Population and community are not the same concept.
Basic introduction Chinese name: population mbth: population status: population is the basic unit of evolution description 1: population density description 2: birth rate and mortality description 3: migration rate and emigration rate description, population density, birth rate and mortality rate, migration rate and emigration rate, sex ratio, age structure, spatial pattern, dynamic change, mathematical model, natural growth and decline, population regulation, intraspecific relationship, clustering. Population and evolution, overview, gene pool, geographical and reproductive isolation, gene frequency, and feature description Population density: Population density refers to the number of individuals per unit area or volume, and "population density" is different from "density", and the former is individual. Both the prediction of agricultural and forestry pests and the determination of fishing intensity need to investigate the population density. In the natural state, the population density of a population often fluctuates greatly, but it is not infinite. Birth rate, death rate, emigration rate and emigration rate all have effects on population density. There are upper and lower limits on the population. The upper limit of population density is determined by the energy flow of the ecosystem where the population is located, and the lower limit is difficult to determine. The stable regulation of ecosystem can keep the population density of dominant organisms within a limited range. Statistics and estimation methods of population density Population density plays an important role in production and life. Here are two commonly used statistical and estimation methods, among which "equal possibility" is the most critical, and human factors cannot be mixed. ① Sampling methods Common sampling methods are suitable for investigating animals and plants with weak activity, such as jumping insects, snails and dandelions. The operation process is as follows: randomly select several completely equal quadrats within the investigation range, count the number of individuals in each quadrat, and calculate the population density of each quadrat, and then calculate the average population density of all quadrats, and take this value as the estimated population density of the investigation population. Common sampling methods include equidistant sampling, five-point sampling and Z-shaped sampling. ② Mark recapture method The mark recapture method is suitable for animals with strong mobility and large range of activities, such as voles, birds and fish. The operation process is as follows: within the activity range of the investigated population, some individuals are captured, marked and put back to the original environment, and recaptured in situ after a period of time. The estimation formula is: total population/number of tagged individuals = number of recaptured individuals/number of recaptured tagged individuals. The estimated value obtained by this estimation method is often large, because many animals will be more difficult to capture after being captured once, resulting in a few "recaptured marked individuals". When marking, it should also be noted that the signs used should be small and light, which cannot affect the biological action; Don't use too eye-catching colors (such as red), otherwise it will make creatures more vulnerable to predators and affect the accuracy of estimation. Birth rate and mortality The birth rate refers to the proportion of newly born individuals in the population to the total number of existing individuals in the population in a specific period; Mortality is the ratio of the number of dead individuals in a population to the total number of existing individuals in a specific time. In the natural state, the birth rate and death rate determine the change of population density. The birth rate is greater than the death rate, and the population density increases. The same is true of other situations. Immigration rate and emigration rate Many biological populations have the phenomenon of immigration and emigration, and the immigration or emigration of a large number of individuals will have a significant impact on population density. For a certain population, the ratio of the number of individuals who move in or out of the population per unit time to the total number of individuals in the population is the migration rate and the emigration rate respectively. In modern ecology, the rate of immigration and emigration plays an important role in the study of urban population. Sex ratio Sex ratio refers to the proportion of male and female individuals in a population. In nature, the normal sex ratio of different populations varies greatly, and the sex ratio has certain influence on the population number. For example, a large number of male individuals are lured by sex attractants and die, which will make many female pests unable to complete mating and lead to a decline in population density. The age structure of the age structure population refers to the number of individuals in the population, including young individuals (pre-reproductive period), adult individuals (reproductive period) and elderly individuals (post-reproductive period). Growth (a) and stable (b) In the growth population, the number of elderly individuals is small, and the number of young individuals is large, which is figuratively pyramid-shaped. In the future, the population density will continue to increase, and there will be more and more individuals within the species. At present, most stable people are stable people, and the age structure of stable people is moderate. In a certain period of time, the number of new individuals is equivalent to the number of dead individuals, and the population density remains relatively stable. The decline in population mainly occurs in endangered species. The number of young individuals is small, the number of old individuals is large, and the mortality rate is greater than