High fitness costs of climate change-induced camouflage mismatch. Ecology Letters. 2016, Vol. 19, 3 (299-307).</li>
	<li>R. Craig Albertson, J. Todd Streelman, and Thomas D. Kocher. Directional selection has shaped the oral jaws of Lake Malawi cichlid fishes. PNAS. 2003, Vol. 100, (9) 5252-7.</li>
	<li>Susan E. Hiby, Richard Apps, Olympe Chazara, Lydia E. Farrell, Per Magnus, Lill Trogstad, H&#229;kon K. Gjessing, Mary Carrington and Ashley Moffett. Maternal KIR in Combination with Paternal HLA-C2 Regulate Human Birth Weight. J. Immunology. 2014, Vol. 192, (11) 5069-73.</li>
	<li>Clabaut C, Herrel A, Sanger TJ, Smith TB, Abzhanov A. Development of beak polymorphism in the African seedcracker, Pyrenestes ostrinus. Evol Dev. 2009, Vol. 11, (6) 636-46.</li>
	<li>Witte, Wolfgang. Medical Consequences of Antibiotic Use in Agriculture. Science. 1998, Vol. 279, (5353) 996-7.</li>
</ol>

Fitness
Widespread variation of phenotypes in natural populations provides the raw material for evolution, which is the change in the inherited traits of ...

Education

JoVE Lab Manual

Evolution

fitness and selection in populations

Fitness and natural selection shape how traits spread through a population over time. Variation in inherited phenotypes gives evolution its raw material. When a trait is heritable and affects survival or reproduction, natural selection can change how common that trait becomes in later generations.

Fitness means reproductive success compared with others in the same population. A heritable trait that improves fitness is called an adaptation. Individuals with higher lifetime reproductive success are considered more fit and better suited to their environment.

Snowshoe hares show how differential fitness works in nature. They are brown in summer and grow a white coat in winter to blend into snow. Hares that keep a brown coat all year are easier for predators to see, so they survive and reproduce less. Climate change now creates a mismatch for hares because warmer winters and less snow can make white coats easier to spot. Researchers have found that these hares did not change their behavior to fix the mismatch, and mismatched hares had lower survival rates.

Fitness can be calculated from survival rate and reproductive rate. Survival rate is the proportion that survives, and reproductive rate is the average number of offspring per individual. Relative fitness, or w, is found by dividing the product of survival and reproduction for one phenotype by the highest product in the group. The selection coefficient, s, is then 1 minus w. A phenotype with w = 1.0 has no selection against it.

Natural selection can act in three main ways. Directional selection favors one extreme of a trait, often after an environmental change. East African cichlid fish show this through different jaw shapes linked to diet. Stabilizing selection favors an intermediate optimum. Human fetus size is a good example because babies that are too small or too large can have lower survival. Disruptive selection favors both extremes and selects against the middle. West African seedcracker finches show this pattern because large beaks crack hard sedge seeds and small beaks handle soft seeds, while medium beaks are less effective.

Natural selection also affects human health. Antibiotic resistance in bacteria is a major example, especially in hospitals and in agriculture where antibiotics are used heavily. This is an unplanned form of selection caused by human activity. A major concern now is whether natural populations can adapt fast enough to climate change. Their chances depend on existing variation in the population, the strength of selection, and whether population sizes remain large enough for change to happen.

Fitness and Selection in Populations

Fitness and natural selection shape how traits spread through a population over time. Variation in inherited phenotypes gives evolution its raw material. When a trait is heritable and affects survival or reproduction, natural selection can change how common that trait becomes in later generations. Fitness means reproductive success compared with others in the same population. A heritable trait that improves fitness is called an adaptation. Individuals with higher lifetime reproductive success...

artificial selection

Selective Breeding in Dogs and Crops

Artificial selection, also called selective breeding, is the human choice of which plants or animals reproduce. It uses natural variation in a population to produce traits people want. Over many generations, this can create domesticated organisms that look very different from their wild ancestors. Natural selection is a different process, but it helps explain how change happens in nature. In adaptive evolution, differences in traits can lead to differences in survival and reproductive success.

hardy-weinberg and genetic drift

Hardy-Weinberg Tests for Population Change

Hardy-Weinberg tests help scientists check whether a population is changing over time. Because evolution happens slowly and over large areas, researchers often use mathematical models to study it. These models use measurable data and computer simulations to predict how allele frequencies may shift across many generations. The Hardy-Weinberg equilibrium equation is one common model. It was developed independently in 1908 by G. H. Hardy and Wilhelm Weinberg. The equation gives the expected...

