Natural Selection Image Map

Population Growth

You’ve learned why sexual reproduction is important for survival in the Natural Selection unit, but now, we will uncover the big picture behind all reproduction…population growth! As a population grows, the chances for evolving new species may also increase, because more individuals with specific gene frequencies are available to be acted on by natural selection.

The growth of populations occurs exponentially unless something limits it, such as running out of food. What does this mean? This means that initial growth is slow, but as time increases, so does the growth rate. How does this type of growth differ from linear growth? If you can't answer this question, Activity 1 might be a good exercise to do now. 

Bacteria provide us with a useful model of population growth. One bacterial cell divides into two cells. Each of these cells then divide, and so on, and so on. In reality, some bacteria divide every eight minutes! So, if you left one bacterial cell to divide when you left for school in the morning, by the time you arrived home from school, over six quadrillion, or 1015, cells could be waiting for you. If lined up, all of these bacteria could circle the earth at the equator fifteen times! 

So, now that we know how fast bacteria can grow and given that they’ve had a few billion years of time, why isn’t our planet mile deep in bacteria? This kind of growth is impossible. Can you think of some reasons why? Refer to Activity #1 to explore bacterial growth further.

What conditions need to be met for growth to proceed at the fastest possible rate? First, a population would need unlimited environmental resources and space. Then, all the organisms in the population must live and reproduce at a maximum rate.

For humans, some resources, such as food, may seem to be unlimited, but in reality, people starve to death every day. The Irish Potato Famine caused the death of over a million people. Could something like that happen in today’s technological society? In May 2002, Zimbabwe declared a state of disaster as drought swept the country, bringing with it widespread famine. Zimbabwe’s agricultural harvest is expected to produce one-quarter of the normal yield, at most. Many other African countries share the same food supply concerns.

Here are some factors that influence population growth:

  • Light
  • Predators
  • Food and Water
  • Contagious Disease
Is light an unlimited resource? Why? How does the amount of light affect population growth of plants? Let’s take a look at a way in which light shapes the ecosystems of a forest. Many large trees take up a majority of the sunlight, leaving only remnants to the underbrush. Also, you may have noticed that very few intermediate sized plants exist. The light that makes it past the high canopy of trees isn’t strong enough to support this type of growth.
In addition to this, many factors that are independent of the density of the population also limit population growth. The most notable examples of these are weather, climate, and most natural disasters. Think about a population of polar bears in the Sahara desert. This population would be severely limited in the desert because bears aren’t adapted to the extremely high temperatures in desert climates. Why not? 

Photo courtesy of Dan Guravich, Polar Bears International

With this knowledge we can talk about communities, or interactions of populations living in the same area. Rarely in the environment do members of a single species live without any influence on the population density and growth of another species. Even in the case with humans, cities might have a disproportionate number of people, but their living space is shared with many other organisms, like plants, dogs, birds, and bacteria. Can you give some examples of how the presence of humans affects the population growth of plants, dogs, birds, and bacteria living in the shared environment of a city?

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