the birth rate. This situation will often lead to a vicious circle and the population will eventually become extinct, but it does not rule out some situations that fundamentally reverse the development trend, such as the sudden improvement of the living environment, a large number of new individuals moving in or artificial reproduction. Spatial pattern The position, state or layout of individuals who make up a population in its space is called population spatial pattern. The spatial pattern of population can be roughly divided into three categories: uniform distribution refers to the spatial pattern produced by the uniform distribution of population at a certain distance in space. The fundamental reason is the balance between intra-species struggle and maximum utilization of resources. The uniform distribution of many populations is man-made, such as the uniform distribution of rice in farmland ecosystem. There are also even distributions in nature, such as the even distribution of some trees in the forest. Random random distribution means that every individual has the same chance to appear at every point in the population field, and the existence of one individual does not affect the distribution of other individuals. Random distribution is rare, because it is easy to produce random distribution when environmental resources are evenly distributed and individuals in the population do not attract or repel each other. For example, some spiders in forest ground cover, Tenebrio molitor in flour, etc. Cluster distribution) Cluster distribution is the most common type of internal distribution. The reasons for the formation of group distribution are: the uneven distribution of environmental resources and the embedding of the rich and the poor; ⑵ The way plants spread seeds is to take the mother plant as the diffusion center; (3) Social behaviors of animals make them form groups. Group distribution can be further divided into uniform group, random group and group according to the distribution of the group itself, and the latter has two-level group distribution. Mathematical model of dynamic change Some simple and typical dynamic changes in population can be measured by mathematical model. There are two common exponential growth models (J-type growth), which were put forward by thomas T.Maithus, a famous demographer. He thinks that population growth is not a simple additive relationship, but a multiple growth. Later, biologist C.R. charles robert C.R.Darwin reconfirmed this growth pattern through the study of elephant population. This objective growth model shows that all populations have the ability of explosive growth. The function of exponential growth is an exponential equation, the variable is time t, and the constant is a multiple of population density growth. This growth model has no upper limit, and complete exponential growth only exists in the ideal situation that there are no natural enemies and food and space are absolutely sufficient (so there is no intraspecific struggle). In real life, when bacteria, invasive organisms (such as Eichhornia crassipes) and cyanobacteria just inoculated in Petri dishes erupt, the population will increase exponentially for a considerable period of time, and then tend to be stable or die in large numbers. Logistic growth ("S" growth) main term: Logistic growth model exponential growth is an ideal situation, and the number of many organisms will remain stable after exponential growth for a period of time, which can be described by another mathematical model. Example: Russian ecologist G.W. Gowther once did an experiment. Five giant paramecium were put into 0.5ml culture medium, and the population density of the population was counted every 24 hours. The result is shown on the right. As can be seen from the figure, after rapid growth, the number of giant paramecium is stable at 75 (K value). The experimental results show that Logistic growth model can better guide the regulation of artificial population. There is a logical upper limit of population carrying capacity, which is called environmental carrying capacity, abbreviated as "K value", representing the maximum carrying capacity of population without destroying the environment, or the maximum number of population in the environment. When the population density is K/2, a population grows fastest, which can guide the collection of economic organisms and keep the population density within the range of K/2 all the time. "Redundant" collection can make economic organisms grow at the fastest speed. Natural growth and decline In nature, the quantitative change of a population is not only an increase, but also does not necessarily conform to the above mathematical model. Its quantitative change has some basic characteristics. ① Seasonal changes generally include seasonal reproduction types. The maximum population density often drops after the last reproduction in a year, and then reproduction stops. The population is sterile because of death, so the population density decreases, which continues until the beginning of the next breeding season, and the population number is the lowest, thus appearing. Example: In the cold regions of Eurasia, many small birds and mammals usually stop breeding in winter, and their populations are the lowest before they start breeding in spring. After breeding began in spring, the population number kept rising, and reached the highest level in a year before stopping breeding due to cold in autumn. Larger animals, such as badgers and marmots, breed only once a year. Their breeding season is in spring, and their number reaches its peak after birth. Later, due to death, the number gradually decreased. When investigating populations with seasonal changes in population density, it is usually done twice. (2) Annual change Under relatively stable environmental conditions, the population density of seed plants and large vertebrates shows periodic changes in a long time span. For example, common trees such as poplar and willow blossom and bear fruit once a year, and the number of seeds is relatively stable; Another example is large ungulates, which generally have 1 ~ 2 babies every year, and the population is relatively stable. The population of Canadian argali has changed in 36 years, and the ratio of the highest to the lowest is only 4.5 times. In the past 20 years, the ratio of the highest number to the lowest number of American red deer in winter is only 1.8 times. There are also some species whose numbers fluctuate violently, but not periodically. The most famous species is the housefly. It lives in houses, farmland and threshing floors. According to the statistical data of Chinese Academy of Sciences 16, the annual average capture rate fluctuates between 0. 