Species

Species

by Matt Toktosun

​Frequency diagrams

If we count up the number of individuals with each version of a variable feature, we can display the results as a frequency diagram.

A species of plant called kidney vetch has flowers that show variation in colour.

A student counted the number of plants with each colour of flower growing in a small area of a field.

​For each plant that she found, she put a stroke "I" in the correct column of the Tally row.

When she had recorded the flower colour of each plant, she added up all the tally strokes and wrote the number in the last row.

• ​Then she used her results to draw a number frequency diagram, like the one on of plants on the right.

Multiple Choice

Which was the most common flower colour?

• yellow

• orange

• pink

• red

• cream

Open Ended

The student recorded her results in a results table, and then in a frequency

diagram. Which of these do you think shows the results more clearly?

Explain your answer.

​Copy to your notebook

• ​Differences between organisms belonging to the same species are called variation.​

• We can show the pattern of variation in a group of organisms using a frequency diagram.

Open Ended

Please write the number of orange, white, red, pink, yellow, and purple tulips.

​Investigating variation

​Often, the variation in a species involves differences that we can count or

measure. We have seen that we can use frequency diagrams to make it easy to see any patterns in this variation.

We can also use the results that we collect to find some useful information about the variation.

For example, imagine that you have counted the numbers of petals on ten

daisy flowers.

These were your results:

18, 21, 19, 20, 20, 17 , 19, 18, 17, 20.

​You could draw a frequency diagram like this.

​The range is the spread of the values - from the smallest number of petals

you counted, to the largest number. The range for the number of petals on

the daisy flowers is 17 to 21.

• The median is the middle value in your results. The median number of

petals on the daisy flowers is 19.

• The mode is the most common value. The mode for the number of petals

on the daisy flowers is 20.

• The mean number of petals is often called the ‘average’ . To find the

mean, add up all the individual values and divide by the number of results.

For the daisy petals, the total number is:

18 + 21 + 19 + 20 + 20 + 17 + 19 + 18 + 17 + 20 = 189

So the mean is:

189 / 10 = 18.9

​To calculate the mean of a set of results, add them all up

and divide by the number of results.

​

We can show the range and pattern of variation in a characteristic using a frequency diagram.

​

The range is the spread of numbers, from the smallest to the largest.

​

The median is the middle value.

​

The mode is the most common value.