Adaptation and Natural Selection

Lecture 3

Topic: Demecology - the ecology of populations

Section objectives:

1. Statistic and dynamic characteristics of population. Population growth.
Growth Curves.

2. Population regulation (Dynamics): Density-dependent and density-independent
factors

1. Statistic and dynamic characteristics of population. Population growth.

Population - is a group of individuals of the same species that live in the same area.

Statistic characteristics:

• Population Size - number of individuals in a given population.

• Population Density - the number of individuals in a population in given area
at a given time.

• Population Demography Structure - distribution of population number by
age accounting individual's sex (Pyramid-like graph).

Patterns of dispersion

Individual members of populations may be distributed over a geographical area in a number of different ways including

1. Clumped distribution (attraction)
2. Uniform distribution (repulsion)
3. Random distribution (minimal interaction/influence)

Survival rate – a number of individuals survived over specific period of time.

Types of bell curves of Survival rate: strait diagonal, convex, concave.

(a) Clumping may result either from individual organisms being attracted to each other, or individual organisms being attracted more to some patches within a range than they are to other patches; the net effect is that some parts of the range will have a large number of individuals whereas others will contain few or none

(b) A uniform distribution means that approximately the same distance may be found between individual organisms; uniform distributions result from individual organisms actively repelling each other

(c) A random distribution means that where individual organisms are found is only minimally influenced by interactions with other members of the same population, and random distributions are un common; "Random spacing occurs in the absence of strong attractions or repulsions among individuals of a population."

(d) Note that both clumping and uniform distributions suggest that individual organisms are either interacting with one another (actively seeking each other out or actively avoiding each other), or are all competing with one another for the same limited resources, regardless of the overall population density (as in the case of clumping which results from geographical patchiness)

DEMOGRAPHICS

– A population's demographics are its vital statistics, particularly those statistics which can impact on present and future population size

– Two statistics that are of particular import are:

- a population's age structure

- a population's sex ratio

Age structure refers to the size of cohorts within a population

A cohort is a group of individuals all of whom have the same age.

In a typical population, the size of cohorts will vary with age. For example, in a typical population, younger cohorts will be larger (i.e., more individuals per cohort) than older cohorts, all else being equal.

Parameters related to age structure include:

Fecundity (birth rate) refers to the average birth rate associated with a population.

The greater a population's fecundity, all else held constant, the faster a population will increase in size. Note that fecundity typically varies with the age of individuals.

Generation time is simply the average span between the birth of individuals and the birth of their offspring.

"Other factors being equal, a shorter generation time will result in faster population growth.“

Note that species which are capable of reproducing more than once will display an overlapping of generations which basically means that parental cohorts and progeny cohorts can be alive (and potentially competing with one another) at the same time.

Note that another way of saying this is that when life expectancies exceed the minimum time between generations, generations will overlap

Death rate Death rate is the rate at which individuals of a certain age die

Note that death rates often vary with age with either the very young or the very old displaying the greatest death rates

Note additionally that population growth occurs when overall birth rates exceed overall death rates

Dynamic characteristics:

• Population growth - is an increase in a population.

• Population growth rate - is the change in the number of individuals in a
population over time

• Biotic potential - is the rate at which a population will grow if all individuals survive and reproduce at maximum capacity.

• Carrying capacity - is the maximum number of individuals that the ecosystem is capable of supporting.

• Growth Curves - is a graph showing the number of individuals in a population over time.

The first significant contribution to the theory of population ecology was that of Thomas Malthus, an English clergyman, who in 1798 published his Essay on the Principle of Population. Malthus introduced the concept that at some point in time an expanding population must exceed supply of prerequisite natural resources, i.e., population increases exponentially resulting in increasing competition for means of subsistence, food, shelter, etc. This concept has been termed the "Struggle for Existence".

The J-shaped curve (Exponential curve) - is a growth curve that tracks two phases of population growth.

Lag-phase - little or no increase occurs in a population.

Exponential phase - increase occurs in a population so rapidly that the number of individuals doubles in a specific time interval and keeps doubling in increasingly shorter periods of time.

The S-shaped curve (Sigmoidal or Logistic curve) - is a growth curve that depicts the period or relative stability in a population that occurs after its lag and exponential phases. The top of this curve indicates the carrying capacity of an ecosystem. Notice that the J-shaped curve is the lower part of the S-shaped curve.

2. Population Regulation (Dynamics): Density-dependent and density-independent factors

Environmental Resistance - factors that tend to reduce population growth rates:

Population Regulation (Dynamics) - periodical or not periodical changes in number, sex and age of a population affected by abiotic and biotic factors (stable, changeable, explosive)

Density-dependent Factors - are factors that affect populations in different ways depending on population density (availability of food, space, light, parasitic infections and disease etc.).

Density-independent factors -are factors that affect populations regardless of population density (natural disasters as forest fire, earthquake, flood etc.)

Intrinsic - attributes of a species

• - slow reproduction

Extrinsic - external to a species

• - predators, competitors,

• environmental risks

Environmental feedbacks:

Populations degrade their environment, utilizing its resources. If the environment is highly favorable, resources are not limiting, i.e., the resources are replaced or recover faster than they are utilized. This favors rapid (exponential) growth of the population. As the rate of utilization of resources approaches the rate of replacement, the rate of population increase slows. Finally, population overshots carrying capacity of environment (resources depleted faster than can be renewed). This results in severe competition and the population collapses. When the population drops below the replacement value the environment recovers.

Adaptation and Natural Selection

Two types of adaptation:

- Acclimation -changes in an individual organism due to non-permanent physiological modifications

- Evolution -gradual changes in a species due to changes in genetic material and competition

Theory of evolution -developed by Charles Darwin and Alfred Wallace.

Natural selection - genetic combinations best adapted

for present environmental conditions tend to become abundant

• Spontaneous, random mutations

• Selective pressure - physiological stress, predation, competition, luck

Domestication -is the process in that plants and animals are adapted or trained to live in a human environment. Domestication gave people a more reliable and constant supply of food, which was one important factor in determining human population size.

Homeostasis – a biological balance between an organism and its environment.

From Greek “homeo” means the “same” and “stasis” means “standing” (“Staying the same”).

Homeostasis – state of dynamic equilibrium.

Questions for review:

1. Name statistic and dynamic characteristics of population.

2. In what ways do J-shaped and S-shaped growth curves differ?

3. What is biotic potential and carrying capacity of an ecosystem?

4. How do biotic potential and/or carrying capacity produce the J-shaped and S-shaped population growth curves?

5. What is the difference between density-dependent and density-independent factors? How are they similar?

Critical Thinking (Writing an essay) :

1. Try to describe, which factors определяют изменения в популяции бездомных собак в вашем городе?

2. Раньше в Алматы была довольно большая популяции горлинок и воробьев, но в настоящее время они практически исчезли из города. С чем это может быть связано.

3. Опишите, чем по вашему мнению определяется carrying capacity для экосистемы города Алматы.

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