10 ~ 17.57, that is, the highest-lowest ratio is several hundred times. Another example is the Brandt vole, which also has irregular quantitative changes. In the lowest year, there were only 1.3 per hectare on average, and in the highest year, there were 786 per hectare, which was more than 600 times worse. There are birth and death in the population, and its members are constantly updated, but this change often revolves around an average density. In other words, when the population is increased or decreased by some interference, it often returns to the original level. This situation is dynamic equilibrium. Any organism with irregular or periodic fluctuations in population outbreaks may have population outbreaks, and red tide is an example of this situation. The main item of population regulation: natural regulation of population In nature, most populations are in a relatively stable state. Due to various factors, the proportion of population relative to other organisms in the biological community is maintained at a certain density level, which is called the natural balance of population and the equilibrium density. Due to the constraints of various factors on the natural population, it is impossible for the population to grow indefinitely and eventually tend to be relatively balanced, and the density factor is an important factor to adjust its balance. The process of population returning to its equilibrium density after leaving is called population regulation. The factors that can restore the population to its original equilibrium density are called regulatory factors. Most biological populations in the world have reached a stable equilibrium period. This balance is dynamic. On the one hand, many physical and biological factors can affect the birth rate and death rate of the population. On the other hand, the population has the ability of self-adjustment to keep the population in balance. There are many factors that affect the number of individuals in a group. The function of some factors varies with population density, which is called density restriction factor. For example, infectious diseases are more likely to spread in high-density people, which has a greater impact on the population, on the contrary, it has a smaller impact in low-density people; For another example, in densely populated areas, the greater the intensity of competition, the greater the impact on the population, and vice versa. Feedback regulation of density constraints: the relative stability of biological population is related to regular fluctuations and the role of density constraints. When the population growth exceeds the carrying capacity of the environment, the impact of density constraints on the population increases, thus increasing the mortality rate and keeping the population below full load. When the population is lower than the carrying capacity, the effect of density constraint is weakened and the population is increased. Now give a few examples to illustrate this feedback adjustment. ① Take the lynx living in Canada as an example to illustrate food. Researchers have studied the number of lynx and mountain hare in 90 years, and found that the population density of lynx is roughly proportional to the population density of mountain hare. The more mountain rabbits there are, the more food the lynx has, and the more population it has. (2) Fertility is also affected by density. The snails in the pond lay more eggs at low density and less eggs at high density. In the forest areas of Britain, the number of eggs laid by great tits per nest decreases or increases with the population density. But this effect may also be caused by the lack of food or other factors in the case of high density. ③ Secretion inhibitors Many organisms have the ability to regulate population density by secretion inhibitors. Among plants, eucalyptus is self-toxic, and when the density is high, it can reduce its number by itself. Bacteria have a similar situation: when they multiply too much, their metabolites will limit the increase in quantity; When the density is reduced, these metabolites are less, which is not enough to inhibit, so the number can be increased. ④ The higher the population density of diseases and parasites, the easier it is for epidemic diseases and parasitic diseases to spread, resulting in more individual deaths and lower population density. The population density is low, but the disease is not easy to spread. As a result, the population density gradually recovers. Although some factors limit the population size, the intensity of non-density constraint has nothing to do with population density. This is the case with climate factors. Wind, rain, snowfall and temperature will all affect the population, but the effect of this factor has nothing to do with population density. Such factors are called non-density constraints. Function: The irregular changes of biological population are often related to non-density constraints. The influence of non-density constraints on population size is always fierce and disastrous. For example, the plague of locusts frequently recorded in the history of China was caused by Locusta migratoria manilensis. One of the physical factors causing locust outbreaks is drought. Locusta migratoria