evolutionary relationships

DNA Evidence in Evolutionary Trees

DNA evidence helps scientists map evolutionary relationships between living things. Early classification began with Aristotle in the 4th century BC and was later improved during the Renaissance and by Carolus Linnaeus in the mid-1700s. These systems grouped species by physical similarity, such as the presence or absence of a backbone in vertebrates and invertebrates. Traits like a backbone are called synapomorphies, which are shared traits that likely came from a common ancestor. Physical...

plant diversity

Plant Adaptations for Land Life

Plant adaptations for land life explain how plants moved from water to land and diversified over time. Kingdom Plantae first appeared about 410 million years ago, when green algae began transitioning from aquatic environments to dry land. That shift was difficult, but land offered more light, more carbon dioxide, and far less competition than crowded oceans. Life on land forced plants to solve new problems. Aquatic plants can absorb water and nutrients directly from the surrounding liquid, use...

animal diversity

Animal Classification and Adaptation

Kingdom Animalia includes a wide range of organisms that share a few basic traits. Most animals are multicellular eukaryotes that move, consume organic matter, and reproduce sexually. Even with those shared features, animals are extremely diverse because each species adapts to a different niche. A niche is the place, role, and set of interactions a species has with living and nonliving parts of its environment. Specialization helps species survive, reproduce, and reduce competition with others...

microbial and fungal diversity

How Scientists Identify Bacteria and Fungi

Bacteria and fungi are highly diverse organisms, and scientists identify them by looking at their structure, growth, and role in health. Some species help people, while others can cause disease, so learning how to tell them apart matters in biology and medicine. Biological taxonomists group organisms by evolutionary relatedness. Life is divided into three domains: Bacteria, Archaea, and Eukaryota. Bacteria belong to the domain Bacteria, which includes single-celled prokaryotes. Prokaryotic...

evolution

Learn how fitness, relative fitness, and selection coefficients explain natural selection, plus examples such as snowshoe hares, cichlid fish, and antibiotic resistance.

Fitness and Selection in Populations | JoVE K12

LS.Bio.9.4

Pennsylvania State Academic Standards

Life Science

Natural Selection and Evolution

3.1.9-12.T

3.1.9-12.U

3.1.9-12.W

Louisiana Student Standards for Science

High School

Life Sciences

Biological Evolution: Unity And Diversity

HS-LS4-2

ohios-learning-standards-and-model-curriculum

Ohio’s Learning Standards and Model Curriculum

Mechanisms

Virginia  Standards of Learning

HIgh School

BIO.7

BIO.7.c

Minnesota Academic Standards in Science

3.2.1 9L.3.2.1.5

4.1.1

new-jersey-student-learning-standards-for-science

New Jersey Student Learning Standards for Science

Biological Evolution: Unity and Diversity

HS-LS4-4

Arizona Science Standards

HS+B.L4

HS+B.L4U1.14

Alabama Course of Study Science

Unity and Diversity

9-12.B.4.14

Next Generation Science Standards

heredity-inheritance-and-variation-of-traits

Heredity: Inheritance and Variation of Traits

HS-LS3-3

HS-LS4-3

Florida Department of Education

SC.912.L.15

SC.912.L.15.13

Texas Essential Knowledge and Skills

BIO.10

BIO.10.B

south-carolina-college-and-career-ready-science-standards

South Carolina College and Career Ready Science Standards

B-LS4-2.

B-LS4-3.

B-LS4-4.

new-york-state-science-learning-standards

New York State Science Learning Standards

Inheritance and Variation of Traits

HS-LS3-3.

HS-LS4-1.

HS-LS4-3.

HS-LS4-4.

Georgia Standards of Excellence

Ecology

SEC2.

SEC2.c

SB6.d

Prepare a natural selection simulation with pipe cleaners, tape, scissors, black and white construction paper, and student tables for partner stations.

Natural Selection Lab Materials Setup | JoVE K12

Model natural selection with pipe cleaners, color backgrounds, and grab-and-signal trials. Compare eyes-open and eyes-closed selection, fitness, and phenotypes.

Natural Selection in Pipe Cleaner Trials | JoVE K12

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LS.Bio.9.2

Use models to illustrate the conditions required for natural selection, including the overproduction of offspring, inherited variation, and the struggle to survive.

LS.Bio.9.3

Construct an explanation to explain how natural selection leads to adaptations within populations.

Extinction

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LS.Bio.10.1

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LS.Bio.10.2

Understand natural selection as a mechanism for biological evolution.

Fitness and Selection in Populations

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