manilensis lays eggs on the grass of Gramineae. If there is much rain, a large number of eggs will die because of floods or mold infections, so it can't be a disaster. Locusts only occur when the climate is dry. Therefore, successive years of drought in China's history are often accompanied by insect disasters. The relationship between them, density constraints and non-density constraints, which has a greater impact on population density, needs specific analysis. Non-density constraints such as physical factors have no feedback effect, but their effects can be adjusted by density constraints, that is, by the feedback mechanism of density constraints. When some physical factors have changed greatly (such as drought and severe cold) or people's activities (such as using pesticides) increase the death rate of the population and greatly decrease the population, the constraint of equal density of food will no longer play a controlling role, so the birth rate will rise and the population will soon return to its original level. By studying the variation law of biological population and the factors affecting the variation, especially the self-regulation ability of the population, we can formulate measures to control the population, predict the variation of the population, and serve the production (such as making plans to prevent pests, predicting the occurrence of pests and harmful animals, and determining the rationality of hunting and logging, etc.). ). Intra-species relationship cluster main item: Cluster (aggregahon or society, group) phenomenon is widespread in natural populations. Different individuals of the same organism will live together for a certain period of time to ensure the survival and normal reproduction of the population, so clustering is an important adaptive feature. In a group, some individuals may live together to form a group, but others may live alone. For example, although most lions live in groups in a family way, some live alone. According to the classification of the duration after clustering, clustering can be divided into temporary and permanent types. (1) permanent cluster permanent cluster exists in social animals (bees, ants, termites, etc. ) and higher animals (primates). Due to the specialization of social insects, different individuals in the same population have different forms. For example, in the ant society, there are a large number of worker ants, soldier ants and queens. Worker ants are responsible for collecting food, raising offspring and building nests. Soldier ants are specially responsible for security work and have powerful mouthparts; The queen ant becomes a reproductive machine specialized in laying eggs, with enlarged gonads and specific sexual behaviors, while its foraging and defense functions are completely degraded. (2) Temporary Clusters Most clusters belong to temporary clusters, and the phenomenon of temporary clusters is common in nature, such as seasonal clusters such as migration clusters and breeding clusters, as well as temporary groups composed of foragers and habitats. The reasons for biological clusters are complex and diverse. It mainly includes the following aspects: ① The demand for food, light, temperature and water in the habitat is the same. For example, the wet habitat makes some snails gather together in groups, and the body of a deer is used as food and shelter, attracting many scavengers to form groups; (2) The same reaction to day and night weather or seasonal climate. For example, the great migration of wildebeest on the African grassland; (3) As a result of reproduction, because parents have the same reaction to an environment, their offspring (eggs or offspring) are born in the same environment, and the offspring form a group together. For example, the migration of Atlantic salmon, the young salmon colony after spawning. Family cluster is also caused by similar reasons, but the individuals in the family have certain kinship; ④ The result of passive transportation. For example, strong winds and rapids can transport some mosquitoes and small fish to a place with slow wind speed or flow speed to form a group; ⑤ As a result of mutual attraction between individuals. Social animals, especially permanent social animals, usually have a strong group desire, which is caused by the mutual attraction between individuals. When a wandering pigeon meets a group of strangers, there is no doubt that the wandering pigeon will soon join the ranks of strangers. Sometimes, due to the strong desire to gather, some animals even join other species' groups to satisfy their desire to gather, such as wandering seagulls joining petrels; The formation of animal groups may be entirely determined by environmental factors or caused by social attraction. According to these two different reasons, animal groups can be divided into two categories, the former is called collection and the latter is called society. Cluster effect and ecological significance Many animal species in the animal kingdom live in groups, which shows that group life has many biological significance, and the adaptive value of group advantage promotes the evolution of animal social structure. At present, it is known that many kinds of insects and vertebrates can produce beneficial effects. The beneficial effect of living with the same animal becomes a cluster effect. The ecological significance of clustering mainly includes the following aspects: ① clustering is conducive to improving predation efficiency; (2) Cluster can * * defend the enemy; ③ Cluster is beneficial to change the niche; ④ Clustering can improve the learning efficiency of some animal species; ⑤ Clustering can promote reproduction. Territory If animals defend an area and do not allow other animals (usually animals of the same species) to enter, then this area or space is called territory, and the behavior of animals occupying territory is called territory. Domain behavior is one of the manifestations of intraspecific struggle. Some areas are temporary, for example, most birds only establish and defend areas during breeding. Some fields are permanent, such as those established by big cat carnivores. The demand for resources of the same species of animals is the same, and the territoriality is caused by intra-species struggle. The owner of the territory excludes other individuals of the same kind from entering, which can reduce competition and gain more resources. In addition, there are territorial behaviors among species with similar behaviors, but this already belongs to the community category. Social class Social class refers to the hierarchical phenomenon that the status of each animal in the animal population has a certain order. The formation of social hierarchy is based on dominant behavior, or master-slave relationship. For example, domestic chicken farmers are very familiar with pecking among chickens. After pecking, they form a hierarchy. After stabilization, low-level general representatives compromise and obey, but sometimes they will change the order level by fighting again. Stable flocks of chickens tend to grow fast and lay more eggs. The reason is that individuals in unstable groups often struggle with each other and consume a lot of energy, which explains the rationality of social hierarchy in evolutionary selection. The superiority of social class also includes that the dominant individuals have priority in food, shelter and mate selection, which ensures that the strong within the species get the opportunity to mate and produce offspring first, thus contributing to the preservation and continuation of the whole race. The formation of communicative social organizations also needs to be based on the mutual transmission of information between individuals. Information transmission, or communication, means that one person sends a signal and another person receives it. And cause the reaction of the latter. According to the nature and reception of signals, communication can be divided into vision, chemistry and hearing. The purpose of information transmission is very extensive, such as the identification of individuals, including the identification of individuals of the same species, the same community and the same family, the communication between parents and offspring, the courtship between the sexes, the intimidation, obedience and compromise between individuals, mutual warning, marking the field and so on. From the perspective of evolution, we should choose a signal that is convenient for transmission, saves energy consumption, has small error and is necessary for survival. Overview of population and evolution In modern biological evolution theory, population is the basic unit of biological evolution, and individuals within a species constantly evolve through non-directional mutation and natural selection. Gene bank All genes of all individuals in a population are called the gene bank of that population. In a population's gene pool, the proportion of a gene to all alleles is called gene frequency. For example, in peas, the allele that determines the color of peas is: R (dominant, green) R (invisible, yellow). Assuming that 200 individuals are randomly selected from a pea population, the number of rr, Rr and RR is 80, 40 and 80 respectively, then it can be calculated that R = 80× 2+40 = 200 (a) R = 40+80× 2 = 200 (a) R = 200 ÷ (200+) Also known as the "law of genetic balance", in 1908, the British mathematician Godfrey Harold Hardy first discovered and proved this law; 1909, German doctor William Weinberg also independently proved this law, hence the name. Hardy-Weinberg law is mainly used to describe the relationship between allele frequency and genotype frequency in population. The contents are as follows: ① An infinite population randomly mates under ideal conditions, and after several generations, the gene frequency and genotype frequency can still maintain a stable balance. ② In the case of a pair of alleles, the gene frequency relationship between gene P (dominant) and gene Q (recessive) is: (P+Q) 2 = 1 binomial expansion: P 2+2pq+Q 2 = 1, where "P 2" is the proportion of dominant homozygotes. Hardy-Weinberg law can also be applied to more complex situations, such as polyploidy. The main items of geographical and reproductive isolation: reproductive isolation, geographical isolation, the situation that individuals of different populations can't mate with each other or produce fertile offspring (such as mules produced by crossing donkeys with horses) is called reproductive isolation, which can distinguish different species or subspecies, that is, reproductive isolation is a sign to distinguish species. The reproductive isolation between Kaibabu squirrel (left) and Abbott squirrel is often caused by geographical isolation. After the same population is forcibly separated due to geographical factors (orogeny, continental drift, etc.). ), after hundreds of millions of years of variation and natural selection, different species will be formed. For example, the Kaibabu squirrel and Abbott squirrel on both sides of the Grand Canyon in the United States were originally a species, and their ancestors were divided into two isolated populations because of the formation of the Grand Canyon River. Finally, the gene frequency of all genes contained in all individuals in a population is called the gene pool of this population. The genes contained in each individual in the population are only a part of the gene pool of the population. The proportion of a gene in a population's gene pool is called the frequency of the gene. The proportion of all different genes in the population gene pool constitutes the population gene frequency. In nature, population gene frequency is always changing due to gene mutation, gene recombination and natural selection. In fact, natural selection is to select some genes and eliminate others, so natural selection will inevitably cause directional changes in the gene frequency of the population and determine the direction of biological evolution.